From 0000000000000000000000000000000000000000 Mon Sep 17 00:00:00 2001
From: Spottedleaf <Spottedleaf@users.noreply.github.com>
Date: Fri, 14 Feb 2020 01:24:39 -0800
Subject: [PATCH] Optimise TickListServer by rewriting it

In my profiling TickListServer showed up as
~10% for saving chunks and ~5% for the scheduling
of items on a server with ~90 players at
view distance = 5. Most of the performance
loss is unneccessary.

TickListServer has numerous performance issues:
 1. Handling scheduled items is O(nlogn)
 2. Getting scheduled items for a chunk is O(n),
    with n being the the number of scheduled items
    for all chunks (hits saving very hard)
 3. Checking if an item is scheduled for the current tick is O(n),
    with n being the number of items scheduled for current tick
 4. Items not in ticking chunks are churned in the scheduler

The biggest issues are 4 & 2.

We solve 1 by splitting up scheduled items into short and long scheduled,
where we expect the vast majority of our entries to be in the short scheduled
set. Handling short scheduled items is O(n) due to how the comparison
process is reduced to mapping. See TickListServerInterval. However,
this isn't memory-efficient - which is why long scheduled exists.
Long scheduled is handled the same as TickListServer.

2 is solved by mapping what entries are in what chunks.

3 is solved by mapping what blocks have what scheduled for them.

4 is solved by moving the items that are not in ticking chunks
into a map of entries for that chunk. Once the chunk is moved
to ticking, the items are re-scheduled.

This patch has also added two flags to debug excessive tick delays:
-Dpaper.ticklist-warn-on-excessive-delay=true (false by default)
and -Dpaper.ticklist-excessive-delay-threshold=ticks which
sets the excessive tick delay to the specified ticks (defaults to
60 * 20 ticks, aka 60 seconds)

diff --git a/src/main/java/com/destroystokyo/paper/PaperConfig.java b/src/main/java/com/destroystokyo/paper/PaperConfig.java
index f4836e2da1061e7aa62ddb01c8ca7b3467b18415..647f6fc8efb350fbd0bc4c40358a998f8b89b96a 100644
--- a/src/main/java/com/destroystokyo/paper/PaperConfig.java
+++ b/src/main/java/com/destroystokyo/paper/PaperConfig.java
@@ -369,6 +369,13 @@ public class PaperConfig {
         maxBookTotalSizeMultiplier = getDouble("settings.book-size.total-multiplier", maxBookTotalSizeMultiplier);
     }
 
+    public static boolean useOptimizedTickList = true;
+    private static void useOptimizedTickList() {
+        if (config.contains("settings.use-optimized-ticklist")) { // don't add default, hopefully temporary config
+            useOptimizedTickList = config.getBoolean("settings.use-optimized-ticklist");
+        }
+    }
+
     public static boolean asyncChunks = false;
     private static void asyncChunks() {
         ConfigurationSection section;
diff --git a/src/main/java/com/destroystokyo/paper/server/ticklist/PaperTickList.java b/src/main/java/com/destroystokyo/paper/server/ticklist/PaperTickList.java
new file mode 100644
index 0000000000000000000000000000000000000000..ce653f6b4be3ab6c6d35cb3e9222e7f8c8759e25
--- /dev/null
+++ b/src/main/java/com/destroystokyo/paper/server/ticklist/PaperTickList.java
@@ -0,0 +1,622 @@
+package com.destroystokyo.paper.server.ticklist;
+
+import net.minecraft.server.MCUtil;
+import net.minecraft.server.MinecraftServer;
+import it.unimi.dsi.fastutil.longs.Long2ObjectMap;
+import it.unimi.dsi.fastutil.longs.Long2ObjectOpenHashMap;
+import it.unimi.dsi.fastutil.objects.ObjectRBTreeSet;
+import net.minecraft.server.BaseBlockPosition;
+import net.minecraft.server.BlockPosition;
+import net.minecraft.server.ChunkCoordIntPair;
+import net.minecraft.server.ChunkProviderServer;
+import net.minecraft.server.CrashReport;
+import net.minecraft.server.CrashReportSystemDetails;
+import net.minecraft.server.IBlockData;
+import net.minecraft.server.MinecraftKey;
+import net.minecraft.server.NBTTagList;
+import net.minecraft.server.NextTickListEntry;
+import net.minecraft.server.ReportedException;
+import net.minecraft.server.StructureBoundingBox;
+import net.minecraft.server.TickListPriority;
+import net.minecraft.server.TickListServer;
+import net.minecraft.server.WorldServer;
+import java.util.ArrayDeque;
