Region profiler

Profiling for a region starts with the /profiler command.
The usage for /profiler:
/profiler <world> <block x> <block z> <time in s> [radius, default 100 blocks]

Any region within the radius of the specified block coordinates
will be profiled. The profiling will stop after the specified
time has passed.

Once the profiler finishes, it will place a report in
the directory ./profiler/<id>.

Since regions can split into smaller regions, or merge into
other regions, the profiler will track this information. If
a profiled region splits, then all of the regions it splits
into are attached to the same profiler instance. If a profiled
region merges into another region, then the merged region is
profiled. This information is tracked and logged into the
"journal.txt" file contained in the report directory. The
journal tracks the region ids for the merge/split operations.

Region profiling is placed into the "region-X.txt" file where
X is the region id inside the profile directory. The header
of the file describes some stats about the region, namely
total profiling duration, ticks, utilisation, TPS, and MSPT.

Then, the timing tree is follows. The format is as specified:

There are two types of data recorded: Timers and Counters.

Timers are specified as follows:
<indent><name> X% total, Y% parent, self A% total, self B% children, avg D sum E, Fms raw sum
The above specifies the format for a named timer.

The <indent> specifies the number of parent timers.

"X" represents the percentage of time the timer took relative
to the entire profiling instance.

"Y" represents the percentage of time the timer took relative
to its _parents_ timer. For example:
```
Full Tick 100.000% total, 100.000% parent, self 0.889% total, self 0.889% children, avg 200.000 sum 200, 401.300ms raw sum
|+++Tick World: minecraft:overworld 81.409% total, 81.409% parent, self 1.873% total, self 2.301% children, avg 1.000 sum 200, 326.694ms raw sum
|---|---Entity Tick 56.784% total, 69.751% parent, self 6.049% total, self 10.652% children, avg 1.000 sum 200, 227.874ms raw sum
```
"Entity Tick" measured 69.751% of the time for the "Tick World: minecraft:overworld" timer.

"A" represents the self time relative to the entire profiling instance.
The self time is the amount of time for a timer that is _not_ measured
by a child timer.

"B" represents the self time relative to its _parents_ timer.

"D" represents the average number of times the timer is invoked relative to
its parent.
For example:
```
|---|---|---Entity Tick: bat 2.642% total, 7.343% parent, self 2.642% total, self 100.000% children, avg 14.975 sum 2,995, 23.127ms raw sum
```
In this case, an average of 14.975 bats were ticked for every
time the "Entity Tick" timer was invoked.

"E" represents the total number of times the timer is invoked.

"F" represents the total raw time accumulated by the timer.

Counters are specified as follows:
<indent>#<name> avg X sum Y

The X is the average number of times the counter is invoked
relative to the parent, exactly similar to the D field of Timers,
where Y is the total number of times the counter is invoked.

