From 43d96e4be29752e4c7f7d11df974daa7bd2a5a4b Mon Sep 17 00:00:00 2001
From: Aikar <aikar@aikar.co>
Date: Sat, 21 Jul 2018 16:55:04 -0400
Subject: [PATCH] Async Chunk Loading and Generation

This brings back parity to 1.12 and older versions in that any
chunk requested as part of the PlayerChunkMap can be loaded
asynchronously, since the chunk isn't needed "immediately".

The previous system used by CraftBukkit has been completely abandoned, as
mojang has put more concurrency checks into the process.

The new process is no longer lock free, but tries to maintain locks as
short as possible.

But with 1.13, we now have Chunk Conversions too. A main issue about
keeping just loading parity to 1.12 is that standard loads now
are treated as generation level events, to run the converter on
another thread.

However mojangs code was pretty bad here and doesn't actually provide
any concurrency...

Mojangs code is still not thread safe, and can only operate on
one world per thread safely, but this is still a major improvement
to get world generation off of the main thread for exploration.

This change brings Chunk Requests triggered by the Chunk Map to be
lazily loaded asynchronously.

Standard chunk loads can load in parallel across a shared executor.

However, chunk conversions and generations must only run one per world
at a time, so we have a single thread executor for those operations
per world, that all of those requests get scheduled to.

getChunkAt method is now thread safe, but has not been tested in
use by other threads for generations, but should be safe to do.

However, we are not encouraging plugins to go getting chunks async,
as while looking the chunk up may be safe, absolutely nothing about
reading or writing to the chunk will be safe, so plugins still
should not be touching chunks asynchronously!

diff --git a/src/main/java/com/destroystokyo/paper/PaperConfig.java b/src/main/java/com/destroystokyo/paper/PaperConfig.java
index 07d7976d2..c25db284f 100644
--- a/src/main/java/com/destroystokyo/paper/PaperConfig.java
+++ b/src/main/java/com/destroystokyo/paper/PaperConfig.java
@@ -376,4 +376,57 @@ public class PaperConfig {
             }
         }
     }
+
+    public static boolean asyncChunks = false;
+    public static boolean asyncChunkGeneration = true;
+    public static boolean asyncChunkGenThreadPerWorld = true;
+    public static int asyncChunkLoadThreads = -1;
+    private static void asyncChunks() {
+        if (version < 15) {
+            boolean enabled = config.getBoolean("settings.async-chunks", true);
+            ConfigurationSection section = config.createSection("settings.async-chunks");
+            section.set("enable", enabled);
+            section.set("load-threads", -1);
+            section.set("generation", true);
+            section.set("thread-per-world-generation", true);
+        }
+
+        asyncChunks = getBoolean("settings.async-chunks.enable", true);
+        asyncChunkGeneration = getBoolean("settings.async-chunks.generation", true);
+        asyncChunkGenThreadPerWorld = getBoolean("settings.async-chunks.thread-per-world-generation", true);
+        asyncChunkLoadThreads = getInt("settings.async-chunks.load-threads", -1);
+        if (asyncChunkLoadThreads <= 0) {
+            asyncChunkLoadThreads = (int) Math.min(Integer.getInteger("paper.maxChunkThreads", 8), Runtime.getRuntime().availableProcessors() * 1.5);
+        }
+
+        // Let Shared Host set some limits
+        String sharedHostEnvGen = System.getenv("PAPER_ASYNC_CHUNKS_SHARED_HOST_GEN");
+        String sharedHostEnvLoad = System.getenv("PAPER_ASYNC_CHUNKS_SHARED_HOST_LOAD");
+        if ("1".equals(sharedHostEnvGen)) {
+            log("Async Chunks - Generation: Your host has requested to use a single thread world generation");
+            asyncChunkGenThreadPerWorld = false;
+        } else if ("2".equals(sharedHostEnvGen)) {
+            log("Async Chunks - Generation: Your host has disabled async world generation - You will experience lag from world generation");
+            asyncChunkGeneration = false;
+        }
+
+        if (sharedHostEnvLoad != null) {
+            try {
+                asyncChunkLoadThreads = Math.max(1, Math.min(asyncChunkLoadThreads, Integer.parseInt(sharedHostEnvLoad)));
+            } catch (NumberFormatException ignored) {}
+        }
+
+        if (!asyncChunks) {
+            log("Async Chunks: Disabled - Chunks will be managed synchronosuly, and will cause tremendous lag.");
+        } else {
+            log("Async Chunks: Enabled - Chunks will be loaded much faster, without lag.");
+            if (!asyncChunkGeneration) {
+                log("Async Chunks - Generation: Disabled - Chunks will be generated synchronosuly, and will cause tremendous lag.");
+            } else if (asyncChunkGenThreadPerWorld) {
+                log("Async Chunks - Generation: Enabled - Chunks will be generated much faster, without lag.");
+            } else {
+                log("Async Chunks - Generation: Enabled (Single Thread) - Chunks will be generated much faster, without lag.");
+            }
+        }
+    }
 }
diff --git a/src/main/java/com/destroystokyo/paper/util/PriorityQueuedExecutor.java b/src/main/java/com/destroystokyo/paper/util/PriorityQueuedExecutor.java
new file mode 100644
index 000000000..8f18c2869
--- /dev/null
+++ b/src/main/java/com/destroystokyo/paper/util/PriorityQueuedExecutor.java
@@ -0,0 +1,347 @@
+package com.destroystokyo.paper.util;
+
+import javax.annotation.Nonnull;
+import java.util.ArrayList;
+import java.util.List;
+import java.util.concurrent.AbstractExecutorService;
+import java.util.concurrent.CompletableFuture;
+import java.util.concurrent.ConcurrentLinkedQueue;
+import java.util.concurrent.RejectedExecutionException;
+import java.util.concurrent.TimeUnit;
+import java.util.concurrent.atomic.AtomicBoolean;
+import java.util.concurrent.atomic.AtomicInteger;
+import java.util.function.Supplier;
+
+/**
+ * Implements an Executor Service that allows specifying Task Priority
+ * and bumping of task priority.
+ *
+ * This is a non blocking executor with 3 priority levels.
+ *
+ * URGENT: Rarely used, something that is critical to take action now.
+ * HIGH: Something with more importance than the base tasks
+ *
+ * @author Daniel Ennis &lt;aikar@aikar.co&gt;
+ */
+@SuppressWarnings({"WeakerAccess", "UnusedReturnValue", "unused"})
+public class PriorityQueuedExecutor extends AbstractExecutorService {
+
+    private final ConcurrentLinkedQueue<Runnable> urgent = new ConcurrentLinkedQueue<>();
+    private final ConcurrentLinkedQueue<Runnable> high = new ConcurrentLinkedQueue<>();
+    private final ConcurrentLinkedQueue<Runnable> normal = new ConcurrentLinkedQueue<>();
+    private final List<Thread> threads = new ArrayList<>();
+    private final RejectionHandler handler;
+
+    private volatile boolean shuttingDown = false;
+    private volatile boolean shuttingDownNow = false;
+
+    public PriorityQueuedExecutor(String name) {
+        this(name, Math.max(1, Runtime.getRuntime().availableProcessors() - 1));
+    }
+
+    public PriorityQueuedExecutor(String name, int threads) {
+        this(name, threads, Thread.NORM_PRIORITY, null);
+    }
+
+    public PriorityQueuedExecutor(String name, int threads, int threadPriority) {
+        this(name, threads, threadPriority, null);
+    }
+
+    public PriorityQueuedExecutor(String name, int threads, RejectionHandler handler) {
+        this(name, threads, Thread.NORM_PRIORITY, handler);
+    }
+
+    public PriorityQueuedExecutor(String name, int threads, int threadPriority, RejectionHandler handler) {
+        for (int i = 0; i < threads; i++) {
+            ExecutorThread thread = new ExecutorThread(this::processQueues);
+            thread.setDaemon(true);
+            thread.setName(threads == 1 ? name  : name + "-" + (i + 1));
+            thread.setPriority(threadPriority);
+            thread.start();
+            this.threads.add(thread);
+        }
+        if (handler == null) {
+            handler = ABORT_POLICY;
+        }
+        this.handler = handler;
+    }
+
+    /**
+     * If the Current thread belongs to a PriorityQueuedExecutor, return that Executro
+     * @return The executor that controls this thread
+     */
+    public static PriorityQueuedExecutor getExecutor() {
+        if (!(Thread.currentThread() instanceof ExecutorThread)) {
+            return null;
+        }
+        return ((ExecutorThread) Thread.currentThread()).getExecutor();
+    }
+
+    public void shutdown() {
+        shuttingDown = true;
+        synchronized (this) {
+            this.notifyAll();
+        }
+    }
+
+    @Nonnull
+    @Override
+    public List<Runnable> shutdownNow() {
+        shuttingDown = true;
+        shuttingDownNow = true;
+        List<Runnable> tasks = new ArrayList<>(high.size() + normal.size());
+        Runnable run;
+        while ((run = getTask()) != null) {
+            tasks.add(run);
+        }
+
+        return tasks;
+    }
+
+    @Override
+    public boolean isShutdown() {
+        return shuttingDown;
+    }
+
+    @Override
+    public boolean isTerminated() {
+        if (!shuttingDown) {
+            return false;
+        }
+        return high.isEmpty() && normal.isEmpty();
+    }
+
+    @Override
+    public boolean awaitTermination(long timeout, @Nonnull TimeUnit unit) {
+        synchronized (this) {
+            this.notifyAll();
+        }
+        final long wait = unit.toNanos(timeout);
+        final long max = System.nanoTime() + wait;
+        for (;!threads.isEmpty() && System.nanoTime() < max;) {
+            threads.removeIf(thread -> !thread.isAlive());
+        }
+        return isTerminated();
+    }
+
+
+    public PendingTask<Void> createPendingTask(Runnable task) {
+        return createPendingTask(task, Priority.NORMAL);
+    }
+    public PendingTask<Void> createPendingTask(Runnable task, Priority priority) {
+        return createPendingTask(() -> {
+            task.run();
+            return null;
+        }, priority);
+    }
+
+    public <T> PendingTask<T> createPendingTask(Supplier<T> task) {
+        return createPendingTask(task, Priority.NORMAL);
+    }
+
+    public <T> PendingTask<T> createPendingTask(Supplier<T> task, Priority priority) {
+        return new PendingTask<>(task, priority);
+    }
+
+    public PendingTask<Void> submitTask(Runnable run) {
+        return createPendingTask(run).submit();
+    }
+
+    public PendingTask<Void> submitTask(Runnable run, Priority priority) {
+        return createPendingTask(run, priority).submit();
+    }
+
+    public <T> PendingTask<T> submitTask(Supplier<T> run) {
+        return createPendingTask(run).submit();
+    }
+
+    public <T> PendingTask<T> submitTask(Supplier<T> run, Priority priority) {
+        PendingTask<T> task = createPendingTask(run, priority);
+        return task.submit();
+    }
+
+    @Override
+    public void execute(@Nonnull Runnable command) {
+        submitTask(command);
+    }
+
+    public boolean isCurrentThread() {
+        final Thread thread = Thread.currentThread();
+        if (!(thread instanceof ExecutorThread)) {
+            return false;
+        }
+        return ((ExecutorThread) thread).getExecutor() == this;
+    }
+
+    public Runnable getUrgentTask() {
+        return urgent.poll();
+    }
+
+    public Runnable getTask() {
+        Runnable run = urgent.poll();
+        if (run != null) {
+            return run;
+        }
+        run = high.poll();
+        if (run != null) {
+            return run;
+        }
+        return normal.poll();
+    }
+
+    private void processQueues() {
+        Runnable run = null;
+        while (true) {
+            if (run != null) {
+                run.run();
+            }
+            if (shuttingDownNow) {
+                return;
+            }
+            if ((run = getTask()) != null) {
+                continue;
+            }
+            synchronized (PriorityQueuedExecutor.this) {
+                if ((run = getTask()) != null) {
+                    continue;
+                }
+
+                if (shuttingDown || shuttingDownNow) {
+                    return;
+                }
+                try {
+                    PriorityQueuedExecutor.this.wait();
+                } catch (InterruptedException ignored) {
+                }
+            }
+        }
+    }
+
+    public boolean processUrgentTasks() {
+        Runnable run;
+        boolean hadTask = false;
+        while ((run = getUrgentTask()) != null) {
+            run.run();
+            hadTask = true;
+        }
+        return hadTask;
+    }
+
+    public enum Priority {
+        NORMAL, HIGH, URGENT
+    }
+
+    public class ExecutorThread extends Thread {
+        public ExecutorThread(Runnable runnable) {
+            super(runnable);
+        }
+
+        public PriorityQueuedExecutor getExecutor() {
+            return PriorityQueuedExecutor.this;
+        }
+    }
+
+    public class PendingTask <T> implements Runnable {
+
+        private final AtomicBoolean hasRan = new AtomicBoolean();
+        private final AtomicInteger submitted = new AtomicInteger(-1);
+        private final AtomicInteger priority;
+        private final Supplier<T> run;
+        private final CompletableFuture<T> future = new CompletableFuture<>();
+        private volatile PriorityQueuedExecutor executor;
+
+        public PendingTask(Supplier<T> run) {
+            this(run, Priority.NORMAL);
+        }
+
+        public PendingTask(Supplier<T> run, Priority priority) {
+            this.priority = new AtomicInteger(priority.ordinal());
+            this.run = run;
+        }
+
+        public boolean cancel() {
+            return hasRan.compareAndSet(false, true);
+        }
+
+        @Override
+        public void run() {
+            if (!hasRan.compareAndSet(false, true)) {
+                return;
+            }
+
+            try {
+                future.complete(run.get());
+            } catch (Throwable e) {
+                future.completeExceptionally(e);
+            }
+        }
+
+        public void bumpPriority() {
+            bumpPriority(Priority.HIGH);
+        }
+
+        public void bumpPriority(Priority newPriority) {
+            for (;;) {
+                int current = this.priority.get();
+                int ordinal = newPriority.ordinal();
+                if (current >= ordinal || priority.compareAndSet(current, ordinal)) {
+                    break;
+                }
+            }
+
+
+            if (this.submitted.get() == -1 || this.hasRan.get()) {
+                return;
+            }
+
+            // Only resubmit if it hasnt ran yet and has been submitted
+            submit();
+        }
+
+        public CompletableFuture<T> onDone() {
+            return future;
+        }
+
+        public PendingTask<T> submit() {
+            if (shuttingDown) {
+                handler.onRejection(this, PriorityQueuedExecutor.this);
+                return this;
+            }
+            for (;;) {
+                final int submitted = this.submitted.get();
+                final int priority = this.priority.get();
+                if (submitted == priority) {
+                    return this;
+                }
+                if (this.submitted.compareAndSet(submitted, priority)) {
+                    if (priority == Priority.URGENT.ordinal()) {
+                        urgent.add(this);
+                    } else if (priority == Priority.HIGH.ordinal()) {
+                        high.add(this);
+                    } else {
+                        normal.add(this);
+                    }
+
+                    break;
+                }
+            }
+
+            synchronized (PriorityQueuedExecutor.this) {
+                // Wake up a thread to take this work
+                PriorityQueuedExecutor.this.notify();
+            }
+            return this;
+        }
+    }
+    public interface RejectionHandler {
+        void onRejection(Runnable run, PriorityQueuedExecutor executor);
+    }
+
+    public static final RejectionHandler ABORT_POLICY = (run, executor) -> {
+        throw new RejectedExecutionException("Executor has been shutdown");
+    };
+    public static final RejectionHandler CALLER_RUNS_POLICY = (run, executor) -> {
+        run.run();
+    };
+
+}
diff --git a/src/main/java/net/minecraft/server/Chunk.java b/src/main/java/net/minecraft/server/Chunk.java
index 4ae0acbf1..4f64072a7 100644
--- a/src/main/java/net/minecraft/server/Chunk.java
+++ b/src/main/java/net/minecraft/server/Chunk.java
@@ -190,6 +190,7 @@ public class Chunk implements IChunkAccess {
 