+import java.util.ArrayList;
+import java.util.Collections;
+import java.util.Comparator;
+import java.util.Iterator;
+import java.util.List;
+import java.util.function.Consumer;
+import java.util.function.Function;
+import java.util.function.Predicate;
+import java.util.stream.Stream;
+
+public final class PaperTickList<T> extends TickListServer<T> { // extend to avoid breaking ABI
+
+    // in the order the state is expected to change (mostly)
+    public static final int STATE_UNSCHEDULED    = 1 << 0;
+    public static final int STATE_SCHEDULED      = 1 << 1; // scheduled for some tick
+    public static final int STATE_PENDING_TICK   = 1 << 2; // for this tick
+    public static final int STATE_TICKING        = 1 << 3;
+    public static final int STATE_TICKED         = 1 << 4; // after this, it gets thrown back to unscheduled
+    public static final int STATE_CANCELLED_TICK = 1 << 5; // still gets moved to unscheduled after tick
+
+    private static final int SHORT_SCHEDULE_TICK_THRESHOLD = 20 * 20 + 1; // 20 seconds
+
+    private final WorldServer world;
+    private final Predicate<T> excludeFromScheduling;
+    private final Function<T, MinecraftKey> getMinecraftKeyFrom;
+    private final Function<MinecraftKey, T> getObjectFronMinecraftKey;
+    private final Consumer<NextTickListEntry<T>> tickFunction;
+
+    private final co.aikar.timings.Timing timingCleanup; // Paper
+    private final co.aikar.timings.Timing timingTicking; // Paper
+    private final co.aikar.timings.Timing timingFinished;
+
+    // note: remove ops / add ops suck on fastutil, a chained hashtable implementation would work better, but Long...
+    // try to alleviate with a very small load factor
+    private final Long2ObjectOpenHashMap<ArrayList<NextTickListEntry<T>>> entriesByBlock = new Long2ObjectOpenHashMap<>(1024, 0.25f);
+    private final Long2ObjectOpenHashMap<ObjectRBTreeSet<NextTickListEntry<T>>> entriesByChunk = new Long2ObjectOpenHashMap<>(1024, 0.25f);
+    private final Long2ObjectOpenHashMap<ArrayList<NextTickListEntry<T>>> pendingChunkTickLoad = new Long2ObjectOpenHashMap<>(1024, 0.5f);
+
+    // fastutil has O(1) first/last while TreeMap/TreeSet are log(n)
+    private final ObjectRBTreeSet<NextTickListEntry<T>> longScheduled = new ObjectRBTreeSet<>(TickListServerInterval.ENTRY_COMPARATOR);
+
+    private final ArrayDeque<NextTickListEntry<T>> toTickThisTick = new ArrayDeque<>();
+
+    private final TickListServerInterval<T>[] shortScheduled = new TickListServerInterval[SHORT_SCHEDULE_TICK_THRESHOLD];
+    {
+        for (int i = 0, len = this.shortScheduled.length; i < len; ++i) {
+            this.shortScheduled[i] = new TickListServerInterval<>();
+        }
+    }
+    private int shortScheduledIndex;
+
+    private long nextTick;
+
+    private static final boolean WARN_ON_EXCESSIVE_DELAY = Boolean.getBoolean("paper.ticklist-warn-on-excessive-delay");
+    private static final long EXCESSIVE_DELAY_THRESHOLD = Long.getLong("paper.ticklist-excessive-delay-threshold", 60 * 20).longValue(); // 1 min dfl
+
+    // assume index < length
+    private static int getWrappedIndex(final int start, final int length, final int index) {
+        final int next = start + index;
+        return next < length ? next : next - length;
+    }
+
+    private static int getNextIndex(final int curr, final int length) {
+        final int next = curr + 1;
+        return next < length ? next : 0;
+    }
+
+    public PaperTickList(final WorldServer world, final Predicate<T> excludeFromScheduling, final Function<T, MinecraftKey> getMinecraftKeyFrom,
+                         final Function<MinecraftKey, T> getObjectFronMinecraftKey, final Consumer<NextTickListEntry<T>> tickFunction, final String timingsType) {
+        super(world, excludeFromScheduling, getMinecraftKeyFrom, getObjectFronMinecraftKey, tickFunction, timingsType);
+        this.world = world;
+        this.excludeFromScheduling = excludeFromScheduling;
+        this.getMinecraftKeyFrom = getMinecraftKeyFrom;
+        this.getObjectFronMinecraftKey = getObjectFronMinecraftKey;
+        this.tickFunction = tickFunction;
+        this.timingCleanup = co.aikar.timings.WorldTimingsHandler.getTickList(world, timingsType + " - Cleanup"); // Paper