patches/server/0003-Threaded-Regions.patch

@@ -1351,485 +1351,6 @@ index 0000000000000000000000000000000000000000..63688716244066581d5b505703576e33
 +
 +    private TimeUtil() {}
 +}
-diff --git a/src/main/java/ca/spottedleaf/leafprofiler/LProfileGraph.java b/src/main/java/ca/spottedleaf/leafprofiler/LProfileGraph.java
-new file mode 100644
-index 0000000000000000000000000000000000000000..1083a3dc0fc824da176e6ee64654b8c01b80ca4b
---- /dev/null
-+++ b/src/main/java/ca/spottedleaf/leafprofiler/LProfileGraph.java
-@@ -0,0 +1,89 @@
-+package ca.spottedleaf.leafprofiler;
-+
-+import it.unimi.dsi.fastutil.ints.Int2IntMap;
-+import it.unimi.dsi.fastutil.ints.Int2IntOpenHashMap;
-+import java.util.ArrayDeque;
-+import java.util.ArrayList;
-+import java.util.Arrays;
-+import java.util.Iterator;
-+import java.util.List;
-+
-+public final class LProfileGraph {
-+
-+    public static final int ROOT_NODE = 0;
-+    // Array idx is the graph node id, where the int->int mapping is a mapping of profile timer id to graph node id
-+    private Int2IntOpenHashMap[] nodes;
-+    private int nodeCount;
-+
-+    public LProfileGraph() {
-+        final Int2IntOpenHashMap[] nodes = new Int2IntOpenHashMap[16];
-+        nodes[ROOT_NODE] = new Int2IntOpenHashMap();
-+
-+        this.nodes = nodes;
-+        this.nodeCount = 1;
-+    }
-+
-+    public static record GraphNode(GraphNode parent, int nodeId, int timerId) {}
-+
-+    public List<GraphNode> getDFS() {
-+        final List<GraphNode> ret = new ArrayList<>();
-+        final ArrayDeque<GraphNode> queue = new ArrayDeque<>();
-+
-+        queue.addFirst(new GraphNode(null, ROOT_NODE, -1));
-+        final Int2IntOpenHashMap[] nodes = this.nodes;
-+
-+        GraphNode graphNode;
-+        while ((graphNode = queue.pollFirst()) != null) {
-+            ret.add(graphNode);
-+
-+            final int parent = graphNode.nodeId;
-+
-+            final Int2IntOpenHashMap children = nodes[parent];
-+
-+            for (final Iterator<Int2IntMap.Entry> iterator = children.int2IntEntrySet().fastIterator(); iterator.hasNext();) {
-+                final Int2IntMap.Entry entry = iterator.next();
-+                queue.addFirst(new GraphNode(graphNode, entry.getIntValue(), entry.getIntKey()));
-+            }
-+        }
-+
-+        return ret;
-+    }
-+
-+    private int createNode(final int parent, final int timerId) {
-+        Int2IntOpenHashMap[] nodes = this.nodes;
-+
-+        final Int2IntOpenHashMap node = nodes[parent];
-+
-+        final int newNode = this.nodeCount;
-+        final int prev = node.putIfAbsent(timerId, newNode);
-+
-+        if (prev != 0) {
-+            // already exists
-+            return prev;
-+        }
-+
-+        // insert new node
-+        ++this.nodeCount;
-+
-+        if (newNode >= nodes.length) {
-+            this.nodes = (nodes = Arrays.copyOf(nodes, nodes.length * 2));
-+        }
-+
-+        nodes[newNode] = new Int2IntOpenHashMap();
-+
-+        return newNode;
-+    }
-+
-+    public int getOrCreateNode(final int parent, final int timerId) {
-+        // note: requires parent node to exist
-+        final Int2IntOpenHashMap[] nodes = this.nodes;
-+
-+        final int mapping = nodes[parent].get(timerId);
-+
-+        if (mapping != 0) {
-+            return mapping;
-+        }
-+
-+        return this.createNode(parent, timerId);
-+    }
-+}
-diff --git a/src/main/java/ca/spottedleaf/leafprofiler/LProfilerRegistry.java b/src/main/java/ca/spottedleaf/leafprofiler/LProfilerRegistry.java
-new file mode 100644
-index 0000000000000000000000000000000000000000..66200c6f4bcf27d060eedf066f56b70bd0cc3929
---- /dev/null
-+++ b/src/main/java/ca/spottedleaf/leafprofiler/LProfilerRegistry.java
-@@ -0,0 +1,59 @@
-+package ca.spottedleaf.leafprofiler;
-+
-+import java.util.Arrays;
-+import java.util.concurrent.ConcurrentHashMap;
-+
-+public final class LProfilerRegistry {
-+
-+    // volatile required to ensure correct publishing when resizing
-+    private volatile ProfilerEntry[] typesById = new ProfilerEntry[16];
-+    private int totalEntries;
-+    private final ConcurrentHashMap<String, ProfilerEntry> nameToEntry = new ConcurrentHashMap<>();
-+
-+    public LProfilerRegistry() {
-+
-+    }
-+
-+    public ProfilerEntry getById(final int id) {
-+        final ProfilerEntry[] entries = this.typesById;
-+
-+        return id < 0 || id >= entries.length ? null : entries[id];
-+    }
-+
-+    public ProfilerEntry getByName(final String name) {
-+        return this.nameToEntry.get(name);
-+    }
-+
-+    public int createType(final ProfileType type, final String name) {
-+        synchronized (this) {
-+            final int id = this.totalEntries;