         for (k = 0; k < this.sections.length; ++k) {
             this.sections[k] = protochunk.getSections()[k];
+            if (this.sections[k] != null) this.sections[k].disableLocks(); // Paper - Async Chunks - disable locks used during world gen
         }
 
         Iterator iterator = protochunk.s().iterator();
diff --git a/src/main/java/net/minecraft/server/ChunkMap.java b/src/main/java/net/minecraft/server/ChunkMap.java
index 2021c0d02..154ab09e0 100644
--- a/src/main/java/net/minecraft/server/ChunkMap.java
+++ b/src/main/java/net/minecraft/server/ChunkMap.java
@@ -14,9 +14,17 @@ public class ChunkMap extends Long2ObjectOpenHashMap<Chunk> {
     }
 
     public Chunk put(long i, Chunk chunk) {
+        org.spigotmc.AsyncCatcher.catchOp("Async Chunk put"); // Paper
         chunk.world.timings.syncChunkLoadPostTimer.startTiming(); // Paper
         lastChunkByPos = chunk; // Paper
-        Chunk chunk1 = (Chunk) super.put(i, chunk);
+        // Paper start
+        Chunk chunk1;
+        synchronized (this) {
+            // synchronize so any async gets are safe
+            chunk1 = (Chunk) super.put(i, chunk);
+        }
+        if (chunk1 == null) { // Paper - we should never be overwriting chunks
+        // Paper end
         ChunkCoordIntPair chunkcoordintpair = new ChunkCoordIntPair(i);
 
         for (int j = chunkcoordintpair.x - 1; j <= chunkcoordintpair.x + 1; ++j) {
@@ -47,7 +55,11 @@ public class ChunkMap extends Long2ObjectOpenHashMap<Chunk> {
                     chunk.setNeighborLoaded(x, z);
                 }
             }
+            // Paper start
+        } } else {
+            a.error("Overwrote existing chunk! (" + chunk.world.getWorld().getName() + ":" + chunk.locX+"," + chunk.locZ + ")", new IllegalStateException());
         }
+        // Paper end
         // Paper start - if this is a spare chunk (not part of any players view distance), go ahead and queue it for unload.
         if (!((WorldServer)chunk.world).getPlayerChunkMap().isChunkInUse(chunk.locX, chunk.locZ)) {
             if (chunk.world.paperConfig.delayChunkUnloadsBy > 0) {
@@ -64,11 +76,19 @@ public class ChunkMap extends Long2ObjectOpenHashMap<Chunk> {
     }
 
     public Chunk put(Long olong, Chunk chunk) {
-        return this.put(olong, chunk);
+        return MCUtil.ensureMain("Chunk Put", () -> this.put(olong.longValue(), chunk)); // Paper
     }
 
     public Chunk remove(long i) {
-        Chunk chunk = (Chunk) super.remove(i);
+        // Paper start
+        org.spigotmc.AsyncCatcher.catchOp("Async Chunk remove");
+        Chunk chunk;
+        synchronized (this) {
+            // synchronize so any async gets are safe
+            chunk = super.remove(i);
+        }
+        if (chunk != null) { // Paper - don't decrement if we didn't remove anything
+        // Paper end
         ChunkCoordIntPair chunkcoordintpair = new ChunkCoordIntPair(i);
 
         for (int j = chunkcoordintpair.x - 1; j <= chunkcoordintpair.x + 1; ++j) {
@@ -84,6 +104,7 @@ public class ChunkMap extends Long2ObjectOpenHashMap<Chunk> {
         }
 
         // Paper start
+        } // close if (chunk != null)
         if (lastChunkByPos != null && i == lastChunkByPos.chunkKey) {
             lastChunkByPos = null;
         }
@@ -93,15 +114,22 @@ public class ChunkMap extends Long2ObjectOpenHashMap<Chunk> {
 
     @Override
     public Chunk get(long l) {
-        if (lastChunkByPos != null && l == lastChunkByPos.chunkKey) {
-            return lastChunkByPos;
+        if (MCUtil.isMainThread()) {
+            if (lastChunkByPos != null && l == lastChunkByPos.chunkKey) {
+                return lastChunkByPos;
+            }
+            final Chunk chunk = super.get(l);
+            return chunk != null ? (lastChunkByPos = chunk) : null;
+        } else {
+            synchronized (this) {
+                return super.get(l);
+            }
         }
-        return lastChunkByPos = super.get(l);
     }
     // Paper end
 
     public Chunk remove(Object object) {
-        return this.remove((Long) object);
+        return MCUtil.ensureMain("Chunk Remove", () -> this.remove(((Long) object).longValue())); // Paper
     }
 
     public void putAll(Map<? extends Long, ? extends Chunk> map) {
diff --git a/src/main/java/net/minecraft/server/ChunkProviderServer.java b/src/main/java/net/minecraft/server/ChunkProviderServer.java
index 186cfda7e..781e06877 100644
--- a/src/main/java/net/minecraft/server/ChunkProviderServer.java
+++ b/src/main/java/net/minecraft/server/ChunkProviderServer.java
@@ -33,12 +33,12 @@ public class ChunkProviderServer implements IChunkProvider {
     private long lastProcessedSaves = 0L; // Paper
     private long lastSaveStatPrinted = System.currentTimeMillis();
     // Paper end
-    public final Long2ObjectMap<Chunk> chunks = Long2ObjectMaps.synchronize(new ChunkMap(8192));
+    public final Long2ObjectMap<Chunk> chunks = new ChunkMap(8192); // Paper - remove synchronize - we keep everything on main for manip
     private Chunk lastChunk;
     private final ChunkTaskScheduler chunkScheduler;
-    private final SchedulerBatch<ChunkCoordIntPair, ChunkStatus, ProtoChunk> batchScheduler;
+    final SchedulerBatch<ChunkCoordIntPair, ChunkStatus, ProtoChunk> batchScheduler; // Paper
     public final WorldServer world;
-    private final IAsyncTaskHandler asyncTaskHandler;
+    final IAsyncTaskHandler asyncTaskHandler; // Paper
 
     public ChunkProviderServer(WorldServer worldserver, IChunkLoader ichunkloader, ChunkGenerator<?> chunkgenerator, IAsyncTaskHandler iasynctaskhandler) {
         this.world = worldserver;
@@ -75,10 +75,77 @@ public class ChunkProviderServer implements IChunkProvider {
         this.unloadQueue.remove(ChunkCoordIntPair.a(i, j));
     }
 
+    // Paper start - defaults if Async Chunks is not enabled
+    boolean chunkGoingToExists(int x, int z) {
+        final long k = ChunkCoordIntPair.asLong(x, z);
+        return chunkScheduler.progressCache.containsKey(k);
+    }
+    public void bumpPriority(ChunkCoordIntPair coords) {
+        // do nothing, override in async
+    }
+
+    public List<ChunkCoordIntPair> getSpiralOutChunks(BlockPosition blockposition, int radius) {
+        List<ChunkCoordIntPair> list = com.google.common.collect.Lists.newArrayList();
+
+        list.add(new ChunkCoordIntPair(blockposition.getX() >> 4, blockposition.getZ() >> 4));
+        for (int r = 1; r <= radius; r++) {
+            int x = -r;
+            int z = r;
+
+            // Iterates the edge of half of the box; then negates for other half.
+            while (x <= r && z > -r) {
+                list.add(new ChunkCoordIntPair((blockposition.getX() + (x << 4)) >> 4, (blockposition.getZ() + (z << 4)) >> 4));
+                list.add(new ChunkCoordIntPair((blockposition.getX() - (x << 4)) >> 4, (blockposition.getZ() - (z << 4)) >> 4));
+
+                if (x < r) {
+                    x++;
+                } else {
+                    z--;
+                }
+            }
+        }
+        return list;
+    }
+
+    public Chunk getChunkAt(int x, int z, boolean load, boolean gen, Consumer<Chunk> consumer) {
+        return getChunkAt(x, z, load, gen, false, consumer);
+    }
+    public Chunk getChunkAt(int x, int z, boolean load, boolean gen, boolean priority, Consumer<Chunk> consumer) {
+        Chunk chunk = getChunkAt(x, z, load, gen);
+        if (consumer != null) {
+            consumer.accept(chunk);
+        }
+        return chunk;
+    }
+
+    PaperAsyncChunkProvider.CancellableChunkRequest requestChunk(int x, int z, boolean gen, boolean priority, Consumer<Chunk> consumer) {
+        Chunk chunk = getChunkAt(x, z, gen, priority, consumer);
+        return new PaperAsyncChunkProvider.CancellableChunkRequest() {
+            @Override
+            public void cancel() {
+
+            }
+
+            @Override
+            public Chunk getChunk() {
+                return chunk;
+            }
+        };
+    }
+    // Paper end
+
     @Nullable
     public Chunk getChunkAt(int i, int j, boolean flag, boolean flag1) {
         IChunkLoader ichunkloader = this.chunkLoader;
         Chunk chunk;
+        // Paper start - do already loaded checks before synchronize
+        long k = ChunkCoordIntPair.a(i, j);
+        chunk = (Chunk) this.chunks.get(k);
+        if (chunk != null) {
+            //this.lastChunk = chunk; // Paper remove vanilla lastChunk
+            return chunk;
+        }
+        // Paper end
 
         synchronized (this.chunkLoader) {
             // Paper start - remove vanilla lastChunk, we do it more accurately
@@ -86,13 +153,15 @@ public class ChunkProviderServer implements IChunkProvider {
                 return this.lastChunk;
             }*/ // Paper end
 
-            long k = ChunkCoordIntPair.a(i, j);
+            // Paper start - move up
+            //long k = ChunkCoordIntPair.a(i, j);
 
-            chunk = (Chunk) this.chunks.get(k);
+            /*chunk = (Chunk) this.chunks.get(k);
             if (chunk != null) {
                 //this.lastChunk = chunk; // Paper remove vanilla lastChunk
                 return chunk;
-            }
+            }*/
+            // Paper end
 
             if (flag) {
                 try (co.aikar.timings.Timing timing = world.timings.syncChunkLoadTimer.startTiming()) { // Paper
@@ -150,7 +219,8 @@ public class ChunkProviderServer implements IChunkProvider {
         return (IChunkAccess) (chunk != null ? chunk : (IChunkAccess) this.chunkScheduler.b(new ChunkCoordIntPair(i, j), flag));
     }
 
-    public CompletableFuture<ProtoChunk> a(Iterable<ChunkCoordIntPair> iterable, Consumer<Chunk> consumer) {
+    public CompletableFuture<Void> loadAllChunks(Iterable<ChunkCoordIntPair> iterable, Consumer<Chunk> consumer) { return a(iterable, consumer).thenCompose(protoChunk -> null); } // Paper - overriden in async chunk provider
+    private CompletableFuture<ProtoChunk> a(Iterable<ChunkCoordIntPair> iterable, Consumer<Chunk> consumer) { // Paper - mark private, use above method
         this.batchScheduler.b();
         Iterator iterator = iterable.iterator();
 
@@ -168,6 +238,7 @@ public class ChunkProviderServer implements IChunkProvider {
         return this.batchScheduler.c();
     }
 