+        this.timingTicking = co.aikar.timings.WorldTimingsHandler.getTickList(world, timingsType + " - Ticking"); // Paper
+        this.timingFinished = co.aikar.timings.WorldTimingsHandler.getTickList(world, timingsType + " - Finish");
+        this.nextTick = this.world.getTime();
+    }
+
+    private void queueEntryForTick(final NextTickListEntry<T> entry, final ChunkProviderServer chunkProvider) {
+        if (entry.tickState == STATE_SCHEDULED) {
+            if (chunkProvider.isTickingReadyMainThread(entry.getPosition())) {
+                this.toTickThisTick.add(entry);
+                entry.tickState = STATE_PENDING_TICK;
+            } else {
+                // we dump them to a map to avoid constantly re-scheduling them
+                this.addToNotTickingReady(entry);
+            }
+        }
+    }
+
+    private void addToNotTickingReady(final NextTickListEntry<T> entry) {
+        this.pendingChunkTickLoad.computeIfAbsent(MCUtil.getCoordinateKey(entry.getPosition()), (long keyInMap) -> {
+            return new ArrayList<>();
+        }).add(entry);
+    }
+
+    private void addToSchedule(final NextTickListEntry<T> entry) {
+        long delay = entry.getTargetTick() - this.nextTick;
+        if (delay < SHORT_SCHEDULE_TICK_THRESHOLD) {
+            if (delay < 0) {
+                // longScheduled orders by tick time, short scheduled does not
+                this.longScheduled.add(entry);
+            } else {
+                this.shortScheduled[getWrappedIndex(this.shortScheduledIndex, SHORT_SCHEDULE_TICK_THRESHOLD, (int)delay)].addEntryLast(entry);
+            }
+        } else {
+            this.longScheduled.add(entry);
+        }
+    }
+
+    private void removeEntry(final NextTickListEntry<T> entry) {
+        entry.tickState = STATE_CANCELLED_TICK;
+        // short/long scheduled will skip the entry
+
+        final BlockPosition pos = entry.getPosition();
+        final long blockKey = MCUtil.getBlockKey(pos);
+
+        final ArrayList<NextTickListEntry<T>> currentEntries = this.entriesByBlock.get(blockKey);
+
+        if (currentEntries.size() == 1) {
+            // it should contain our entry
+            this.entriesByBlock.remove(blockKey);
+        } else {
+            // it's more likely that this entry is at the start of the list than the end
+            for (int i = 0, len = currentEntries.size(); i < len; ++i) {
+                final NextTickListEntry<T> currentEntry = currentEntries.get(i);
+                if (currentEntry == entry) {
+                    currentEntries.remove(i);
+                    break;
+                }
+            }
+        }
+
+        final long chunkKey = MCUtil.getCoordinateKey(entry.getPosition());
+
+        ObjectRBTreeSet<NextTickListEntry<T>> set = this.entriesByChunk.get(chunkKey);
+
+        set.remove(entry);
+
+        if (set.isEmpty()) {
+            this.entriesByChunk.remove(chunkKey);
+        }
+
+        ArrayList<NextTickListEntry<T>> pendingTickingLoad = this.pendingChunkTickLoad.get(chunkKey);
+
+        if (pendingTickingLoad != null) {
+            for (int i = 0, len = pendingTickingLoad.size(); i < len; ++i) {
+                if (pendingTickingLoad.get(i) == entry) {
+                    pendingTickingLoad.remove(i);
+                    break;
+                }
+            }
+
+            if (pendingTickingLoad.isEmpty()) {
+                this.pendingChunkTickLoad.remove(chunkKey);
+            }
+        }
+
+        long delay = entry.getTargetTick() - this.nextTick;
+        if (delay >= SHORT_SCHEDULE_TICK_THRESHOLD) {
+            this.longScheduled.remove(entry);
+        }
+    }
+
+    public void onChunkSetTicking(final int chunkX, final int chunkZ) {
+        final ArrayList<NextTickListEntry<T>> pending = this.pendingChunkTickLoad.remove(MCUtil.getCoordinateKey(chunkX, chunkZ));
+        if (pending == null) {
+            return;
+        }
+
+        for (int i = 0, size = pending.size(); i < size; ++i) {
+            final NextTickListEntry<T> entry = pending.get(i);
+            // already in all the relevant reference maps, just need to add to longScheduled or shortScheduled