-+
-+            final ProfilerEntry ret = new ProfilerEntry(type, name, id);
-+
-+            final ProfilerEntry prev = this.nameToEntry.putIfAbsent(name, ret);
-+
-+            if (prev != null) {
-+                throw new IllegalStateException("Entry already exists: " + prev);
-+            }
-+
-+            ++this.totalEntries;
-+
-+            ProfilerEntry[] entries = this.typesById;
-+
-+            if (id >= entries.length) {
-+                this.typesById = entries = Arrays.copyOf(entries, entries.length * 2);
-+            }
-+
-+            // should be opaque, but I don't think that matters here.
-+            entries[id] = ret;
-+
-+            return id;
-+        }
-+    }
-+
-+    public static enum ProfileType {
-+        COUNTER, TIMER;
-+    }
-+
-+    public static record ProfilerEntry(ProfileType type, String name, int id) {}
-+}
-diff --git a/src/main/java/ca/spottedleaf/leafprofiler/LeafProfiler.java b/src/main/java/ca/spottedleaf/leafprofiler/LeafProfiler.java
-new file mode 100644
-index 0000000000000000000000000000000000000000..24d041db762f82c16a735271dd4266b8630666ca
---- /dev/null
-+++ b/src/main/java/ca/spottedleaf/leafprofiler/LeafProfiler.java
-@@ -0,0 +1,313 @@
-+package ca.spottedleaf.leafprofiler;
-+
-+import it.unimi.dsi.fastutil.ints.IntArrayFIFOQueue;
-+import it.unimi.dsi.fastutil.longs.LongArrayFIFOQueue;
-+import it.unimi.dsi.fastutil.objects.Reference2ReferenceOpenHashMap;
-+import java.text.DecimalFormat;
-+import java.util.ArrayDeque;
-+import java.util.ArrayList;
-+import java.util.Arrays;
-+import java.util.List;
-+
-+public final class LeafProfiler {
-+
-+    private static final ThreadLocal<DecimalFormat> THREE_DECIMAL_PLACES = ThreadLocal.withInitial(() -> {
-+        return new DecimalFormat("#,##0.000");
-+    });
-+    private static final ThreadLocal<DecimalFormat> NO_DECIMAL_PLACES = ThreadLocal.withInitial(() -> {
-+        return new DecimalFormat("#,##0");
-+    });
-+
-+    public final LProfilerRegistry registry;
-+    private final LProfileGraph graph;
-+
-+    private long[] timers = new long[16];
-+    private long[] counters = new long[16];
-+    private final IntArrayFIFOQueue callStack = new IntArrayFIFOQueue();
-+    private int topOfStack = LProfileGraph.ROOT_NODE;
-+    private final LongArrayFIFOQueue timerStack = new LongArrayFIFOQueue();
-+    private long lastTimerStart = 0L;
-+
-+    public LeafProfiler(final LProfilerRegistry registry, final LProfileGraph graph) {
-+        this.registry = registry;
-+        this.graph = graph;
-+    }
-+
-+    private long[] resizeTimers(final long[] old, final int least) {
-+        return this.timers = Arrays.copyOf(old, Math.max(old.length * 2, least * 2));
-+    }
-+
-+    private void incrementTimersDirect(final int nodeId, final long count) {
-+        final long[] timers = this.timers;
-+        if (nodeId >= timers.length) {
-+            this.resizeTimers(timers, nodeId)[nodeId] += count;
-+        } else {
-+            timers[nodeId] += count;
-+        }
-+    }
-+
-+    private long[] resizeCounters(final long[] old, final int least) {
-+        return this.counters = Arrays.copyOf(old, Math.max(old.length * 2, least * 2));
-+    }
-+
-+    private void incrementCountersDirect(final int nodeId, final long count) {
-+        final long[] counters = this.counters;
-+        if (nodeId >= counters.length) {
-+            this.resizeTimers(counters, nodeId)[nodeId] += count;
-+        } else {
-+            counters[nodeId] += count;
-+        }
-+    }
-+
-+    public void incrementCounter(final int timerId, final long count) {
-+        final int node = this.graph.getOrCreateNode(this.topOfStack, timerId);
-+        this.incrementCountersDirect(node, count);
-+    }
-+
-+    public void incrementTimer(final int timerId, final long count) {
-+        final int node = this.graph.getOrCreateNode(this.topOfStack, timerId);
-+        this.incrementTimersDirect(node, count);
-+    }
-+
-+    public void startTimer(final int timerId, final long startTime) {
-+        final long lastTimerStart = this.lastTimerStart;
-+        final LProfileGraph graph = this.graph;
-+        final int parentNode = this.topOfStack;
-+        final IntArrayFIFOQueue callStack = this.callStack;
-+        final LongArrayFIFOQueue timerStack = this.timerStack;
-+
-+        this.lastTimerStart = startTime;
-+        this.topOfStack = graph.getOrCreateNode(parentNode, timerId);
-+
-+        callStack.enqueue(parentNode);
-+        timerStack.enqueue(lastTimerStart);
-+    }
-+
-+    public void stopTimer(final int timerId, final long endTime) {
-+        final long lastStart = this.lastTimerStart;