+    ReportedException generateChunkError(int i, int j, Throwable throwable) { return a(i, j, throwable); } // Paper - OBFHELPER
     private ReportedException a(int i, int j, Throwable throwable) {
         CrashReport crashreport = CrashReport.a(throwable, "Exception generating new chunk");
         CrashReportSystemDetails crashreportsystemdetails = crashreport.a("Chunk to be generated");
@@ -289,11 +360,13 @@ public class ChunkProviderServer implements IChunkProvider {
     }
 
     public void close() {
-        try {
+        // Paper start - we do not need to wait for chunk generations to finish on close
+        /*try {
             this.batchScheduler.a();
         } catch (InterruptedException interruptedexception) {
             ChunkProviderServer.a.error("Couldn't stop taskManager", interruptedexception);
-        }
+        }*/
+        // Paper end
 
     }
 
diff --git a/src/main/java/net/minecraft/server/ChunkRegionLoader.java b/src/main/java/net/minecraft/server/ChunkRegionLoader.java
index d938eb374..7734712af 100644
--- a/src/main/java/net/minecraft/server/ChunkRegionLoader.java
+++ b/src/main/java/net/minecraft/server/ChunkRegionLoader.java
@@ -119,7 +119,7 @@ public class ChunkRegionLoader implements IChunkLoader, IAsyncChunkSaver {
     // CraftBukkit start
     private boolean check(ChunkProviderServer cps, int x, int z) throws IOException {
         if (cps != null) {
-            com.google.common.base.Preconditions.checkState(org.bukkit.Bukkit.isPrimaryThread(), "primary thread");
+            //com.google.common.base.Preconditions.checkState(org.bukkit.Bukkit.isPrimaryThread(), "primary thread"); // Paper - this is safe
             if (cps.isLoaded(x, z)) {
                 return true;
             }
@@ -204,10 +204,13 @@ public class ChunkRegionLoader implements IChunkLoader, IAsyncChunkSaver {
         }
     }
 
+    private final Object legacyStructureLock = new Object(); // Paper
+    public void getPersistentStructureLegacy(DimensionManager dimensionmanager, @Nullable PersistentCollection persistentcollection) { this.a(dimensionmanager, persistentcollection); } // Paper
     public void a(DimensionManager dimensionmanager, @Nullable PersistentCollection persistentcollection) {
         if (this.e == null) {
+        synchronized (legacyStructureLock){ if (this.e == null) { // Paper
             this.e = PersistentStructureLegacy.a(dimensionmanager, persistentcollection);
-        }
+        } } } // Paper
 
     }
 
@@ -385,11 +388,12 @@ public class ChunkRegionLoader implements IChunkLoader, IAsyncChunkSaver {
                     }
                 };
             } else {
+                /* // Paper start - we will never invoke this in an unsafe way
                 NBTTagCompound nbttagcompound2 = this.a(world, chunkcoordintpair.x, chunkcoordintpair.z);
 
                 if (nbttagcompound2 != null && this.a(nbttagcompound2) == ChunkStatus.Type.LEVELCHUNK) {
                     return;
-                }
+                }*/ // Paper end
 
                 completion = new Supplier<NBTTagCompound>() {
                     public NBTTagCompound get() {
diff --git a/src/main/java/net/minecraft/server/ChunkSection.java b/src/main/java/net/minecraft/server/ChunkSection.java
index 2af07ae59..9c6844d44 100644
--- a/src/main/java/net/minecraft/server/ChunkSection.java
+++ b/src/main/java/net/minecraft/server/ChunkSection.java
@@ -25,7 +25,17 @@ public class ChunkSection {
             this.skyLight = new NibbleArray();
         }
 
+        // Paper start - Async Chunks - Lock during world gen
+        if (chunk instanceof ProtoChunk) {
+            this.blockIds.enableLocks();
+        } else {
+            this.blockIds.disableLocks();
+        }
+    }
+    void disableLocks() {
+        this.blockIds.disableLocks();
     }
+    // Paper end
 
     public IBlockData getType(int i, int j, int k) {
         return (IBlockData) this.blockIds.a(i, j, k);
diff --git a/src/main/java/net/minecraft/server/ChunkTaskScheduler.java b/src/main/java/net/minecraft/server/ChunkTaskScheduler.java
index d3898599f..8f061f5ca 100644
--- a/src/main/java/net/minecraft/server/ChunkTaskScheduler.java
+++ b/src/main/java/net/minecraft/server/ChunkTaskScheduler.java
@@ -17,13 +17,14 @@ public class ChunkTaskScheduler extends Scheduler<ChunkCoordIntPair, ChunkStatus
     private final ChunkGenerator<?> d;
     private final IChunkLoader e;
     private final IAsyncTaskHandler f;
-    private final Long2ObjectMap<Scheduler<ChunkCoordIntPair, ChunkStatus, ProtoChunk>.a> progressCache = new ExpiringMap<Scheduler<ChunkCoordIntPair, ChunkStatus, ProtoChunk>.a>(8192, 5000) {
+    protected final Long2ObjectMap<Scheduler<ChunkCoordIntPair, ChunkStatus, ProtoChunk>.a> progressCache = new ExpiringMap<Scheduler<ChunkCoordIntPair, ChunkStatus, ProtoChunk>.a>(8192, 5000) { // Paper - protected
         protected boolean a(Scheduler<ChunkCoordIntPair, ChunkStatus, ProtoChunk>.a scheduler_a) {
             ProtoChunk protochunk = (ProtoChunk) scheduler_a.a();
 
             return !protochunk.ab_() /*&& !protochunk.h()*/; // Paper
         }
     };
+    private final Long2ObjectMap<java.util.concurrent.CompletableFuture<Scheduler.a>> pendingSchedulers = new it.unimi.dsi.fastutil.longs.Long2ObjectOpenHashMap<>(); // Paper
 
     public ChunkTaskScheduler(int i, World world, ChunkGenerator<?> chunkgenerator, IChunkLoader ichunkloader, IAsyncTaskHandler iasynctaskhandler) {
         super("WorldGen", i, ChunkStatus.FINALIZED, () -> {
@@ -47,8 +48,28 @@ public class ChunkTaskScheduler extends Scheduler<ChunkCoordIntPair, ChunkStatus
     protected Scheduler<ChunkCoordIntPair, ChunkStatus, ProtoChunk>.a a(ChunkCoordIntPair chunkcoordintpair, boolean flag) {
         IChunkLoader ichunkloader = this.e;
 
-        synchronized (this.e) {
-            return flag ? (Scheduler.a) this.progressCache.computeIfAbsent(chunkcoordintpair.a(), (i) -> {
+        // Paper start - refactor a lot of this - avoid generating a chunk while holding lock on expiring map
+        java.util.concurrent.CompletableFuture<Scheduler.a> pending = null;
+        boolean created = false;
+        long key = chunkcoordintpair.a();
+        synchronized (pendingSchedulers) {
+            Scheduler.a existing = this.progressCache.get(key);
+            if (existing != null) {
+                return existing;
+            }
+            pending = this.pendingSchedulers.get(key);
+            if (pending == null) {
+                if (!flag) {
+                    return null;
+                }
+                created = true;
+                pending = new java.util.concurrent.CompletableFuture<>();
+                pendingSchedulers.put(key, pending);
+            }
+        }
+        if (created) {
+            java.util.function.Function<Long, Scheduler.a> get = (i) -> {
+                // Paper end
                 ProtoChunk protochunk;
 
                 try {
@@ -67,8 +88,18 @@ public class ChunkTaskScheduler extends Scheduler<ChunkCoordIntPair, ChunkStatus
                 } else {
                     return new Scheduler.a(chunkcoordintpair, new ProtoChunk(chunkcoordintpair, ChunkConverter.a, this.getWorld()), ChunkStatus.EMPTY); // Paper - Anti-Xray
                 }
-            }) : (Scheduler.a) this.progressCache.get(chunkcoordintpair.a());
+                // Paper start
+            };
+            Scheduler.a scheduler = get.apply(key);
+            progressCache.put(key, scheduler);
+            pending.complete(scheduler);
+            synchronized (pendingSchedulers) {
+                pendingSchedulers.remove(key);
+            }
+            return scheduler;
         }
+        return pending.join();
+        // Paper end
     }
 
     protected ProtoChunk a(ChunkCoordIntPair chunkcoordintpair, ChunkStatus chunkstatus, Map<ChunkCoordIntPair, ProtoChunk> map) {
diff --git a/src/main/java/net/minecraft/server/DataPaletteBlock.java b/src/main/java/net/minecraft/server/DataPaletteBlock.java
index 454903a0e..dcbcb655c 100644
--- a/src/main/java/net/minecraft/server/DataPaletteBlock.java
+++ b/src/main/java/net/minecraft/server/DataPaletteBlock.java
@@ -3,7 +3,7 @@ package net.minecraft.server;
 import com.destroystokyo.paper.antixray.ChunkPacketInfo; // Paper - Anti-Xray
 import java.util.Arrays;
 import java.util.Objects;
-import java.util.concurrent.locks.ReentrantLock;
+import java.util.concurrent.locks.ReentrantReadWriteLock;
 import java.util.function.Function;
 import java.util.stream.Collectors;
 
@@ -21,26 +21,43 @@ public class DataPaletteBlock<T> implements DataPaletteExpandable<T> {
     protected DataBits a; protected DataBits getDataBits() { return this.a; } // Paper - OBFHELPER
     private DataPalette<T> h; private DataPalette<T> getDataPalette() { return this.h; } // Paper - OBFHELPER
     private int i; private int getBitsPerObject() { return this.i; } // Paper - OBFHELPER
-    private final ReentrantLock j = new ReentrantLock();
+    // Paper start - use read write locks only during generation, disable once back on main thread
+    private static final NoopLock NOOP_LOCK = new NoopLock();
+    private java.util.concurrent.locks.Lock readLock = NOOP_LOCK;
+    private java.util.concurrent.locks.Lock writeLock = NOOP_LOCK;
+
+    private static class NoopLock extends ReentrantReadWriteLock.WriteLock {
+        private NoopLock() {
+            super(new ReentrantReadWriteLock());
+        }
+
+        @Override
+        public final void lock() {
+        }
+
+        @Override
+        public final void unlock() {
 
-    private void b() {
-        if (this.j.isLocked() && !this.j.isHeldByCurrentThread()) {
-            String s = (String) Thread.getAllStackTraces().keySet().stream().filter(Objects::nonNull).map((thread) -> {
-                return thread.getName() + ": \n\tat " + (String) Arrays.stream(thread.getStackTrace()).map(Object::toString).collect(Collectors.joining("\n\tat "));
-            }).collect(Collectors.joining("\n"));
-            CrashReport crashreport = new CrashReport("Writing into PalettedContainer from multiple threads", new IllegalStateException());
-            CrashReportSystemDetails crashreportsystemdetails = crashreport.a("Thread dumps");
-
-            crashreportsystemdetails.a("Thread dumps", (Object) s);
-            throw new ReportedException(crashreport);
-        } else {
-            this.j.lock();
         }
     }
 
+    synchronized void enableLocks() {
+        ReentrantReadWriteLock lock = new ReentrantReadWriteLock();
+        readLock = lock.readLock();
+        writeLock = lock.writeLock();
+    }
+    synchronized void disableLocks() {
+        readLock = NOOP_LOCK;
+        writeLock = NOOP_LOCK;
+    }
+
+    private void b() {
+        writeLock.lock();
+    }
     private void c() {
-        this.j.unlock();
+        writeLock.unlock();
     }
+    // Paper end
 
     public DataPaletteBlock(DataPalette<T> datapalette, RegistryBlockID<T> registryblockid, Function<NBTTagCompound, T> function, Function<T, NBTTagCompound> function1, T t0) {
         // Paper start - Anti-Xray - Support default constructor
@@ -155,9 +172,13 @@ public class DataPaletteBlock<T> implements DataPaletteExpandable<T> {
     }
 
     protected T a(int i) {
-        T t0 = this.h.a(this.a.a(i));
-
-        return t0 == null ? this.g : t0;
+        try { // Paper start - read lock
+            readLock.lock();
+            T object = this.h.a(this.a.a(i)); // Paper - decompile fix
+            return (T)(object == null ? this.g : object);
+        } finally {
+            readLock.unlock();
+        } // Paper end
     }
 
