From 0000000000000000000000000000000000000000 Mon Sep 17 00:00:00 2001
From: Aikar <aikar@aikar.co>
Date: Fri, 29 May 2020 23:32:14 -0400
Subject: [PATCH] Improve Chunk Status Transition Speed

When a chunk is loaded from disk that has already been generated,
the server has to promote the chunk through the system to reach
it's current desired status level.

This results in every single status transition going from the main thread
to the world gen threads, only to discover it has no work it actually
needs to do.... and then it returns back to main.

This back and forth costs a lot of time and can really delay chunk loads
when the server is under high TPS due to their being a lot of time in
between chunk load times, as well as hogs up the chunk threads from doing
actual generation and light work.

Additionally, the whole task system uses a lot of CPU on the server threads anyways.

So by optimizing status transitions for status's that are already complete,
we can run them to the desired level while on main thread (where it has
to happen anyways) instead of ever jumping to world gen thread.

This will improve chunk loading effeciency to be reduced down to the following
scenario / path:

1) MAIN: Chunk Requested, Load Request sent to ChunkTaskManager / IO Queue
2) IO: Once position in queue comes, submit read IO data and schedule to chunk task thread
3) CHUNK: Once IO is loaded and position in queue comes, deserialize the chunk data, process conversions, submit to main queue
4) MAIN: next Chunk Task process (Mid Tick or End Of Tick), load chunk data into world (POI, main thread tasks)
5) MAIN: process status transitions all the way to LIGHT, light schedules Threaded task
6) SERVER: Light tasks register light enablement for chunk and any lighting needing to be done
7) MAIN: Task returns to main, finish processing to FULL/TICKING status

Previously would have hopped to SERVER around 12+ times there extra.

diff --git a/src/main/java/net/minecraft/server/PlayerChunk.java b/src/main/java/net/minecraft/server/PlayerChunk.java
index 04dcb79c6033f1dec62c5df49937a4ef067a2cb8..f8820f24075e7f42f67426fc9ecf5238f4499b72 100644
--- a/src/main/java/net/minecraft/server/PlayerChunk.java
+++ b/src/main/java/net/minecraft/server/PlayerChunk.java
@@ -56,6 +56,13 @@ public class PlayerChunk {
         this.playersInChunkTickRange = this.chunkMap.playerChunkTickRangeMap.getObjectsInRange(key);
     }
     // Paper end - optimise isOutsideOfRange
+    // Paper start - optimize chunk status progression without jumping through thread pool
+    public boolean canAdvanceStatus() {
+        ChunkStatus status = getChunkHolderStatus();
+        IChunkAccess chunk = getAvailableChunkNow();
+        return chunk != null && (status == null || chunk.getChunkStatus().isAtLeastStatus(getNextStatus(status)));
+    }
+    // Paper end
 
     // Paper start - no-tick view distance
     public final Chunk getSendingChunk() {
diff --git a/src/main/java/net/minecraft/server/PlayerChunkMap.java b/src/main/java/net/minecraft/server/PlayerChunkMap.java
index be4078fc0fc67ab0fd281e3b7781fe1b22bde809..9914f6dd4a7ea9b5ed1f2b25707f18b00ce69dec 100644
--- a/src/main/java/net/minecraft/server/PlayerChunkMap.java
+++ b/src/main/java/net/minecraft/server/PlayerChunkMap.java
@@ -88,6 +88,7 @@ public class PlayerChunkMap extends IChunkLoader implements PlayerChunk.d {
     public final WorldServer world;
     private final LightEngineThreaded lightEngine;
     private final IAsyncTaskHandler<Runnable> executor;
+    final java.util.concurrent.Executor mainInvokingExecutor; // Paper
     public final ChunkGenerator<?> chunkGenerator;
     private final Supplier<WorldPersistentData> l; public final Supplier<WorldPersistentData> getWorldPersistentDataSupplier() { return this.l; } // Paper - OBFHELPER
     private final VillagePlace m;
@@ -285,6 +286,15 @@ public class PlayerChunkMap extends IChunkLoader implements PlayerChunk.d {
         this.world = worldserver;
         this.chunkGenerator = chunkgenerator;
         this.executor = iasynctaskhandler;
+        // Paper start - optimize chunk status progression without jumping through thread pool
+        this.mainInvokingExecutor = (run) -> {
+            if (MCUtil.isMainThread()) {
+                run.run();
+            } else {
+                iasynctaskhandler.execute(run);
+            }
+        };
+        // Paper end
         ThreadedMailbox<Runnable> threadedmailbox = ThreadedMailbox.a(executor, "worldgen");
 
         iasynctaskhandler.getClass();
@@ -707,7 +717,7 @@ public class PlayerChunkMap extends IChunkLoader implements PlayerChunk.d {
             return either.mapLeft((list) -> {
                 return (Chunk) list.get(list.size() / 2);
             });
-        }, this.executor);
+        }, this.mainInvokingExecutor); // Paper
     }
 
     @Nullable
@@ -1073,7 +1083,7 @@ public class PlayerChunkMap extends IChunkLoader implements PlayerChunk.d {
                         return this.b(playerchunk, chunkstatus);
                     }
                 }
-            }, this.executor);
+            }, this.mainInvokingExecutor); // Paper - optimize chunk status progression without jumping through thread pool
         }
     }
 
@@ -1184,6 +1194,12 @@ public class PlayerChunkMap extends IChunkLoader implements PlayerChunk.d {
                 return CompletableFuture.completedFuture(Either.right(playerchunk_failure));
             });
         }, (runnable) -> {
+            // Paper start - optimize chunk status progression without jumping through thread pool
+            if (playerchunk.canAdvanceStatus()) {
+                this.mainInvokingExecutor.execute(runnable);
+                return;
+            }
+            // Paper end
             this.mailboxWorldGen.a(ChunkTaskQueueSorter.a(playerchunk, runnable)); // CraftBukkit - decompile error
         });
     }