-+        final int currentNode = this.topOfStack;
-+        final IntArrayFIFOQueue callStack = this.callStack;
-+        final LongArrayFIFOQueue timerStack = this.timerStack;
-+        this.lastTimerStart = timerStack.dequeueLastLong();
-+        this.topOfStack = callStack.dequeueLastInt();
-+
-+        this.incrementTimersDirect(currentNode, endTime - lastStart);
-+        this.incrementCountersDirect(currentNode, 1L);
-+    }
-+
-+    private static final char[][] INDENT_PATTERNS = new char[][] {
-+        "|---".toCharArray(),
-+        "|+++".toCharArray(),
-+    };
-+
-+    public List<String> dumpToString() {
-+        final List<LProfileGraph.GraphNode> graphDFS = this.graph.getDFS();
-+        final Reference2ReferenceOpenHashMap<LProfileGraph.GraphNode, ProfileNode> nodeMap = new Reference2ReferenceOpenHashMap<>();
-+
-+        final ArrayDeque<ProfileNode> orderedNodes = new ArrayDeque<>();
-+
-+        for (int i = 0, len = graphDFS.size(); i < len; ++i) {
-+            final LProfileGraph.GraphNode graphNode = graphDFS.get(i);
-+            final ProfileNode parent = nodeMap.get(graphNode.parent());
-+            final int nodeId = graphNode.nodeId();
-+
-+            final long totalTime = this.timers[nodeId];
-+            final long totalCount = this.counters[nodeId];
-+            final LProfilerRegistry.ProfilerEntry profiler = this.registry.getById(graphNode.timerId());
-+
-+            final ProfileNode profileNode = new ProfileNode(parent, nodeId, profiler, totalTime, totalCount);
-+
-+            if (parent != null) {
-+                parent.childrenTimingCount += totalTime;
-+                parent.children.add(profileNode);
-+            } else if (i != 0) { // i == 0 is root
-+                throw new IllegalStateException("Node " + nodeId + " must have parent");
-+            } else {
-+                // set up
-+                orderedNodes.add(profileNode);
-+            }
-+
-+            nodeMap.put(graphNode, profileNode);
-+        }
-+
-+        final List<String> ret = new ArrayList<>();
-+
-+        long totalTime = 0L;
-+
-+        // totalTime = sum of times for root node's children
-+        for (final ProfileNode node : orderedNodes.peekFirst().children) {
-+            totalTime += node.totalTime;
-+        }
-+
-+        ProfileNode profileNode;
-+        final StringBuilder builder = new StringBuilder();
-+        while ((profileNode = orderedNodes.pollFirst()) != null) {
-+            if (profileNode.nodeId != LProfileGraph.ROOT_NODE && profileNode.totalCount == 0L) {
-+                // skip nodes not recorded
-+                continue;
-+            }
-+
-+            final int depth = profileNode.depth;
-+            profileNode.children.sort((final ProfileNode p1, final ProfileNode p2) -> {
-+                final int typeCompare = p1.profiler.type().compareTo(p2.profiler.type());
-+                if (typeCompare != 0) {
-+                    // first count, then profiler
-+                    return typeCompare;
-+                }
-+
-+                if (p1.profiler.type() == LProfilerRegistry.ProfileType.COUNTER) {
-+                    // highest count first
-+                    return Long.compare(p2.totalCount, p1.totalCount);
-+                } else {
-+                    // highest time first
-+                    return Long.compare(p2.totalTime, p1.totalTime);
-+                }
-+            });
-+
-+            for (int i = profileNode.children.size() - 1; i >= 0; --i) {
-+                final ProfileNode child = profileNode.children.get(i);
-+                child.depth = depth + 1;
-+                orderedNodes.addFirst(child);
-+            }
-+
-+            if (profileNode.nodeId == LProfileGraph.ROOT_NODE) {
-+                // don't display root
-+                continue;
-+            }
-+
-+            final boolean noParent = profileNode.parent == null || profileNode.parent.nodeId == LProfileGraph.ROOT_NODE;
-+
-+            final long parentTime = noParent ? totalTime : profileNode.parent.totalTime;
-+            final LProfilerRegistry.ProfilerEntry profilerEntry = profileNode.profiler;
-+
-+            // format:
-+            // For profiler type:
-+            // <indent><name> X% of total, Y% of parent, self A% of total, self B% of children, Dms raw sum
-+            // For counter type:
-+            // <indent>#<name> avg X sum Y
-+            builder.setLength(0);
-+            // prepare indent
-+            final char[] indent = INDENT_PATTERNS[ret.size() % INDENT_PATTERNS.length];
-+            for (int i = 0; i < depth; ++i) {
-+                builder.append(indent);
-+            }
-+
-+            switch (profilerEntry.type()) {