     // Paper start - Anti-Xray - Support default methods
diff --git a/src/main/java/net/minecraft/server/DefinedStructureManager.java b/src/main/java/net/minecraft/server/DefinedStructureManager.java
index f5a6387f2..f45685099 100644
--- a/src/main/java/net/minecraft/server/DefinedStructureManager.java
+++ b/src/main/java/net/minecraft/server/DefinedStructureManager.java
@@ -21,7 +21,7 @@ import org.apache.logging.log4j.Logger;
 public class DefinedStructureManager implements IResourcePackListener {
 
     private static final Logger a = LogManager.getLogger();
-    private final Map<MinecraftKey, DefinedStructure> b = Maps.newHashMap();
+    private final Map<MinecraftKey, DefinedStructure> b = Maps.newConcurrentMap(); // Paper
     private final DataFixer c;
     private final MinecraftServer d;
     private final java.nio.file.Path e;
diff --git a/src/main/java/net/minecraft/server/Entity.java b/src/main/java/net/minecraft/server/Entity.java
index 3d90fdb64..d6d0dd6d8 100644
--- a/src/main/java/net/minecraft/server/Entity.java
+++ b/src/main/java/net/minecraft/server/Entity.java
@@ -207,7 +207,7 @@ public abstract class Entity implements INamableTileEntity, ICommandListener, Ke
         this.random = SHARED_RANDOM; // Paper
         this.fireTicks = -this.getMaxFireTicks();
         this.justCreated = true;
-        this.uniqueID = MathHelper.a(this.random);
+        this.uniqueID = MathHelper.a(java.util.concurrent.ThreadLocalRandom.current()); // Paper
         this.au = this.uniqueID.toString();
         this.aJ = Sets.newHashSet();
         this.aL = new double[] { 0.0D, 0.0D, 0.0D};
diff --git a/src/main/java/net/minecraft/server/IChunkLoader.java b/src/main/java/net/minecraft/server/IChunkLoader.java
index 4698ee99f..431f4ab18 100644
--- a/src/main/java/net/minecraft/server/IChunkLoader.java
+++ b/src/main/java/net/minecraft/server/IChunkLoader.java
@@ -6,6 +6,9 @@ import javax.annotation.Nullable;
 
 public interface IChunkLoader {
 
+    void getPersistentStructureLegacy(DimensionManager dimensionmanager, @Nullable PersistentCollection persistentcollection); // Paper
+    void loadEntities(NBTTagCompound nbttagcompound, Chunk chunk); // Paper - Async Chunks
+    Object[] loadChunk(GeneratorAccess generatoraccess, int i, int j, Consumer<Chunk> consumer) throws IOException; // Paper - Async Chunks
     @Nullable
     Chunk a(GeneratorAccess generatoraccess, int i, int j, Consumer<Chunk> consumer) throws IOException;
 
diff --git a/src/main/java/net/minecraft/server/MathHelper.java b/src/main/java/net/minecraft/server/MathHelper.java
index 8bb2593aa..67bb28954 100644
--- a/src/main/java/net/minecraft/server/MathHelper.java
+++ b/src/main/java/net/minecraft/server/MathHelper.java
@@ -7,7 +7,7 @@ import java.util.function.IntPredicate;
 public class MathHelper {
 
     public static final float a = c(2.0F);
-    private static final float[] b = (float[]) SystemUtils.a((Object) (new float[65536]), (afloat) -> {
+    private static final float[] b = (float[]) SystemUtils.a((new float[65536]), (afloat) -> { // Paper - Decompile fix
         for (int i = 0; i < afloat.length; ++i) {
             afloat[i] = (float) Math.sin((double) i * 3.141592653589793D * 2.0D / 65536.0D);
         }
@@ -136,6 +136,7 @@ public class MathHelper {
         return Math.floorMod(i, j);
     }
 
+    public static float normalizeYaw(float fx) { return g(fx); } // Paper - OBFHELPER
     public static float g(float f) {
         f %= 360.0F;
         if (f >= 180.0F) {
diff --git a/src/main/java/net/minecraft/server/MinecraftServer.java b/src/main/java/net/minecraft/server/MinecraftServer.java
index d2ee4e578..236fbafeb 100644
--- a/src/main/java/net/minecraft/server/MinecraftServer.java
+++ b/src/main/java/net/minecraft/server/MinecraftServer.java
@@ -499,6 +499,7 @@ public abstract class MinecraftServer implements IAsyncTaskHandler, IMojangStati
 
         // CraftBukkit start - fire WorldLoadEvent and handle whether or not to keep the spawn in memory
         Stopwatch stopwatch = Stopwatch.createStarted();
+        boolean waitForChunks = Boolean.getBoolean("paper.waitforchunks"); // Paper
         for (WorldServer worldserver : this.getWorlds()) {
             MinecraftServer.LOGGER.info("Preparing start region for level " + worldserver.dimension + " (Seed: " + worldserver.getSeed() + ")");
             if (!worldserver.getWorld().getKeepSpawnInMemory()) {
@@ -506,29 +507,24 @@ public abstract class MinecraftServer implements IAsyncTaskHandler, IMojangStati
             }
 
             BlockPosition blockposition = worldserver.getSpawn();
-            List<ChunkCoordIntPair> list = Lists.newArrayList();
+            List<ChunkCoordIntPair> list = worldserver.getChunkProvider().getSpiralOutChunks(blockposition,  worldserver.paperConfig.keepLoadedRange >> 4); // Paper
             Set<ChunkCoordIntPair> set = Sets.newConcurrentHashSet();
 
-            // Paper start
-            short radius = worldserver.paperConfig.keepLoadedRange;
-            for (int i = -radius; i <= radius && this.isRunning(); i += 16) {
-                for (int j = -radius; j <= radius && this.isRunning(); j += 16) {
-                    // Paper end
-                    list.add(new ChunkCoordIntPair(blockposition.getX() + i >> 4, blockposition.getZ() + j >> 4));
-                }
-            } // Paper
+            // Paper - remove arraylist creation, call spiral above
             if (this.isRunning()) { // Paper
                 int expected = list.size(); // Paper
 
-
-                CompletableFuture completablefuture = worldserver.getChunkProvider().a((Iterable) list, (chunk) -> {
+                CompletableFuture completablefuture = worldserver.getChunkProvider().loadAllChunks(list, (chunk) -> { // Paper
                     set.add(chunk.getPos());
-                    if (set.size() < expected && set.size() % 25 == 0) this.a(new ChatMessage("menu.preparingSpawn", new Object[0]), set.size() * 100 / expected); // Paper
+                    if (waitForChunks && (set.size() == expected || (set.size() < expected && set.size() % (set.size() / 10) == 0))) {
+                        this.a(new ChatMessage("menu.preparingSpawn", new Object[0]), set.size() * 100 / expected); // Paper
+                    }
                 });
 
-                while (!completablefuture.isDone()) {
+                while (waitForChunks && !completablefuture.isDone() && isRunning()) { // Paper
                     try {
-                        completablefuture.get(1L, TimeUnit.SECONDS);
+                        PaperAsyncChunkProvider.processMainThreadQueue(this); // Paper
+                        completablefuture.get(50L, TimeUnit.MILLISECONDS); // Paper
                     } catch (InterruptedException interruptedexception) {
                         throw new RuntimeException(interruptedexception);
                     } catch (ExecutionException executionexception) {
@@ -538,11 +534,11 @@ public abstract class MinecraftServer implements IAsyncTaskHandler, IMojangStati
 
                         throw new RuntimeException(executionexception.getCause());
                     } catch (TimeoutException timeoutexception) {
-                        this.a(new ChatMessage("menu.preparingSpawn", new Object[0]), set.size() * 100 / expected); // Paper
+                        //this.a(new ChatMessage("menu.preparingSpawn", new Object[0]), set.size() * 100 / expected); // Paper
                     }
                 }
 
-                this.a(new ChatMessage("menu.preparingSpawn", new Object[0]), set.size() * 100 / expected); // Paper
+                if (waitForChunks) this.a(new ChatMessage("menu.preparingSpawn", new Object[0]), set.size() * 100 / expected); // Paper
             }
         }
 
@@ -646,6 +642,7 @@ public abstract class MinecraftServer implements IAsyncTaskHandler, IMojangStati
             if (hasStopped) return;
             hasStopped = true;
         }
+        PaperAsyncChunkProvider.stop(this); // Paper
         // CraftBukkit end
         MinecraftServer.LOGGER.info("Stopping server");
         MinecraftTimings.stopServer(); // Paper
@@ -1013,6 +1010,7 @@ public abstract class MinecraftServer implements IAsyncTaskHandler, IMojangStati
         while ((futuretask = (FutureTask) this.f.poll()) != null) {
             SystemUtils.a(futuretask, MinecraftServer.LOGGER);
         }
+        PaperAsyncChunkProvider.processMainThreadQueue(this); // Paper
         MinecraftTimings.minecraftSchedulerTimer.stopTiming(); // Paper
 