+            this.addToSchedule(entry);
+        }
+    }
+
+    private void prepare() {
+        final long currentTick = this.world.getTime();
+
+        final ChunkProviderServer chunkProvider = this.world.getChunkProvider();
+
+        // here we setup what's going to tick
+
+        // we don't remove items from shortScheduled (but do from longScheduled) because they're cleared at the end of
+        // this tick
+        if (this.longScheduled.isEmpty() || this.longScheduled.first().getTargetTick() > currentTick) {
+            // nothing in longScheduled to worry about
+            final TickListServerInterval<T> interval = this.shortScheduled[this.shortScheduledIndex];
+            for (int i = 0, len = interval.byPriority.length; i < len; ++i) {
+                for (final Iterator<NextTickListEntry<T>> iterator = interval.byPriority[i].iterator(); iterator.hasNext();) {
+                    this.queueEntryForTick(iterator.next(), chunkProvider);
+                }
+            }
+        } else {
+            final TickListServerInterval<T> interval = this.shortScheduled[this.shortScheduledIndex];
+
+            // combine interval and longScheduled, keeping order
+            final Comparator<NextTickListEntry<T>> comparator = (Comparator)TickListServerInterval.ENTRY_COMPARATOR;
+            final Iterator<NextTickListEntry<T>> longScheduledIterator = this.longScheduled.iterator();
+            NextTickListEntry<T> longCurrent = longScheduledIterator.next();
+
+            for (int i = 0, len = interval.byPriority.length; i < len; ++i) {
+                for (final Iterator<NextTickListEntry<T>> iterator = interval.byPriority[i].iterator(); iterator.hasNext();) {
+                    final NextTickListEntry<T> shortCurrent = iterator.next();
+                    if (longCurrent != null) {
+                        // drain longCurrent until we can add shortCurrent
+                        while (comparator.compare(longCurrent, shortCurrent) <= 0) {
+                            this.queueEntryForTick(longCurrent, chunkProvider);
+                            longScheduledIterator.remove();
+                            if (longScheduledIterator.hasNext()) {
+                                longCurrent = longScheduledIterator.next();
+                                if (longCurrent.getTargetTick() > currentTick) {
+                                    longCurrent = null;
+                                    break;
+                                }
+                            } else {
+                                longCurrent = null;
+                                break;
+                            }
+                        }
+                    }
+                    this.queueEntryForTick(shortCurrent, chunkProvider);
+                }
+            }
+
+            // add remaining from long scheduled
+            for (;;) {
+                if (longCurrent == null || longCurrent.getTargetTick() > currentTick) {
+                    break;
+                }
+                longScheduledIterator.remove();
+                this.queueEntryForTick(longCurrent, chunkProvider);
+
+                if (longScheduledIterator.hasNext()) {
+                    longCurrent = longScheduledIterator.next();
+                } else {
+                    break;
+                }
+            }
+        }
+    }
+
+    @Override
+    public void tick() {
+        final ChunkProviderServer chunkProvider = this.world.getChunkProvider();
+
+        this.world.getMethodProfiler().enter("cleaning");
+        this.timingCleanup.startTiming();
+
+        this.prepare();
+
+        // this must be done here in case something schedules in the tick code
+        this.shortScheduled[this.shortScheduledIndex].clear();
+        this.shortScheduledIndex = getNextIndex(this.shortScheduledIndex, SHORT_SCHEDULE_TICK_THRESHOLD);
+        this.nextTick = this.world.getTime() + 1;
+
+        this.timingCleanup.stopTiming();
+        this.world.getMethodProfiler().exitEnter("ticking");
+        this.timingTicking.startTiming();
+
+        for (final NextTickListEntry<T> toTick : this.toTickThisTick) {
+            if (toTick.tickState != STATE_PENDING_TICK) {
+                // onTickEnd gets called at end of tick