-+                case TIMER: {
-+                    ret.add(
-+                        builder
-+                            .append(profilerEntry.name())
-+                            .append(' ')
-+                            .append(THREE_DECIMAL_PLACES.get().format(((double)profileNode.totalTime / (double)totalTime) * 100.0))
-+                            .append("% of total, ")
-+                            .append(THREE_DECIMAL_PLACES.get().format(((double)profileNode.totalTime / (double)parentTime) * 100.0))
-+                            .append("% of parent, self ")
-+                            .append(THREE_DECIMAL_PLACES.get().format(((double)(profileNode.totalTime - profileNode.childrenTimingCount) / (double)totalTime) * 100.0))
-+                            .append("% of total, self ")
-+                            .append(THREE_DECIMAL_PLACES.get().format(((double)(profileNode.totalTime - profileNode.childrenTimingCount) / (double)profileNode.totalTime) * 100.0))
-+                            .append("% of children, ")
-+                            .append(THREE_DECIMAL_PLACES.get().format((double)profileNode.totalTime / 1.0E6))
-+                            .append("ms raw sum")
-+                            .toString()
-+                    );
-+                    break;
-+                }
-+                case COUNTER: {
-+                    ret.add(
-+                        builder
-+                            .append('#')
-+                            .append(profilerEntry.name())
-+                            .append(" avg ")
-+                            .append(THREE_DECIMAL_PLACES.get().format((double)profileNode.totalCount / (double)(noParent ? 1L : profileNode.parent.totalCount)))
-+                            .append(" sum ")
-+                            .append(NO_DECIMAL_PLACES.get().format(profileNode.totalCount))
-+                            .toString()
-+                    );
-+                    break;
-+                }
-+                default: {
-+                    throw new IllegalStateException("Unknown type " + profilerEntry.type());
-+                }
-+            }
-+        }
-+
-+        return ret;
-+    }
-+
-+    private static final class ProfileNode {
-+
-+        public final ProfileNode parent;
-+        public final int nodeId;
-+        public final LProfilerRegistry.ProfilerEntry profiler;
-+        public final long totalTime;
-+        public final long totalCount;
-+        public final List<ProfileNode> children = new ArrayList<>();
-+        public long childrenTimingCount;
-+        public int depth = -1;
-+
-+        private ProfileNode(final ProfileNode parent, final int nodeId, final LProfilerRegistry.ProfilerEntry profiler,
-+                            final long totalTime, final long totalCount) {
-+            this.parent = parent;
-+            this.nodeId = nodeId;
-+            this.profiler = profiler;
-+            this.totalTime = totalTime;
-+            this.totalCount = totalCount;
-+        }
-+    }
-+
-+    /*
-+    public static void main(final String[] args) throws Throwable {
-+        final Thread timerHack = new Thread("Timer hack thread") {
-+            @Override
-+            public void run() {
-+                for (;;) {
-+                    try {
-+                        Thread.sleep(Long.MAX_VALUE);
-+                    } catch (final InterruptedException ex) {
-+                        continue;
-+                    }
-+                }
-+            }
-+        };
-+        timerHack.setDaemon(true);
-+        timerHack.start();
-+
-+        final LProfilerRegistry registry = new LProfilerRegistry();
-+
-+        final int tickId = registry.createType(LProfilerRegistry.ProfileType.TIMER, "tick");
-+        final int entityTickId = registry.createType(LProfilerRegistry.ProfileType.TIMER, "entity tick");
-+        final int getEntitiesId = registry.createType(LProfilerRegistry.ProfileType.COUNTER, "getEntities call");
-+        final int tileEntityId = registry.createType(LProfilerRegistry.ProfileType.TIMER, "tile entity tick");
-+        final int creeperEntityId = registry.createType(LProfilerRegistry.ProfileType.TIMER, "creeper entity tick");
-+        final int furnaceId = registry.createType(LProfilerRegistry.ProfileType.TIMER, "furnace tile entity tick");
-+
-+        final LeafProfiler profiler = new LeafProfiler(registry, new LProfileGraph());
-+
-+        profiler.startTimer(tickId, System.nanoTime());
-+        Thread.sleep(10L);
-+
-+        profiler.startTimer(entityTickId, System.nanoTime());
-+        Thread.sleep(1L);
-+
-+        profiler.startTimer(creeperEntityId, System.nanoTime());