         this.methodProfiler.exitEnter("commandFunctions");
@@ -1049,6 +1047,7 @@ public abstract class MinecraftServer implements IAsyncTaskHandler, IMojangStati
         // CraftBukkit - dropTickTime
         for (Iterator iterator = this.getWorlds().iterator(); iterator.hasNext();) {
              WorldServer worldserver = (WorldServer) iterator.next();
+            PaperAsyncChunkProvider.processMainThreadQueue(worldserver); // Paper
             worldserver.hasPhysicsEvent =  org.bukkit.event.block.BlockPhysicsEvent.getHandlerList().getRegisteredListeners().length > 0; // Paper
             TileEntityHopper.skipHopperEvents = worldserver.paperConfig.disableHopperMoveEvents || org.bukkit.event.inventory.InventoryMoveItemEvent.getHandlerList().getRegisteredListeners().length == 0; // Paper
             i = SystemUtils.getMonotonicNanos();
diff --git a/src/main/java/net/minecraft/server/PaperAsyncChunkProvider.java b/src/main/java/net/minecraft/server/PaperAsyncChunkProvider.java
new file mode 100644
index 000000000..851756d65
--- /dev/null
+++ b/src/main/java/net/minecraft/server/PaperAsyncChunkProvider.java
@@ -0,0 +1,658 @@
+/*
+ * This file is licensed under the MIT License (MIT).
+ *
+ * Copyright (c) 2018 Daniel Ennis <http://aikar.co>
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy
+ * of this software and associated documentation files (the "Software"), to deal
+ * in the Software without restriction, including without limitation the rights
+ * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+ * copies of the Software, and to permit persons to whom the Software is
+ * furnished to do so, subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in
+ * all copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+ * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+ * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+ * THE SOFTWARE.
+ */
+package net.minecraft.server;
+
+import com.destroystokyo.paper.PaperConfig;
+import com.destroystokyo.paper.util.PriorityQueuedExecutor;
+import com.destroystokyo.paper.util.PriorityQueuedExecutor.Priority;
+import it.unimi.dsi.fastutil.longs.Long2ObjectMap;
+import it.unimi.dsi.fastutil.longs.Long2ObjectMaps;
+import it.unimi.dsi.fastutil.longs.Long2ObjectOpenHashMap;
+import org.bukkit.Bukkit;
+import org.bukkit.craftbukkit.generator.CustomChunkGenerator;
+import org.bukkit.craftbukkit.generator.InternalChunkGenerator;
+
+import javax.annotation.Nullable;
+import java.io.IOException;
+import java.util.ArrayList;
+import java.util.Iterator;
+import java.util.List;
+import java.util.concurrent.CompletableFuture;
+import java.util.concurrent.ConcurrentLinkedDeque;
+import java.util.concurrent.atomic.AtomicBoolean;
+import java.util.concurrent.atomic.AtomicInteger;
+import java.util.function.Consumer;
+
+@SuppressWarnings("unused")
+public class PaperAsyncChunkProvider extends ChunkProviderServer {
+
+    private static final int GEN_THREAD_PRIORITY = Integer.getInteger("paper.genThreadPriority", 3);
+    private static final int LOAD_THREAD_PRIORITY = Integer.getInteger("paper.loadThreadPriority", 4);
+    private static final PriorityQueuedExecutor EXECUTOR = new PriorityQueuedExecutor("PaperChunkLoader", PaperConfig.asyncChunks ? PaperConfig.asyncChunkLoadThreads : 0, LOAD_THREAD_PRIORITY);
+    private static final PriorityQueuedExecutor SINGLE_GEN_EXECUTOR = new PriorityQueuedExecutor("PaperChunkGenerator", PaperConfig.asyncChunks && PaperConfig.asyncChunkGeneration && !PaperConfig.asyncChunkGenThreadPerWorld ? 1 : 0, GEN_THREAD_PRIORITY);
+    private static final ConcurrentLinkedDeque<Runnable> MAIN_THREAD_QUEUE = new ConcurrentLinkedDeque<>();
+
+    private final PriorityQueuedExecutor generationExecutor;
+    //private static final PriorityQueuedExecutor generationExecutor = new PriorityQueuedExecutor("PaperChunkGen", 1);
+    private final Long2ObjectMap<PendingChunk> pendingChunks = Long2ObjectMaps.synchronize(new Long2ObjectOpenHashMap<>());
+    private final IAsyncTaskHandler asyncHandler;
+
+    private final WorldServer world;
+    private final IChunkLoader chunkLoader;
+    private final MinecraftServer server;
+    private final boolean shouldGenSync;
+
+    public PaperAsyncChunkProvider(WorldServer world, IChunkLoader chunkLoader, InternalChunkGenerator generator, MinecraftServer server) {
+        super(world, chunkLoader, generator, server);
+
+        this.server = world.getMinecraftServer();
+        this.world = world;
+        this.asyncHandler = server;
+        this.chunkLoader = chunkLoader;
+        String worldName = this.world.getWorld().getName();
+        this.shouldGenSync = generator instanceof CustomChunkGenerator && !(((CustomChunkGenerator) generator).asyncSupported) || !PaperConfig.asyncChunkGeneration;
+        this.generationExecutor = PaperConfig.asyncChunkGenThreadPerWorld ? new PriorityQueuedExecutor("PaperChunkGen-" + worldName, shouldGenSync ? 0 : 1, GEN_THREAD_PRIORITY) : SINGLE_GEN_EXECUTOR;
+    }
+
+    private static Priority calculatePriority(boolean isBlockingMain, boolean priority) {
+        if (isBlockingMain) {
+            return Priority.URGENT;
+        }
+
+        if (priority) {
+            return Priority.HIGH;
+        }
+
+        return Priority.NORMAL;
+    }
+
+    static void stop(MinecraftServer server) {
+        for (WorldServer world : server.getWorlds()) {
+            world.getPlayerChunkMap().shutdown();
+        }
+    }
+
+    static void processMainThreadQueue(MinecraftServer server) {
+        for (WorldServer world : server.getWorlds()) {
+            processMainThreadQueue(world);
+        }
+    }
+
+    static void processMainThreadQueue(World world) {
+        IChunkProvider chunkProvider = world.getChunkProvider();
+        if (chunkProvider instanceof PaperAsyncChunkProvider) {
+            ((PaperAsyncChunkProvider) chunkProvider).processMainThreadQueue();
+        }
+    }
+
+    private void processMainThreadQueue() {
+        processMainThreadQueue((PendingChunk) null);
+    }
+    private boolean processMainThreadQueue(PendingChunk pending) {
+        Runnable run;
+        boolean hadLoad = false;
+        while ((run = MAIN_THREAD_QUEUE.poll()) != null) {
+            run.run();
+            hadLoad = true;
+            if (pending != null && pending.hasPosted) {
+                break;
+            }
+        }
+        return hadLoad;
+    }
+
+    @Override
+    public void bumpPriority(ChunkCoordIntPair coords) {
+        final PendingChunk pending = pendingChunks.get(coords.asLong());
+        if (pending != null) {
+            pending.bumpPriority(Priority.HIGH);
+        }
+    }
+
+    @Nullable
+    @Override
+    public Chunk getChunkAt(int x, int z, boolean load, boolean gen) {
+        return getChunkAt(x, z, load, gen, null);
+    }
+
+    @Nullable
+    @Override
+    public Chunk getChunkAt(int x, int z, boolean load, boolean gen, boolean priority, Consumer<Chunk> consumer) {
+        final long key = ChunkCoordIntPair.asLong(x, z);
+        final Chunk chunk = this.chunks.get(key);
+        if (chunk != null || !load) { // return null if we aren't loading
+            if (consumer != null) {
+                consumer.accept(chunk);
+            }
+            return chunk;
+        }
+        return loadOrGenerateChunk(x, z, gen, priority, consumer); // Async overrides this method
+    }
+
+    private Chunk loadOrGenerateChunk(int x, int z, boolean gen, boolean priority, Consumer<Chunk> consumer) {
+        return requestChunk(x, z, gen, priority, consumer).getChunk();
+    }
+
+    final PendingChunkRequest requestChunk(int x, int z, boolean gen, boolean priority, Consumer<Chunk> consumer) {
+        try (co.aikar.timings.Timing timing = world.timings.syncChunkLoadTimer.startTiming()) {
+            final long key = ChunkCoordIntPair.asLong(x, z);
+            final boolean isChunkThread = isChunkThread();
+            final boolean isBlockingMain = consumer == null && server.isMainThread();
+            final boolean loadOnThisThread = isChunkThread || isBlockingMain;
+            final Priority taskPriority = calculatePriority(isBlockingMain, priority);
+
+            // Obtain a PendingChunk
+            final PendingChunk pending;
+            synchronized (pendingChunks) {
+                PendingChunk pendingChunk = pendingChunks.get(key);
+                if (pendingChunk == null) {
+                    pending = new PendingChunk(x, z, key, gen, taskPriority);
+                    pendingChunks.put(key, pending);
+                } else if (pendingChunk.hasFinished && gen && !pendingChunk.canGenerate && pendingChunk.chunk == null) {
+                    // need to overwrite the old
+                    pending = new PendingChunk(x, z, key, true, taskPriority);
+                    pendingChunks.put(key, pending);
+                } else {
+                    pending = pendingChunk;
+                    if (pending.taskPriority != taskPriority) {
+                        pending.bumpPriority(taskPriority);
+                    }
+                }
+            }
+
+            // Listen for when result is ready
+            final CompletableFuture<Chunk> future = new CompletableFuture<>();
+            final PendingChunkRequest request = pending.addListener(future, gen, !loadOnThisThread);
+
+            // Chunk Generation can trigger Chunk Loading, those loads may need to convert, and could be slow
+            // Give an opportunity for urgent tasks to jump in at these times
+            if (isChunkThread) {
+                processUrgentTasks();
+            }
+
+            if (loadOnThisThread) {
+                // do loads on main if blocking, or on current if we are a load/gen thread
+                // gen threads do trigger chunk loads
+                pending.loadTask.run();
+            }
+
+            if (isBlockingMain) {
+                while (!future.isDone()) {
+                    // We aren't done, obtain lock on queue
+                    synchronized (MAIN_THREAD_QUEUE) {
+                        // We may of received our request now, check it
+                        if (processMainThreadQueue(pending)) {
+                            // If we processed SOMETHING, don't wait
+                            continue;
+                        }
+                        try {
+                            // We got nothing from the queue, wait until something has been added
+                            MAIN_THREAD_QUEUE.wait(1);
+                        } catch (InterruptedException ignored) {
+                        }
+                    }
+                    // Queue has been notified or timed out, process it
+                    processMainThreadQueue(pending);
+                }
+                // We should be done AND posted into chunk map now, return it
+                request.initialReturnChunk = pending.postChunk();
+            } else if (consumer == null) {
+                // This is on another thread
+                request.initialReturnChunk = future.join();
+            } else {
+                future.thenAccept((c) -> this.asyncHandler.postToMainThread(() -> consumer.accept(c)));
+            }
+
+            return request;
+        }
+    }
+
+    private void processUrgentTasks() {
+        final PriorityQueuedExecutor executor = PriorityQueuedExecutor.getExecutor();
+        if (executor != null) {
+            executor.processUrgentTasks();
+        }
+    }
+
+    @Override
+    public CompletableFuture<Void> loadAllChunks(Iterable<ChunkCoordIntPair> iterable, Consumer<Chunk> consumer) {
+        final Iterator<ChunkCoordIntPair> iterator = iterable.iterator();
+
+        final List<CompletableFuture<Chunk>> all = new ArrayList<>();
+        while (iterator.hasNext()) {
+            final ChunkCoordIntPair chunkcoordintpair = iterator.next();
+            final CompletableFuture<Chunk> future = new CompletableFuture<>();
+            all.add(future);
+            this.getChunkAt(chunkcoordintpair.x, chunkcoordintpair.z, true, true, chunk -> {
+                future.complete(chunk);
+                if (consumer != null) {
+                    consumer.accept(chunk);
+                }
+            });
+        }
+        return CompletableFuture.allOf(all.toArray(new CompletableFuture[0]));
+    }
+
+    boolean chunkGoingToExists(int x, int z) {
+        synchronized (pendingChunks) {
+            PendingChunk pendingChunk = pendingChunks.get(ChunkCoordIntPair.asLong(x, z));
+            return pendingChunk != null && pendingChunk.canGenerate;
+        }
+    }
+
+    private enum PendingStatus {
+        /**
+         * Request has just started
+         */
+        STARTED,
+        /**
+         * Chunk is attempting to be loaded from disk
+         */
+        LOADING,
+        /**
+         * Chunk must generate on main and is pending main
+         */
+        GENERATION_PENDING,
+        /**
+         * Chunk is generating
+         */
+        GENERATING,
+        /**
+         * Chunk is ready and is pending post to main
+         */
+        PENDING_MAIN,
+        /**
+         * Could not load chunk, and did not need to generat
+         */
+        FAIL,
+        /**
+         * Fully done with this request (may or may not of loaded)
+         */
+        DONE,
+        /**
+         * Chunk load was cancelled (no longer needed)
+         */
+        CANCELLED
+    }
+
+    public interface CancellableChunkRequest {
+        void cancel();
+        Chunk getChunk();
+    }
+
+    public static class PendingChunkRequest implements CancellableChunkRequest {
+        private final PendingChunk pending;
+        private final AtomicBoolean cancelled = new AtomicBoolean(false);
+        private volatile boolean generating;
+        private volatile Chunk initialReturnChunk;
+
+        private PendingChunkRequest(PendingChunk pending) {
+            this.pending = pending;
+            this.cancelled.set(true);
+        }
+
+        private PendingChunkRequest(PendingChunk pending, boolean gen) {
+            this.pending = pending;
+            this.generating = gen;
+        }
+
+        public void cancel() {
+            this.pending.cancel(this);
+        }
+
+        /**
+         * Will be null on asynchronous loads
+         */
+        @Override @Nullable
+        public Chunk getChunk() {
+            return initialReturnChunk;
+        }
+    }
+
+    private boolean isLoadThread() {
+        return EXECUTOR.isCurrentThread();
+    }
+
+    private boolean isGenThread() {
+        return generationExecutor.isCurrentThread();
+    }
+    private boolean isChunkThread() {
+        return isLoadThread() || isGenThread();
+    }
+
+    private class PendingChunk implements Runnable {
+        private final int x;
+        private final int z;
+        private final long key;
+        private final long started = System.currentTimeMillis();
+        private final CompletableFuture<Chunk> loadOnly = new CompletableFuture<>();
+        private final CompletableFuture<Chunk> generate = new CompletableFuture<>();
+        private final AtomicInteger requests = new AtomicInteger(0);
+
+        private volatile PendingStatus status = PendingStatus.STARTED;
+        private volatile PriorityQueuedExecutor.PendingTask<Void> loadTask;
+        private volatile PriorityQueuedExecutor.PendingTask<Chunk> genTask;
+        private volatile Priority taskPriority;
+        private volatile boolean generating;
+        private volatile boolean canGenerate;
+        private volatile boolean isHighPriority;
+        private volatile boolean hasPosted;
+        private volatile boolean hasFinished;
+        private volatile Chunk chunk;
+        private volatile NBTTagCompound pendingLevel;
+
+        PendingChunk(int x, int z, long key, boolean canGenerate, boolean priority) {
+            this.x = x;
+            this.z = z;
+            this.key = key;
+            this.canGenerate = canGenerate;
+            taskPriority = priority ? Priority.HIGH : Priority.NORMAL;
+        }
+
+        PendingChunk(int x, int z, long key, boolean canGenerate, Priority taskPriority) {
+            this.x = x;
+            this.z = z;
+            this.key = key;
+            this.canGenerate = canGenerate;
+            this.taskPriority = taskPriority;
+        }
+
+        private synchronized void setStatus(PendingStatus status) {
+            this.status = status;
+        }
+
+        private Chunk loadChunk(int x, int z) throws IOException {
+            setStatus(PendingStatus.LOADING);
+            Object[] data = chunkLoader.loadChunk(world, x, z, null);
+            if (data != null) {
+                // Level must be loaded on main
+                this.pendingLevel = ((NBTTagCompound) data[1]).getCompound("Level");
+                return (Chunk) data[0];
+            } else {
+                return null;
+            }
+        }
+
+        private Chunk generateChunk() {
+            synchronized (this) {
+                if (requests.get() <= 0) {
+                    return null;
+                }
+            }
+
+            try {
+                CompletableFuture<Chunk> pending = new CompletableFuture<>();
+                batchScheduler.startBatch();
+                batchScheduler.add(new ChunkCoordIntPair(x, z));
+
+                ProtoChunk protoChunk = batchScheduler.executeBatch().join();
+                boolean saved = false;
+                if (!Bukkit.isPrimaryThread()) {
+                    // If we are async, dispatch later
+                    try {
+                        chunkLoader.saveChunk(world, protoChunk, true);
+                        saved = true;
+                    } catch (IOException | ExceptionWorldConflict e) {
+                        e.printStackTrace();
+                    }
+                }
+                Chunk chunk = new Chunk(world, protoChunk, x, z);
+                if (saved) {
+                    chunk.setLastSaved(world.getTime());
+                }
+                generateFinished(chunk);
+
+                return chunk;
+            } catch (Throwable e) {
+                MinecraftServer.LOGGER.error("Couldn't generate chunk (" +world.getWorld().getName() + ":" + x + "," + z + ")", e);
+                generateFinished(null);
+                return null;
+            }
+        }
+
+        boolean loadFinished(Chunk chunk) {
+            if (chunk != null) {
+                postChunkToMain(chunk);
+                return false;
+            }
+            loadOnly.complete(null);
+
+            synchronized (this) {
+                boolean cancelled = requests.get() <= 0;
+                if (!canGenerate || cancelled) {
+                    if (!cancelled) {
+                        setStatus(PendingStatus.FAIL);
+                    }
+                    this.chunk = null;
+                    this.hasFinished = true;
+                    pendingChunks.remove(key);
+                    return false;
+                } else {
+                    setStatus(PendingStatus.GENERATING);
+                    generating = true;
+                    return true;
+                }
+            }
+        }
+
+        void generateFinished(Chunk chunk) {
+            synchronized (this) {
+                this.chunk = chunk;
+                this.hasFinished = true;
+            }
+            if (chunk != null) {
+                postChunkToMain(chunk);
+            } else {
+                synchronized (this) {
+                    pendingChunks.remove(key);
+                    completeFutures(null);
+                }
+            }
+        }
+
+        synchronized private void completeFutures(Chunk chunk) {
+            loadOnly.complete(chunk);
+            generate.complete(chunk);
+        }
+
+        private void postChunkToMain(Chunk chunk) {
+            synchronized (this) {
+                setStatus(PendingStatus.PENDING_MAIN);
+                this.chunk = chunk;
+                this.hasFinished = true;
+            }
+
+            if (server.isMainThread()) {
+                postChunk();
+                return;
+            }
+
+            // Don't post here, even if on main, it must enter the queue so we can exit any open batch
+            // schedulers, as post stage may trigger a new generation and cause errors
+            synchronized (MAIN_THREAD_QUEUE) {
+                if (this.taskPriority == Priority.URGENT) {
+                    MAIN_THREAD_QUEUE.addFirst(this::postChunk);
+                } else {
+                    MAIN_THREAD_QUEUE.addLast(this::postChunk);
+                }
+                MAIN_THREAD_QUEUE.notify();
+            }
+        }
+
+        Chunk postChunk() {
+            if (!server.isMainThread()) {
+                throw new IllegalStateException("Must post from main");
+            }
+            synchronized (this) {
+                if (hasPosted || requests.get() <= 0) { // if pending is 0, all were cancelled
+                    return chunk;
+                }
+                hasPosted = true;
+            }
+            try {
+                if (chunk == null) {
+                    chunk = chunks.get(key);
+                    completeFutures(chunk);
+                    return chunk;
+                }
+                if (pendingLevel != null) {
+                    chunkLoader.loadEntities(pendingLevel, chunk);
+                    pendingLevel = null;
+                }
+                synchronized (chunks) {
+                    final Chunk other = chunks.get(key);
+                    if (other != null) {
+                        this.chunk = other;
+                        completeFutures(other);
+                        return other;
+                    }
+                    if (chunk != null) {
+                        chunks.put(key, chunk);
+                    }
+                }
+
+                chunk.addEntities();
+
+                completeFutures(chunk);
+                return chunk;
+            } finally {
+                pendingChunks.remove(key);
+                setStatus(PendingStatus.DONE);
+            }
+        }
+
+        synchronized PendingChunkRequest addListener(CompletableFuture<Chunk> future, boolean gen, boolean autoSubmit) {
+            requests.incrementAndGet();
+            if (loadTask == null) {
+                // Take care of a race condition in that a request could be cancelled after the synchronize
+                // on pendingChunks, but before a listener is added, which would erase these pending tasks.
+                genTask = generationExecutor.createPendingTask(this::generateChunk, taskPriority);
+                loadTask = EXECUTOR.createPendingTask(this, taskPriority);
+                if (autoSubmit) {
+                    // We will execute it outside of the synchronized context immediately after
+                    loadTask.submit();
+                }
+            }
+
+            if (hasFinished) {
+                future.complete(chunk);
+                return new PendingChunkRequest(this);
+            } else if (gen) {
+                canGenerate = true;
+                generate.thenAccept(future::complete);
+            } else {
+                if (generating) {
+                    future.complete(null);
+                    return new PendingChunkRequest(this);
+                } else {
+                    loadOnly.thenAccept(future::complete);
+                }
+            }
+
+            return new PendingChunkRequest(this, gen);
+        }
+
+        @Override
+        public void run() {
+            try {
+                if (!loadFinished(loadChunk(x, z))) {
+                    return;
+                }
+            } catch (Throwable ex) {
+                MinecraftServer.LOGGER.error("Couldn't load chunk (" +world.getWorld().getName() + ":" + x + "," + z + ")", ex);
+                if (ex instanceof IOException) {
+                    generateFinished(null);
+                    return;
+                }
+            }
+
+            if (shouldGenSync) {
+                synchronized (this) {
+                    setStatus(PendingStatus.GENERATION_PENDING);
+                    if (this.taskPriority == Priority.URGENT) {
+                        MAIN_THREAD_QUEUE.addFirst(() -> generateFinished(this.generateChunk()));
+                    } else {
+                        MAIN_THREAD_QUEUE.addLast(() -> generateFinished(this.generateChunk()));
+                    }
+
+                }
+                synchronized (MAIN_THREAD_QUEUE) {
+                    MAIN_THREAD_QUEUE.notify();
+                }
+            } else {
+                if (isGenThread()) {
+                    // ideally we should never run into 1 chunk generating another chunk...
+                    // but if we do, let's apply same solution
+                    genTask.run();
+                } else {
+                    genTask.submit();
+                }
+            }
+        }
+
+        void bumpPriority() {
+            bumpPriority(Priority.HIGH);
+        }
+
+        void bumpPriority(Priority newPriority) {
+            if (taskPriority.ordinal() >= newPriority.ordinal()) {
+                return;
+            }
+
+            this.taskPriority = newPriority;
+            PriorityQueuedExecutor.PendingTask<Void> loadTask = this.loadTask;
+            PriorityQueuedExecutor.PendingTask<Chunk> genTask = this.genTask;
+            if (loadTask != null) {
+                loadTask.bumpPriority(newPriority);
+            }
+            if (genTask != null) {
+                genTask.bumpPriority(newPriority);
+            }
+        }
+
+        public synchronized boolean isCancelled() {
+            return requests.get() <= 0;
+        }
+
+        public synchronized void cancel(PendingChunkRequest request) {
+            synchronized (pendingChunks) {
+                if (!request.cancelled.compareAndSet(false, true)) {
+                    return;
+                }
+
+                if (requests.decrementAndGet() > 0) {
+                    return;
+                }
+
+                boolean c1 = genTask.cancel();
+                boolean c2 = loadTask.cancel();
+                loadTask = null;
+                genTask = null;
+                pendingChunks.remove(key);
+                setStatus(PendingStatus.CANCELLED);
+            }
+        }
+    }
+
+}
diff --git a/src/main/java/net/minecraft/server/PlayerChunk.java b/src/main/java/net/minecraft/server/PlayerChunk.java
index 240f59066..e4cf8548d 100644
--- a/src/main/java/net/minecraft/server/PlayerChunk.java
+++ b/src/main/java/net/minecraft/server/PlayerChunk.java
@@ -22,6 +22,52 @@ public class PlayerChunk {
     private long i;
     private boolean done;
     boolean chunkExists; // Paper
+    // Paper start
+    PaperAsyncChunkProvider.CancellableChunkRequest chunkRequest;
+    private java.util.function.Consumer<Chunk> chunkLoadedConsumer = chunk -> {
+        chunkRequest = null;
+        PlayerChunk pChunk = PlayerChunk.this;
+        pChunk.chunk = chunk;
+        markChunkUsed(); // Paper - delay chunk unloads
+    };
+    private boolean markedHigh = false;
+    void checkHighPriority(EntityPlayer player) {
+        if (done || markedHigh || chunk != null) {
+            return;
+        }
+        final double dist = getDistance(player.locX, player.locZ);
+        if (dist > 8) {
+            return;
+        }
+        if (dist >= 3 && getDistance(player, 5) > 3.5) {
+            return;
+        }
+
+        markedHigh = true;
+        playerChunkMap.getWorld().getChunkProvider().bumpPriority(location);
+        if (chunkRequest == null) {
+            requestChunkIfNeeded(PlayerChunkMap.CAN_GEN_CHUNKS.test(player));
+        }
+    }
+    private void requestChunkIfNeeded(boolean flag) {
+        if (chunkRequest == null) {
+            chunkRequest = this.playerChunkMap.getWorld().getChunkProvider().requestChunk(this.location.x, this.location.z, flag, markedHigh, chunkLoadedConsumer);
+            this.chunk = chunkRequest.getChunk(); // Paper)
+            markChunkUsed(); // Paper - delay chunk unloads
+        }
+    }
+    private double getDistance(EntityPlayer player, int inFront) {
+        final float yaw = MathHelper.normalizeYaw(player.yaw);
+        final double x = player.locX + (inFront * Math.cos(Math.toRadians(yaw)));
+        final double z = player.locZ + (inFront * Math.sin(Math.toRadians(yaw)));
+        return getDistance(x, z);
+    }
+
+    private double getDistance(double blockX, double blockZ) {
+        final double x = location.x - ((int)Math.floor(blockX) >> 4);
+        final double z = location.z - ((int)Math.floor(blockZ) >> 4);
+        return Math.sqrt((x * x) + (z * z));
+    }
 