+                continue;
+            }
+            try {
+                if (chunkProvider.isTickingReadyMainThread(toTick.getPosition())) {
+                    toTick.tickState = STATE_TICKING;
+                    this.tickFunction.accept(toTick);
+                    if (toTick.tickState == STATE_TICKING) {
+                        toTick.tickState = STATE_TICKED;
+                    } // else it's STATE_CANCELLED_TICK
+                } else {
+                    // re-schedule eventually
+                    toTick.tickState = STATE_SCHEDULED;
+                    this.addToNotTickingReady(toTick);
+                }
+            } catch (final Throwable thr) {
+                // start copy from TickListServer // TODO check on update
+                CrashReport crashreport = CrashReport.a(thr, "Exception while ticking");
+                CrashReportSystemDetails crashreportsystemdetails = crashreport.a("Block being ticked");
+
+                CrashReportSystemDetails.a(crashreportsystemdetails, toTick.getPosition(), (IBlockData) null);
+                throw new ReportedException(crashreport);
+                // end copy from TickListServer
+            }
+        }
+
+        this.timingTicking.stopTiming();
+        this.world.getMethodProfiler().exit();
+        this.timingFinished.startTiming();
+
+        // finished ticking, actual cleanup time
+        for (int i = 0, len = this.toTickThisTick.size(); i < len; ++i) {
+            final NextTickListEntry<T> entry = this.toTickThisTick.poll();
+            if (entry.tickState != STATE_SCHEDULED) {
+                // some entries get re-scheduled due to their chunk not being loaded/at correct status, so do not
+                // call onTickEnd for them
+                this.onTickEnd(entry);
+            }
+        }
+
+        this.timingFinished.stopTiming();
+    }
+
+    private void onTickEnd(final NextTickListEntry<T> entry) {
+        if (entry.tickState == STATE_CANCELLED_TICK) {
+            return;
+        }
+        entry.tickState = STATE_UNSCHEDULED;
+
+        final BlockPosition pos = entry.getPosition();
+        final long blockKey = MCUtil.getBlockKey(pos);
+
+        final ArrayList<NextTickListEntry<T>> currentEntries = this.entriesByBlock.get(blockKey);
+
+        if (currentEntries.size() == 1) {
+            // it should contain our entry
+            this.entriesByBlock.remove(blockKey);
+        } else {
+            // it's more likely that this entry is at the start of the list than the end
+            for (int i = 0, len = currentEntries.size(); i < len; ++i) {
+                final NextTickListEntry<T> currentEntry = currentEntries.get(i);
+                if (currentEntry == entry) {
+                    currentEntries.remove(i);
+                    break;
+                }
+            }
+        }
+
+        final long chunkKey = MCUtil.getCoordinateKey(entry.getPosition());
+
+        ObjectRBTreeSet<NextTickListEntry<T>> set = this.entriesByChunk.get(chunkKey);
+
+        set.remove(entry);
+
+        if (set.isEmpty()) {
+            this.entriesByChunk.remove(chunkKey);
+        }
+
+        // already removed from longScheduled or shortScheduled
+    }
+
+    @Override
+    public boolean isPendingTickThisTick(final BlockPosition blockposition, final T data) {
+        final ArrayList<NextTickListEntry<T>> entries = this.entriesByBlock.get(MCUtil.getBlockKey(blockposition));
+
+        if (entries == null) {
+            return false;
+        }
+
+        for (int i = 0, size = entries.size(); i < size; ++i) {
+            final NextTickListEntry<T> entry = entries.get(i);
+            if (entry.getData() == data && entry.tickState == STATE_PENDING_TICK) {
+                return true;
+            }
+        }
+
+        return false;
+    }
+
+    @Override
+    public boolean isScheduledForTick(final BlockPosition blockposition, final T data) {
+        final ArrayList<NextTickListEntry<T>> entries = this.entriesByBlock.get(MCUtil.getBlockKey(blockposition));
+
+        if (entries == null) {
+            return false;
+        }
+
+        for (int i = 0, size = entries.size(); i < size; ++i) {
+            final NextTickListEntry<T> entry = entries.get(i);