-+        Thread.sleep(15L);
-+        profiler.incrementCounter(getEntitiesId, 50L);
-+        profiler.stopTimer(creeperEntityId, System.nanoTime());
-+
-+        profiler.stopTimer(entityTickId, System.nanoTime());
-+
-+        profiler.startTimer(tileEntityId, System.nanoTime());
-+        Thread.sleep(1L);
-+
-+        profiler.startTimer(furnaceId, System.nanoTime());
-+        Thread.sleep(20L);
-+        profiler.stopTimer(furnaceId, System.nanoTime());
-+
-+        profiler.stopTimer(tileEntityId, System.nanoTime());
-+
-+        profiler.stopTimer(tickId, System.nanoTime());
-+
-+        System.out.println("Done.");
-+    }
-+     */
-+}
 diff --git a/src/main/java/com/destroystokyo/paper/util/RedstoneWireTurbo.java b/src/main/java/com/destroystokyo/paper/util/RedstoneWireTurbo.java
 index 22a2547810d0c029f29685faddf7ac21cde2df0b..e36b4053eb2676e934b8c9c401bf58cfa7dd969c 100644
 --- a/src/main/java/com/destroystokyo/paper/util/RedstoneWireTurbo.java
@@ -5557,10 +5078,10 @@ index 0000000000000000000000000000000000000000..7b31c4ea6d01f936271bdadc3626201d
 +}
 diff --git a/src/main/java/io/papermc/paper/threadedregions/ThreadedRegionizer.java b/src/main/java/io/papermc/paper/threadedregions/ThreadedRegionizer.java
 new file mode 100644
-index 0000000000000000000000000000000000000000..72a2b81a0a4dc6aab02d0dbad713ea882887d85f
+index 0000000000000000000000000000000000000000..531aa50f2c84e13358e8918bb0c15ea3cd036cb5
 --- /dev/null
 +++ b/src/main/java/io/papermc/paper/threadedregions/ThreadedRegionizer.java
-@@ -0,0 +1,1328 @@
+@@ -0,0 +1,1405 @@
 +package io.papermc.paper.threadedregions;
 +
 +import ca.spottedleaf.concurrentutil.map.SWMRLong2ObjectHashTable;
@@ -5583,6 +5104,7 @@ index 0000000000000000000000000000000000000000..72a2b81a0a4dc6aab02d0dbad713ea88
 +import java.util.Arrays;
 +import java.util.Iterator;
 +import java.util.List;
++import java.util.Set;
 +import java.util.concurrent.atomic.AtomicLong;
 +import java.util.concurrent.locks.StampedLock;
 +import java.util.function.BooleanSupplier;
@@ -5761,6 +5283,34 @@ index 0000000000000000000000000000000000000000..72a2b81a0a4dc6aab02d0dbad713ea88
 +        this.regionsById.forEachValue(consumer);
 +    }
 +
++    public int computeForRegions(final int fromChunkX, final int fromChunkZ, final int toChunkX, final int toChunkZ,
++                                  final Consumer<Set<ThreadedRegion<R, S>>> consumer) {
++        final int shift = this.sectionChunkShift;
++        final int fromSectionX = fromChunkX >> shift;
++        final int fromSectionZ = fromChunkZ >> shift;
++        final int toSectionX = toChunkX >> shift;
++        final int toSectionZ = toChunkZ >> shift;
++        this.acquireWriteLock();
++        try {
++            final ReferenceOpenHashSet<ThreadedRegion<R, S>> set = new ReferenceOpenHashSet<>();
++
++            for (int currZ = fromSectionZ; currZ <= toSectionZ; ++currZ) {
++                for (int currX = fromSectionX; currX <= toSectionX; ++currX) {
++                    final ThreadedRegionSection<R, S> section = this.sections.get(CoordinateUtils.getChunkKey(currX, currZ));
++                    if (section != null) {
++                        set.add(section.getRegionPlain());
++                    }
++                }
++            }
++
++            consumer.accept(set);
++
++            return set.size();
++        } finally {
++            this.releaseWriteLock();
++        }
++    }
++
 +    public ThreadedRegion<R, S> getRegionAtUnsynchronised(final int chunkX, final int chunkZ) {
 +        final int sectionX = chunkX >> this.sectionChunkShift;
 +        final int sectionZ = chunkZ >> this.sectionChunkShift;
@@ -5941,12 +5491,10 @@ index 0000000000000000000000000000000000000000..72a2b81a0a4dc6aab02d0dbad713ea88
 +                    if (region == regionOfInterest) {
 +                        continue;
 +                    }
-+                    // need the relaxed check, as the region may already be
-+                    // a merge target
-+                    if (!region.tryKill()) {
++
++                    if (!region.killAndMergeInto(regionOfInterest)) {
++                        // note: the region may already be a merge target
 +                        regionOfInterest.mergeIntoLater(region);
-+                    } else {
-+                        region.mergeInto(regionOfInterest);
 +                    }
 +                }
 +
@@ -6044,10 +5592,9 @@ index 0000000000000000000000000000000000000000..72a2b81a0a4dc6aab02d0dbad713ea88
 +            // merge the regions into this one
 +            final ReferenceOpenHashSet<ThreadedRegion<R, S>> expectingMergeFrom = region.expectingMergeFrom.clone();