     public PlayerChunk(PlayerChunkMap playerchunkmap, int i, int j) {
         this.playerChunkMap = playerchunkmap;
@@ -29,13 +75,17 @@ public class PlayerChunk {
         ChunkProviderServer chunkproviderserver = playerchunkmap.getWorld().getChunkProvider();
 
         chunkproviderserver.a(i, j);
-        this.chunk = chunkproviderserver.getChunkAt(i, j, true, false);
-        this.chunkExists = this.chunk != null || org.bukkit.craftbukkit.chunkio.ChunkIOExecutor.hasQueuedChunkLoad(playerChunkMap.getWorld(), i, j); // Paper
+        this.chunk = chunkproviderserver.getChunkAt(i, j, false, false); // Paper
+        this.chunkExists = this.chunk != null || chunkproviderserver.chunkGoingToExists(i, j); // Paper
         markChunkUsed(); // Paper - delay chunk unloads
     }
 
     // Paper start
     private void markChunkUsed() {
+        if (!chunkHasPlayers && chunkRequest != null) {
+            chunkRequest.cancel();
+            chunkRequest = null;
+        }
         if (chunk == null) {
             return;
         }
@@ -65,7 +115,7 @@ public class PlayerChunk {
             this.players.add(entityplayer);
             if (this.done) {
                 this.sendChunk(entityplayer);
-            }
+            } else checkHighPriority(entityplayer); // Paper
 
         }
     }
@@ -90,8 +140,9 @@ public class PlayerChunk {
         if (this.chunk != null) {
             return true;
         } else {
-            this.chunk = this.playerChunkMap.getWorld().getChunkProvider().getChunkAt(this.location.x, this.location.z, true, flag);
-            markChunkUsed(); // Paper - delay chunk unloads
+            // Paper start - async chunks
+            requestChunkIfNeeded(flag);
+            // Paper end
             return this.chunk != null;
         }
     }
diff --git a/src/main/java/net/minecraft/server/PlayerChunkMap.java b/src/main/java/net/minecraft/server/PlayerChunkMap.java
index 679488a3c..b7dda8e28 100644
--- a/src/main/java/net/minecraft/server/PlayerChunkMap.java
+++ b/src/main/java/net/minecraft/server/PlayerChunkMap.java
@@ -25,10 +25,10 @@ public class PlayerChunkMap {
     };
     private static final Predicate<EntityPlayer> b = (entityplayer) -> {
         return entityplayer != null && (!entityplayer.isSpectator() || entityplayer.getWorldServer().getGameRules().getBoolean("spectatorsGenerateChunks"));
-    };
+    }; static final Predicate<EntityPlayer> CAN_GEN_CHUNKS = b; // Paper - OBFHELPER
     private final WorldServer world;
     private final List<EntityPlayer> managedPlayers = Lists.newArrayList();
-    private final Long2ObjectMap<PlayerChunk> e = new Long2ObjectOpenHashMap(4096);
+    private final Long2ObjectMap<PlayerChunk> e = new Long2ObjectOpenHashMap(4096); Long2ObjectMap<PlayerChunk> getChunks() { return e; } // Paper - OBFHELPER
     private final Set<PlayerChunk> f = Sets.newHashSet();
     private final List<PlayerChunk> g = Lists.newLinkedList();
     private final List<PlayerChunk> h = Lists.newLinkedList();
@@ -341,7 +341,13 @@ public class PlayerChunkMap {
                             if (playerchunk != null) {
                                 playerchunk.b(entityplayer);
                             }
+                        } else { // Paper start
+                            PlayerChunk playerchunk = this.getChunk(l1 - j1, i2 - k1);
+                            if (playerchunk != null) {
+                                playerchunk.checkHighPriority(entityplayer); // Paper
+                            }
                         }
+                        // Paper end
                     }
                 }
 
@@ -352,7 +358,11 @@ public class PlayerChunkMap {
                 // CraftBukkit start - send nearest chunks first
                 Collections.sort(chunksToLoad, new ChunkCoordComparator(entityplayer));
                 for (ChunkCoordIntPair pair : chunksToLoad) {
-                    this.c(pair.x, pair.z).a(entityplayer);
+                    // Paper start
+                    PlayerChunk c = this.c(pair.x, pair.z);
+                    c.checkHighPriority(entityplayer);
+                    c.a(entityplayer);
+                    // Paper end
                 }
                 // CraftBukkit end
             }
@@ -424,6 +434,15 @@ public class PlayerChunkMap {
             }
         }
     }
+
+    void shutdown() {
+        getChunks().values().forEach(pchunk -> {
+                PaperAsyncChunkProvider.CancellableChunkRequest chunkRequest = pchunk.chunkRequest;
+                if (chunkRequest != null) {
+                    chunkRequest.cancel();
+                }
+        });
+    }
     // Paper end
 
     private void e() {
diff --git a/src/main/java/net/minecraft/server/RegionLimitedWorldAccess.java b/src/main/java/net/minecraft/server/RegionLimitedWorldAccess.java
index de6dd4fed..da6df06d8 100644
--- a/src/main/java/net/minecraft/server/RegionLimitedWorldAccess.java
+++ b/src/main/java/net/minecraft/server/RegionLimitedWorldAccess.java
@@ -34,7 +34,7 @@ public class RegionLimitedWorldAccess implements GeneratorAccess {
         this.d = l;
         this.e = i;
         this.f = j;
-        this.g = world;
+        this.g = world.regionLimited(this); // Paper
         this.h = world.getSeed();
         this.m = world.getChunkProvider().getChunkGenerator().getSettings();
         this.i = world.getSeaLevel();
diff --git a/src/main/java/net/minecraft/server/SchedulerBatch.java b/src/main/java/net/minecraft/server/SchedulerBatch.java
index 8e909d9ca..f214a74a2 100644
--- a/src/main/java/net/minecraft/server/SchedulerBatch.java
+++ b/src/main/java/net/minecraft/server/SchedulerBatch.java
@@ -10,6 +10,7 @@ public class SchedulerBatch<K, T extends SchedulerTask<K, T>, R> {
     private final Scheduler<K, T, R> b;
     private boolean c;
     private int d = 1000;
+    private final java.util.concurrent.locks.ReentrantLock lock = new java.util.concurrent.locks.ReentrantLock(true); // Paper
 
     public SchedulerBatch(Scheduler<K, T, R> scheduler) {
         this.b = scheduler;
@@ -19,8 +20,10 @@ public class SchedulerBatch<K, T extends SchedulerTask<K, T>, R> {
         this.b.b();
     }
 
+    public void startBatch() { b(); } // Paper - OBFHELPER
     public void b() {
-        if (this.c) {
+        lock.lock(); // Paper
+        if (false && this.c) { // Paper
             throw new RuntimeException("Batch already started.");
         } else {
             this.d = 1000;
@@ -28,6 +31,7 @@ public class SchedulerBatch<K, T extends SchedulerTask<K, T>, R> {
         }
     }
 
+    public CompletableFuture<R> add(K k0) { return a(k0); } // Paper - OBFHELPER
     public CompletableFuture<R> a(K k0) {
         if (!this.c) {
             throw new RuntimeException("Batch not properly started. Please use startBatch to create a new batch.");
@@ -44,8 +48,14 @@ public class SchedulerBatch<K, T extends SchedulerTask<K, T>, R> {
         }
     }
 
+    public CompletableFuture<R> executeBatch() { return c(); } // Paper - OBFHELPER
     public CompletableFuture<R> c() {
-        if (!this.c) {
+        // Paper start
+        if (!lock.isHeldByCurrentThread()) {
+            throw new IllegalStateException("Current thread does not hold the write lock");
+        }
+        try {// Paper end
+        if (false && !this.c) { // Paper
             throw new RuntimeException("Batch not properly started. Please use startBatch to create a new batch.");
         } else {
             if (this.d != 1000) {
@@ -55,5 +65,6 @@ public class SchedulerBatch<K, T extends SchedulerTask<K, T>, R> {
             this.c = false;
             return this.b.c();
         }
+        } finally { lock.unlock(); } // Paper
     }
 }
diff --git a/src/main/java/net/minecraft/server/StructurePiece.java b/src/main/java/net/minecraft/server/StructurePiece.java
index d9def7135..945a005e9 100644
--- a/src/main/java/net/minecraft/server/StructurePiece.java
+++ b/src/main/java/net/minecraft/server/StructurePiece.java
@@ -16,7 +16,7 @@ public abstract class StructurePiece {
     private EnumBlockMirror b;
     private EnumBlockRotation c;
     protected int o;
-    private static final Set<Block> d = ImmutableSet.builder().add(Blocks.NETHER_BRICK_FENCE).add(Blocks.TORCH).add(Blocks.WALL_TORCH).add(Blocks.OAK_FENCE).add(Blocks.SPRUCE_FENCE).add(Blocks.DARK_OAK_FENCE).add(Blocks.ACACIA_FENCE).add(Blocks.BIRCH_FENCE).add(Blocks.JUNGLE_FENCE).add(Blocks.LADDER).add(Blocks.IRON_BARS).build();
+    private static final Set<Block> d = ImmutableSet.<Block>builder().add(Blocks.NETHER_BRICK_FENCE).add(Blocks.TORCH).add(Blocks.WALL_TORCH).add(Blocks.OAK_FENCE).add(Blocks.SPRUCE_FENCE).add(Blocks.DARK_OAK_FENCE).add(Blocks.ACACIA_FENCE).add(Blocks.BIRCH_FENCE).add(Blocks.JUNGLE_FENCE).add(Blocks.LADDER).add(Blocks.IRON_BARS).build(); // Paper - decompile error
 
     public StructurePiece() {}
 
@@ -66,9 +66,11 @@ public abstract class StructurePiece {
     }
 
     public static StructurePiece a(List<StructurePiece> list, StructureBoundingBox structureboundingbox) {
+        StructurePiece structurepiece; // Paper
+        synchronized (list) { // Paper - synchronize main structure list
         Iterator iterator = list.iterator();
 
-        StructurePiece structurepiece;
+        //StructurePiece structurepiece; // Paper - move up
 
         do {
             if (!iterator.hasNext()) {
@@ -77,7 +79,7 @@ public abstract class StructurePiece {
 
             structurepiece = (StructurePiece) iterator.next();
         } while (structurepiece.d() == null || !structurepiece.d().a(structureboundingbox));
-
+        } // Paper
         return structurepiece;
     }
 
diff --git a/src/main/java/net/minecraft/server/StructureStart.java b/src/main/java/net/minecraft/server/StructureStart.java
index 284e96710..8b08efe1f 100644
--- a/src/main/java/net/minecraft/server/StructureStart.java
+++ b/src/main/java/net/minecraft/server/StructureStart.java
@@ -7,7 +7,7 @@ import java.util.Random;
 
 public abstract class StructureStart {
 
-    protected final List<StructurePiece> a = Lists.newArrayList();
+    protected final List<StructurePiece> a = java.util.Collections.synchronizedList(Lists.newArrayList()); // Paper
     protected StructureBoundingBox b;
     protected int c;
     protected int d;
@@ -51,13 +51,14 @@ public abstract class StructureStart {
 
     protected void a(IBlockAccess iblockaccess) {
         this.b = StructureBoundingBox.a();
+        synchronized (this.a) { // Paper - synchronize
         Iterator iterator = this.a.iterator();
 
         while (iterator.hasNext()) {
             StructurePiece structurepiece = (StructurePiece) iterator.next();
 
             this.b.b(structurepiece.d());
-        }
+        }} // Paper
 
     }
 
@@ -126,13 +127,14 @@ public abstract class StructureStart {
         int l = k - this.b.e;
 
         this.b.a(0, l, 0);
+        synchronized (this.a) { // Paper - synchronize
         Iterator iterator = this.a.iterator();
 
         while (iterator.hasNext()) {
             StructurePiece structurepiece = (StructurePiece) iterator.next();
 
             structurepiece.a(0, l, 0);
-        }
+        }} // Paper
 
     }
 
@@ -149,13 +151,14 @@ public abstract class StructureStart {
         int i1 = l - this.b.b;
 
         this.b.a(0, i1, 0);
+        synchronized (this.a) {
         Iterator iterator = this.a.iterator();
 
         while (iterator.hasNext()) {
             StructurePiece structurepiece = (StructurePiece) iterator.next();
 
             structurepiece.a(0, i1, 0);
-        }
+        }} // Paper
 
     }
 
diff --git a/src/main/java/net/minecraft/server/World.java b/src/main/java/net/minecraft/server/World.java
index 0d45e2157..97a0fbd55 100644
--- a/src/main/java/net/minecraft/server/World.java
+++ b/src/main/java/net/minecraft/server/World.java
@@ -41,7 +41,7 @@ import org.bukkit.event.entity.CreatureSpawnEvent.SpawnReason;
 import org.bukkit.generator.ChunkGenerator;
 // CraftBukkit end
 
-public abstract class World implements IEntityAccess, GeneratorAccess, IIBlockAccess, AutoCloseable {
+public abstract class World implements IEntityAccess, GeneratorAccess, IIBlockAccess, AutoCloseable, Cloneable { // Paper
 
     protected static final Logger e = LogManager.getLogger();
     private static final EnumDirection[] a = EnumDirection.values();
@@ -104,6 +104,24 @@ public abstract class World implements IEntityAccess, GeneratorAccess, IIBlockAc
     protected PersistentVillage villages;
     public final MethodProfiler methodProfiler;
     public final boolean isClientSide;
+    // Paper start - yes this is hacky as shit
+    RegionLimitedWorldAccess regionLimited;
+    World originalWorld;
+    public World regionLimited(RegionLimitedWorldAccess limitedWorldAccess) {
+        try {
+            World clone = (World) super.clone();
+            clone.regionLimited = limitedWorldAccess;
+            clone.originalWorld = this;
+            return clone;
+        } catch (CloneNotSupportedException e1) {
+        }
+        return null;
+    }
+    ChunkCoordIntPair[] strongholdCoords;
+    final java.util.concurrent.atomic.AtomicBoolean
+        strongholdInit = new java.util.concurrent.atomic.AtomicBoolean
+        (false);
+    // Paper end
     public boolean allowMonsters;
     public boolean allowAnimals;
     private boolean J;
@@ -744,17 +762,42 @@ public abstract class World implements IEntityAccess, GeneratorAccess, IIBlockAc
 
     }
 
-    public IBlockData getType(BlockPosition blockposition) {
-        // CraftBukkit start - tree generation
+    // Paper - async variant
+    public java.util.concurrent.CompletableFuture<IBlockData> getTypeAsync(BlockPosition blockposition) {
+        int x = blockposition.getX();
+        int z = blockposition.getZ();
         if (captureTreeGeneration) {
             Iterator<CraftBlockState> it = capturedBlockStates.iterator();
             while (it.hasNext()) {
                 CraftBlockState previous = it.next();
-                if (previous.getPosition().equals(blockposition)) {
-                    return previous.getHandle();
+                if (previous.getX() == x && previous.getY() == blockposition.getY() && previous.getZ() == z) {
+                    return java.util.concurrent.CompletableFuture.completedFuture(previous.getHandle());
                 }
             }
         }
+        if (blockposition.isInvalidYLocation()) {
+            return java.util.concurrent.CompletableFuture.completedFuture(Blocks.VOID_AIR.getBlockData());
+        } else {
+            java.util.concurrent.CompletableFuture<IBlockData> future = new java.util.concurrent.CompletableFuture<>();
+            ((ChunkProviderServer) chunkProvider).getChunkAt(x << 4, z << 4, true, true, (chunk) -> {
+                future.complete(chunk.getType(blockposition));
+            });
+            return future;
+        }
+    }
+    // Paper end
+
+    public IBlockData getType(BlockPosition blockposition) {
+        // CraftBukkit start - tree generation
+        if (captureTreeGeneration) { // If any of this logic updates, update async variant above
+            Iterator<CraftBlockState> it = capturedBlockStates.iterator();
+            while (it.hasNext()) { // If any of this logic updates, update async variant above
+                CraftBlockState previous = it.next();
+                if (previous.getX() == blockposition.getX() && previous.getY() == blockposition.getY() && previous.getZ() == blockposition.getZ()) {
+                    return previous.getHandle(); // If any of this logic updates, update async variant above
+                }
+            } // If any of this logic updates, update async variant above
+        }
         // CraftBukkit end
         if (blockposition.isInvalidYLocation()) { // Paper
             return Blocks.VOID_AIR.getBlockData();
@@ -1023,6 +1066,11 @@ public abstract class World implements IEntityAccess, GeneratorAccess, IIBlockAc
     }
 
     public boolean addEntity(Entity entity, SpawnReason spawnReason) { // Changed signature, added SpawnReason
+        // Paper start
+        if (regionLimited != null) {
+            return regionLimited.addEntity(entity, spawnReason);
+        }
+        // Paper end
         org.spigotmc.AsyncCatcher.catchOp( "entity add"); // Spigot
         if (entity.valid) { MinecraftServer.LOGGER.error("Attempted Double World add on " + entity, new Throwable()); return true; } // Paper
         if (!CraftEventFactory.doEntityAddEventCalling(this, entity, spawnReason)) {
diff --git a/src/main/java/net/minecraft/server/WorldGenStronghold.java b/src/main/java/net/minecraft/server/WorldGenStronghold.java
index 69d8a25bd..d0eaa9e9f 100644
--- a/src/main/java/net/minecraft/server/WorldGenStronghold.java
+++ b/src/main/java/net/minecraft/server/WorldGenStronghold.java
@@ -10,23 +10,28 @@ import javax.annotation.Nullable;
 
 public class WorldGenStronghold extends StructureGenerator<WorldGenFeatureStrongholdConfiguration> {
 
-    private boolean b;
-    private ChunkCoordIntPair[] c;
-    private long d;
+    // Paper start - no shared state
+    //private boolean b;
+    //private ChunkCoordIntPair[] c;
+    //private long d;
+    // Paper end
 
     public WorldGenStronghold() {}
 
     protected boolean a(ChunkGenerator<?> chunkgenerator, Random random, int i, int j) {
-        if (this.d != chunkgenerator.getSeed()) {
+        // Paper start
+        /*if (this.d != chunkgenerator.getSeed()) {
             this.c();
-        }
+        }*/
 
-        if (!this.b) {
+        synchronized (chunkgenerator.getWorld().strongholdInit) {
+        if (chunkgenerator.getWorld().strongholdInit.compareAndSet(false, true)) { // Paper
             this.a(chunkgenerator);
-            this.b = true;
-        }
+            //this.b = true;
+        }} // Paper
+        // Paper end
 
-        ChunkCoordIntPair[] achunkcoordintpair = this.c;
+        ChunkCoordIntPair[] achunkcoordintpair = chunkgenerator.getWorld().strongholdCoords; // Paper
         int k = achunkcoordintpair.length;
 
         for (int l = 0; l < k; ++l) {
@@ -41,8 +46,8 @@ public class WorldGenStronghold extends StructureGenerator<WorldGenFeatureStrong
     }
 
     private void c() {
-        this.b = false;
-        this.c = null;
+        //this.b = false; // Paper
+        //this.c = null; // Paper
     }
 
     protected boolean a(GeneratorAccess generatoraccess) {
@@ -76,23 +81,30 @@ public class WorldGenStronghold extends StructureGenerator<WorldGenFeatureStrong
         if (!chunkgenerator.getWorldChunkManager().a(this)) {
             return null;
         } else {
-            if (this.d != world.getSeed()) {
-                this.c();
-            }
+            // Paper start
+        /*if (this.d != chunkgenerator.getSeed()) {
+            this.c();
+        }*/
 
-            if (!this.b) {
-                this.a(chunkgenerator);
-                this.b = true;
-            }
+            synchronized (chunkgenerator.getWorld().strongholdInit) {
+                if (chunkgenerator.getWorld().strongholdInit.compareAndSet(false, true)) { // Paper
+                    this.a(chunkgenerator);
+                    //this.b = true;
+                }} // Paper
+            // Paper end
 
             BlockPosition blockposition1 = null;
             BlockPosition.MutableBlockPosition blockposition_mutableblockposition = new BlockPosition.MutableBlockPosition(0, 0, 0);
             double d0 = Double.MAX_VALUE;
+            // Paper start
+            /*
             ChunkCoordIntPair[] achunkcoordintpair = this.c;
             int j = achunkcoordintpair.length;
 
             for (int k = 0; k < j; ++k) {
-                ChunkCoordIntPair chunkcoordintpair = achunkcoordintpair[k];
+            */
+            for (ChunkCoordIntPair chunkcoordintpair : world.strongholdCoords) {
+                // Paper end
 
                 blockposition_mutableblockposition.c((chunkcoordintpair.x << 4) + 8, 32, (chunkcoordintpair.z << 4) + 8);
                 double d1 = blockposition_mutableblockposition.n(blockposition);
@@ -111,7 +123,7 @@ public class WorldGenStronghold extends StructureGenerator<WorldGenFeatureStrong
     }
 
     private void a(ChunkGenerator<?> chunkgenerator) {
-        this.d = chunkgenerator.getSeed();
+        //this.d = chunkgenerator.getSeed(); // Paper
         List<BiomeBase> list = Lists.newArrayList();
         Iterator iterator = IRegistry.BIOME.iterator();
 