 +            for (final ThreadedRegion<R, S> mergeFrom : expectingMergeFrom) {
-+                if (!mergeFrom.tryKill()) {
++                if (!mergeFrom.killAndMergeInto(region)) {
 +                    throw new IllegalStateException("Merge from region " + mergeFrom + " should be killable! Trying to merge into " + region);
 +                }
-+                mergeFrom.mergeInto(region);
 +            }
 +
 +            if (!region.expectingMergeFrom.isEmpty()) {
@@ -6165,17 +5712,24 @@ index 0000000000000000000000000000000000000000..72a2b81a0a4dc6aab02d0dbad713ea88
 +            return;
 +        }
 +
++        final List<ThreadedRegion<R, S>> newRegionObjects = new ArrayList<>(newRegions.size());
++        for (int i = 0, len = newRegions.size(); i < len; ++i) {
++            newRegionObjects.add(new ThreadedRegion<>(this));
++        }
++
++        this.callbacks.preSplit(region, newRegionObjects);
++
 +        // need to split the region, so we need to kill the old one first
 +        region.state = ThreadedRegion.STATE_DEAD;
 +        region.onRemove(true);
 +
 +        // create new regions
 +        final Long2ReferenceOpenHashMap<ThreadedRegion<R, S>> newRegionsMap = new Long2ReferenceOpenHashMap<>();
-+        final ReferenceOpenHashSet<ThreadedRegion<R, S>> newRegionsSet = new ReferenceOpenHashSet<>();
++        final ReferenceOpenHashSet<ThreadedRegion<R, S>> newRegionsSet = new ReferenceOpenHashSet<>(newRegionObjects);
 +
-+        for (final List<ThreadedRegionSection<R, S>> sections : newRegions) {
-+            final ThreadedRegion<R, S> newRegion = new ThreadedRegion<>(this);
-+            newRegionsSet.add(newRegion);
++        for (int i = 0, len = newRegions.size(); i < len; i++) {
++            final List<ThreadedRegionSection<R, S>> sections = newRegions.get(i);
++            final ThreadedRegion<R, S> newRegion = newRegionObjects.get(i);
 +
 +            for (final ThreadedRegionSection<R, S> section : sections) {
 +                section.setRegionRelease(null);
@@ -6359,6 +5913,20 @@ index 0000000000000000000000000000000000000000..72a2b81a0a4dc6aab02d0dbad713ea88
 +            }
 +        }
 +
++        boolean killAndMergeInto(final ThreadedRegion<R, S> mergeTarget) {
++            if (this.state == STATE_TICKING) {
++                return false;
++            }
++
++            this.regioniser.callbacks.preMerge(this, mergeTarget);
++
++            this.tryKill();
++
++            this.mergeInto(mergeTarget);
++
++            return true;
++        }
++
 +        private void mergeInto(final ThreadedRegion<R, S> mergeTarget) {
 +            if (this == mergeTarget) {
 +                throw new IllegalStateException("Cannot merge a region onto itself");
@@ -6887,6 +6455,36 @@ index 0000000000000000000000000000000000000000..72a2b81a0a4dc6aab02d0dbad713ea88
 +         * @param region The region that is now inactive.
 +         */
 +        public void onRegionInactive(final ThreadedRegion<R, S> region);
++
++        /**
++         * Callback for when a region (from) is about to be merged into a target region (into). Note that
++         * {@code from} is still alive and is a distinct region.
++         * <p>
++         * <b>Note:</b>
++         * </p>
++         * <p>
++         * This function is always called while holding critical locks and as such should not attempt to block on anything, and
++         * should NOT retrieve or modify ANY world state.
++         * </p>
++         * @param from The region that will be merged into the target.
++         * @param into The target of the merge.
++         */
++        public void preMerge(final ThreadedRegion<R, S> from, final ThreadedRegion<R, S> into);
++
++        /**
++         * Callback for when a region (from) is about to be split into a list of target region (into). Note that
++         * {@code from} is still alive, while the list of target regions are not initialised.
++         * <p>
++         * <b>Note:</b>
++         * </p>
++         * <p>
++         * This function is always called while holding critical locks and as such should not attempt to block on anything, and
++         * should NOT retrieve or modify ANY world state.
++         * </p>
++         * @param from The region that will be merged into the target.
++         * @param into The list of regions to split into.
++         */
++        public void preSplit(final ThreadedRegion<R, S> from, final List<ThreadedRegion<R, S>> into);
 +    }
 +}
 diff --git a/src/main/java/io/papermc/paper/threadedregions/TickData.java b/src/main/java/io/papermc/paper/threadedregions/TickData.java
@@ -7230,7 +6828,7 @@ index 0000000000000000000000000000000000000000..29f9fed5f02530b3256e6b993e607d46
 +}
 diff --git a/src/main/java/io/papermc/paper/threadedregions/TickRegionScheduler.java b/src/main/java/io/papermc/paper/threadedregions/TickRegionScheduler.java
 new file mode 100644
-index 0000000000000000000000000000000000000000..ee9f5e1f3387998cddbeb1dc6dc6e2b1ea7cd670
+index 0000000000000000000000000000000000000000..150610d7bf25416dbbde7f003c47da562acc68ba
 --- /dev/null
 +++ b/src/main/java/io/papermc/paper/threadedregions/TickRegionScheduler.java
 @@ -0,0 +1,565 @@
@@ -7551,10 +7149,6 @@ index 0000000000000000000000000000000000000000..ee9f5e1f3387998cddbeb1dc6dc6e2b1
 +                // don't release region for another tick
 +                return null;
 +            } finally {
-+                TickRegionScheduler.setTickTask(null);
-+                if (this.region != null) {
-+                    TickRegionScheduler.setTickingRegion(null);
-+                }
 +                final long tickEnd = System.nanoTime();
 +                final long cpuEnd = MEASURE_CPU_TIME ? THREAD_MX_BEAN.getCurrentThreadCpuTime() : 0L;
 +
@@ -7564,6 +7158,10 @@ index 0000000000000000000000000000000000000000..ee9f5e1f3387998cddbeb1dc6dc6e2b1
 +                );
 +
 +                this.addTickTime(time);
++                TickRegionScheduler.setTickTask(null);
++                if (this.region != null) {
++                    TickRegionScheduler.setTickingRegion(null);
++                }
 +            }
 +
 +            return !this.markNotTicking() || this.cancelled.get() ? null : Boolean.valueOf(ret);
@@ -7627,10 +7225,6 @@ index 0000000000000000000000000000000000000000..ee9f5e1f3387998cddbeb1dc6dc6e2b1
 +                // regionFailed will schedule a shutdown, so we should avoid letting this region tick further
 +                return false;
 +            } finally {
-+                TickRegionScheduler.setTickTask(null);
-+                if (this.region != null) {
-+                    TickRegionScheduler.setTickingRegion(null);
-+                }
 +                final long tickEnd = System.nanoTime();
 +                final long cpuEnd = MEASURE_CPU_TIME ? THREAD_MX_BEAN.getCurrentThreadCpuTime() : 0L;
 +
@@ -7645,6 +7239,10 @@ index 0000000000000000000000000000000000000000..ee9f5e1f3387998cddbeb1dc6dc6e2b1
 +                );
 +
 +                this.addTickTime(time);
++                TickRegionScheduler.setTickTask(null);
++                if (this.region != null) {
++                    TickRegionScheduler.setTickingRegion(null);
++                }
 +            }
 +
 +            // Only AFTER updating the tickStart
@@ -7654,7 +7252,7 @@ index 0000000000000000000000000000000000000000..ee9f5e1f3387998cddbeb1dc6dc6e2b1
 +        /**
 +         * Only safe to call if this tick data matches the current ticking region.
 +         */
-+        private void addTickTime(final TickTime time) {
++        protected void addTickTime(final TickTime time) {
 +            synchronized (this) {
 +                this.currentTickData = null;
 +                this.currentTickingThread = null;
@@ -7800,10 +7398,10 @@ index 0000000000000000000000000000000000000000..ee9f5e1f3387998cddbeb1dc6dc6e2b1
 +    }
 +}
 diff --git a/src/main/java/io/papermc/paper/threadedregions/TickRegions.java b/src/main/java/io/papermc/paper/threadedregions/TickRegions.java
-index d5d39e9c1f326e91010237b0db80d527ac52f4d6..6c76c70574642aa4f3a8fce74e4608781ce132ec 100644
+index d5d39e9c1f326e91010237b0db80d527ac52f4d6..902e82854c89779c7e23c63d1be5b04dad2a61e3 100644
 --- a/src/main/java/io/papermc/paper/threadedregions/TickRegions.java
 +++ b/src/main/java/io/papermc/paper/threadedregions/TickRegions.java
-@@ -1,9 +1,392 @@
+@@ -1,9 +1,404 @@
  package io.papermc.paper.threadedregions;
  
 -// placeholder class for Folia
@@ -7903,6 +7501,18 @@ index d5d39e9c1f326e91010237b0db80d527ac52f4d6..6c76c70574642aa4f3a8fce74e460878
 +        data.tickHandle = data.tickHandle.copy();
 +    }
 +
++    @Override
++    public void preMerge(final ThreadedRegionizer.ThreadedRegion<TickRegionData, TickRegionSectionData> from,
++                         final ThreadedRegionizer.ThreadedRegion<TickRegionData, TickRegionSectionData> into) {
++
++    }
++
++    @Override
++    public void preSplit(final ThreadedRegionizer.ThreadedRegion<TickRegionData, TickRegionSectionData> from,
++                         final java.util.List<ThreadedRegionizer.ThreadedRegion<TickRegionData, TickRegionSectionData>> into) {
++
++    }
++
 +    public static final class TickRegionSectionData implements ThreadedRegionizer.ThreadedRegionSectionData {}
 +
 +    public static final class RegionStats {