@@ -127,7 +139,7 @@ public class WorldGenStronghold extends StructureGenerator<WorldGenFeatureStrong
         int j = chunkgenerator.getSettings().f();
         int k = chunkgenerator.getSettings().g();
 
-        this.c = new ChunkCoordIntPair[j];
+        ChunkCoordIntPair[] strongholdCoords = chunkgenerator.getWorld().strongholdCoords = new ChunkCoordIntPair[j]; // Paper
         int l = 0;
         Long2ObjectMap<StructureStart> long2objectmap = chunkgenerator.getStructureStartCache(this);
 
@@ -137,8 +149,8 @@ public class WorldGenStronghold extends StructureGenerator<WorldGenFeatureStrong
             while (objectiterator.hasNext()) {
                 StructureStart structurestart = (StructureStart) objectiterator.next();
 
-                if (l < this.c.length) {
-                    this.c[l++] = new ChunkCoordIntPair(structurestart.e(), structurestart.f());
+                if (l < strongholdCoords.length) { // Paper
+                    strongholdCoords[l++] = new ChunkCoordIntPair(structurestart.e(), structurestart.f()); // Paper
                 }
             }
         }
@@ -149,11 +161,11 @@ public class WorldGenStronghold extends StructureGenerator<WorldGenFeatureStrong
         double d0 = random.nextDouble() * 3.141592653589793D * 2.0D;
         int i1 = long2objectmap.size();
 
-        if (i1 < this.c.length) {
+        if (i1 < strongholdCoords.length) { // Paper
             int j1 = 0;
             int k1 = 0;
 
-            for (int l1 = 0; l1 < this.c.length; ++l1) {
+            for (int l1 = 0; l1 < strongholdCoords.length; ++l1) { // Paper
                 double d1 = (double) (4 * i + i * k1 * 6) + (random.nextDouble() - 0.5D) * (double) i * 2.5D;
                 int i2 = (int) Math.round(Math.cos(d0) * d1);
                 int j2 = (int) Math.round(Math.sin(d0) * d1);
@@ -165,7 +177,7 @@ public class WorldGenStronghold extends StructureGenerator<WorldGenFeatureStrong
                 }
 
                 if (l1 >= i1) {
-                    this.c[l1] = new ChunkCoordIntPair(i2, j2);
+                    strongholdCoords[l1] = new ChunkCoordIntPair(i2, j2); // Paper
                 }
 
                 d0 += 6.283185307179586D / (double) k;
@@ -174,7 +186,7 @@ public class WorldGenStronghold extends StructureGenerator<WorldGenFeatureStrong
                     ++k1;
                     j1 = 0;
                     k += 2 * k / (k1 + 1);
-                    k = Math.min(k, this.c.length - l1);
+                    k = Math.min(k, strongholdCoords.length - l1); // Paper
                     d0 += random.nextDouble() * 3.141592653589793D * 2.0D;
                 }
             }
diff --git a/src/main/java/net/minecraft/server/WorldServer.java b/src/main/java/net/minecraft/server/WorldServer.java
index 0ff3fe03d..e71a40580 100644
--- a/src/main/java/net/minecraft/server/WorldServer.java
+++ b/src/main/java/net/minecraft/server/WorldServer.java
@@ -80,6 +80,7 @@ public class WorldServer extends World implements IAsyncTaskHandler {
         this.P();
         this.Q();
         this.getWorldBorder().a(minecraftserver.au());
+        MCUtil.scheduleAsyncTask(() -> this.getChunkProvider().chunkLoader.getPersistentStructureLegacy(dimensionmanager, worldMaps)); // Paper
     }
 
     public WorldServer i_() {
@@ -714,7 +715,7 @@ public class WorldServer extends World implements IAsyncTaskHandler {
             gen = new org.bukkit.craftbukkit.generator.NormalChunkGenerator(this, this.getSeed());
         }
 
-        return new ChunkProviderServer(this, ichunkloader, gen, this.server);
+        return com.destroystokyo.paper.PaperConfig.asyncChunks ? new PaperAsyncChunkProvider(this, ichunkloader, gen, this.server) : new ChunkProviderServer(this, ichunkloader, gen, this.server); // Paper - async chunks
         // CraftBukkit end
     }
 
diff --git a/src/main/java/org/bukkit/craftbukkit/CraftServer.java b/src/main/java/org/bukkit/craftbukkit/CraftServer.java
index d4d8fbb55..af065bd70 100644
--- a/src/main/java/org/bukkit/craftbukkit/CraftServer.java
+++ b/src/main/java/org/bukkit/craftbukkit/CraftServer.java
@@ -1022,8 +1022,12 @@ public final class CraftServer implements Server {
         if (internal.getWorld().getKeepSpawnInMemory()) {
             short short1 = internal.paperConfig.keepLoadedRange; // Paper
             long i = System.currentTimeMillis();
-            for (int j = -short1; j <= short1; j += 16) {
-                for (int k = -short1; k <= short1; k += 16) {
+            // Paper start
+            for (ChunkCoordIntPair coords : internal.getChunkProvider().getSpiralOutChunks(internal.getSpawn(), short1 >> 4)) {{
+                    int j = coords.x;
+                    int k = coords.z;
+                // Paper end
+
                     long l = System.currentTimeMillis();
 
                     if (l < i) {
@@ -1039,7 +1043,7 @@ public final class CraftServer implements Server {
                     }
 
                     BlockPosition chunkcoordinates = internal.getSpawn();
-                    internal.getChunkProvider().getChunkAt(chunkcoordinates.getX() + j >> 4, chunkcoordinates.getZ() + k >> 4, true, true);
+                    internal.getChunkProvider().getChunkAt(chunkcoordinates.getX() + j >> 4, chunkcoordinates.getZ() + k >> 4, true, true, c -> {}); // Paper
                 }
             }
         }
diff --git a/src/main/java/org/bukkit/craftbukkit/CraftWorld.java b/src/main/java/org/bukkit/craftbukkit/CraftWorld.java
index 0e4455d66..eacecccfd 100644
--- a/src/main/java/org/bukkit/craftbukkit/CraftWorld.java
+++ b/src/main/java/org/bukkit/craftbukkit/CraftWorld.java
@@ -163,6 +163,16 @@ public class CraftWorld implements World {
         }
     }
 
+    // Paper start - Async chunk load API
+    @Override
+    public java.util.concurrent.CompletableFuture<Chunk> getChunkAtAsync(final int x, final int z, final boolean gen) {
+        final ChunkProviderServer cps = this.world.getChunkProvider();
+        java.util.concurrent.CompletableFuture<Chunk> future = new java.util.concurrent.CompletableFuture<>();
+        cps.getChunkAt(x, z, true, gen, chunk -> future.complete(chunk != null ? chunk.bukkitChunk : null));
+        return future;
+    }
+    // Paper end
+
     public Chunk getChunkAt(int x, int z) {
         return this.world.getChunkProvider().getChunkAt(x, z, true, true).bukkitChunk;
     }
@@ -1466,10 +1476,13 @@ public class CraftWorld implements World {
         int chunkCoordZ = chunkcoordinates.getZ() >> 4;
         // Cycle through the 25x25 Chunks around it to load/unload the chunks.
         int radius = world.paperConfig.keepLoadedRange / 16; // Paper
-        for (int x = -radius; x <= radius; x++) { // Paper
-            for (int z = -radius; z <= radius; z++) { // Paper
+        // Paper start
+        for (ChunkCoordIntPair coords : world.getChunkProvider().getSpiralOutChunks(world.getSpawn(), radius)) {{
+            int x = coords.x;
+            int z = coords.z;
+            // Paper end
                 if (keepLoaded) {
-                    loadChunk(chunkCoordX + x, chunkCoordZ + z);
+                    getChunkAtAsync(chunkCoordX + x, chunkCoordZ + z, chunk -> {}); // Paper - Async Chunks
                 } else {
                     if (isChunkLoaded(chunkCoordX + x, chunkCoordZ + z)) {
                         unloadChunk(chunkCoordX + x, chunkCoordZ + z);
diff --git a/src/main/java/org/bukkit/craftbukkit/event/CraftEventFactory.java b/src/main/java/org/bukkit/craftbukkit/event/CraftEventFactory.java
index 68e30185a..7905420ca 100644
--- a/src/main/java/org/bukkit/craftbukkit/event/CraftEventFactory.java
+++ b/src/main/java/org/bukkit/craftbukkit/event/CraftEventFactory.java
@@ -82,6 +82,7 @@ public class CraftEventFactory {
     public static final DamageSource POISON = CraftDamageSource.copyOf(DamageSource.MAGIC);
     public static org.bukkit.block.Block blockDamage; // For use in EntityDamageByBlockEvent
     public static Entity entityDamage; // For use in EntityDamageByEntityEvent
+    public static boolean isWorldGen(GeneratorAccess world) { return world instanceof net.minecraft.server.RegionLimitedWorldAccess; } // Paper
 
     // helper methods
     private static boolean canBuild(CraftWorld world, Player player, int x, int z) {
@@ -474,6 +475,7 @@ public class CraftEventFactory {
         CraftServer craftServer = (CraftServer) entity.getServer();
 
         CreatureSpawnEvent event = new CreatureSpawnEvent(entity, spawnReason);
+        if (isWorldGen(entityliving.world)) return event; // Paper - do not call during world gen
         craftServer.getPluginManager().callEvent(event);
         return event;
     }
@@ -1128,6 +1130,7 @@ public class CraftEventFactory {
         }
 
         BlockIgniteEvent event = new BlockIgniteEvent(bukkitWorld.getBlockAt(block.getX(), block.getY(), block.getZ()), cause, igniter);
+        if (isWorldGen(world)) return event; // Paper - do not call during world gen
         world.getServer().getPluginManager().callEvent(event);
         return event;
     }
@@ -1152,6 +1155,7 @@ public class CraftEventFactory {
         }
 
         BlockIgniteEvent event = new BlockIgniteEvent(bukkitWorld.getBlockAt(pos.getX(), pos.getY(), pos.getZ()), cause, bukkitIgniter);
+        if (isWorldGen(world)) return event; // Paper - do not call during world gen
         world.getServer().getPluginManager().callEvent(event);
         return event;
     }
@@ -1359,7 +1363,8 @@ public class CraftEventFactory {
     public static BlockPhysicsEvent callBlockPhysicsEvent(GeneratorAccess world, BlockPosition blockposition) {
         org.bukkit.block.Block block = CraftBlock.at(world, blockposition);
         BlockPhysicsEvent event = new BlockPhysicsEvent(block, block.getBlockData());
-        world.getMinecraftWorld().getMinecraftServer().server.getPluginManager().callEvent(event);
+        if (isWorldGen(world)) return event; // Paper - do not call during world gen
+        Bukkit.getPluginManager().callEvent(event); // Paper
         return event;
     }
 
@@ -1395,6 +1400,7 @@ public class CraftEventFactory {
         }
 
         EntityPotionEffectEvent event = new EntityPotionEffectEvent((LivingEntity) entity.getBukkitEntity(), bukkitOldEffect, bukkitNewEffect, cause, action, willOverride);
+        if (isWorldGen(entity.world)) return event; // Paper - do not call during world gen
         Bukkit.getPluginManager().callEvent(event);
 
         return event;
@@ -1413,6 +1419,7 @@ public class CraftEventFactory {
         blockState.setData(block);
 
         BlockFormEvent event = (entity == null) ? new BlockFormEvent(blockState.getBlock(), blockState) : new EntityBlockFormEvent(entity.getBukkitEntity(), blockState.getBlock(), blockState);
+        if (isWorldGen(world)) return true; // Paper - do not call during world gen
         world.getServer().getPluginManager().callEvent(event);
 
         if (!event.isCancelled()) {
diff --git a/src/main/java/org/bukkit/craftbukkit/generator/CustomChunkGenerator.java b/src/main/java/org/bukkit/craftbukkit/generator/CustomChunkGenerator.java
index 9e553866e..04c0adf7c 100644
--- a/src/main/java/org/bukkit/craftbukkit/generator/CustomChunkGenerator.java
+++ b/src/main/java/org/bukkit/craftbukkit/generator/CustomChunkGenerator.java
@@ -21,6 +21,7 @@ public class CustomChunkGenerator extends InternalChunkGenerator<GeneratorSettin
     private final WorldServer world;
     private final long seed;
     private final Random random;
+    public final boolean asyncSupported; // Paper
     private final WorldChunkManager chunkManager;
     private final WorldGenStronghold strongholdGen = new WorldGenStronghold();
     private final GeneratorSettingsDefault settings = new GeneratorSettingsDefault();
@@ -43,6 +44,15 @@ public class CustomChunkGenerator extends InternalChunkGenerator<GeneratorSettin
         this.world = (WorldServer) world;
         this.generator = generator;
         this.seed = seed;
+        // Paper start
+        boolean asyncSupported = false;
+        try {
+            java.lang.reflect.Field asyncSafe = generator.getClass().getDeclaredField("PAPER_ASYNC_SAFE");
+            asyncSafe.setAccessible(true);
+            asyncSupported = asyncSafe.getBoolean(generator);
+        } catch (NoSuchFieldException | IllegalAccessException ignored) {}
+        this.asyncSupported = asyncSupported;
+        // Paper end
 
         this.random = new Random(seed);
         this.chunkManager = world.worldProvider.getChunkGenerator().getWorldChunkManager();
-- 
2.21.0