+            if (entry.getData() == data && entry.tickState == STATE_SCHEDULED) {
+                return true;
+            }
+        }
+
+        return false;
+    }
+
+    @Override
+    public void schedule(BlockPosition blockPosition, T t, int i, TickListPriority tickListPriority) {
+        this.schedule(blockPosition, t, i + this.world.getTime(), tickListPriority);
+    }
+
+    public void schedule(final NextTickListEntry<T> entry) {
+        this.schedule(entry.getPosition(), entry.getData(), entry.getTargetTick(), entry.getPriority());
+    }
+
+    public void schedule(final BlockPosition pos, final T data, final long targetTick, final TickListPriority priority) {
+        final NextTickListEntry<T> entry = new NextTickListEntry<>(pos, data, targetTick, priority);
+        if (this.excludeFromScheduling.test(entry.getData())) {
+            return;
+        }
+
+        if (WARN_ON_EXCESSIVE_DELAY) {
+            final long delay = entry.getTargetTick() - this.nextTick;
+            if (delay >= EXCESSIVE_DELAY_THRESHOLD) {
+                MinecraftServer.LOGGER.warn("Entry " + entry.toString() + " has been scheduled with an excessive delay of: " + delay, new Throwable());
+            }
+        }
+
+        final long blockKey = MCUtil.getBlockKey(pos);
+
+        final ArrayList<NextTickListEntry<T>> currentEntries = this.entriesByBlock.computeIfAbsent(blockKey, (long keyInMap) -> new ArrayList<>(3));
+
+        if (currentEntries.isEmpty()) {
+            currentEntries.add(entry);
+        } else {
+            for (int i = 0, size = currentEntries.size(); i < size; ++i) {
+                final NextTickListEntry<T> currentEntry = currentEntries.get(i);
+
+                // entries are only blocked from scheduling if currentEntry.equals(toSchedule) && currentEntry is scheduled to tick (NOT including pending)
+                if (currentEntry.getData() == entry.getData() && currentEntry.tickState == STATE_SCHEDULED) {
+                    // can't add
+                    return;
+                }
+            }
+            currentEntries.add(entry);
+        }
+
+        entry.tickState = STATE_SCHEDULED;
+
+        this.entriesByChunk.computeIfAbsent(MCUtil.getCoordinateKey(entry.getPosition()), (final long keyInMap) -> {
+            return new ObjectRBTreeSet<>(TickListServerInterval.ENTRY_COMPARATOR);
+        }).add(entry);
+
+        this.addToSchedule(entry);
+    }
+
+    @Override
+    public void scheduleAll(final Stream<NextTickListEntry<T>> stream) {
+        this.scheduleAll(stream.iterator());
+    }
+    public void scheduleAll(final Iterator<NextTickListEntry<T>> iterator) {
+        while (iterator.hasNext()) {
+            this.schedule(iterator.next());
+        }
+    }
+
+    // this is not the standard interception calculation, but it's the one vanilla uses
+    // i.e the y value is ignored? the x, z calc isn't correct?
+    // however for the copy op they use the correct intersection, after using this one of course...
+    private static boolean isBlockInSortof(final StructureBoundingBox boundingBox, final BlockPosition pos) {
+        return pos.getX() >= boundingBox.getMinX() && pos.getX() < boundingBox.getMaxX() && pos.getZ() >= boundingBox.getMinZ() && pos.getZ() < boundingBox.getMaxZ();
+    }
+
+    @Override
+    public List<NextTickListEntry<T>> getEntriesInBoundingBox(final StructureBoundingBox structureboundingbox, final boolean removeReturned, final boolean excludeTicked) {
+        if (structureboundingbox.getMinX() == structureboundingbox.getMaxX() || structureboundingbox.getMinZ() == structureboundingbox.getMaxZ()) {
+            return Collections.emptyList(); // vanilla behaviour, check isBlockInSortof above
+        }
+
+        final int lowerChunkX = structureboundingbox.getMinX() >> 4;
+        final int upperChunkX = (structureboundingbox.getMaxX() - 1) >> 4; // subtract 1 since maxX is exclusive
+        final int lowerChunkZ = structureboundingbox.getMinZ() >> 4;
+        final int upperChunkZ = (structureboundingbox.getMaxZ() - 1) >> 4; // subtract 1 since maxZ is exclusive
+
+        final int xChunksLength = (upperChunkX - lowerChunkX + 1);
+        final int zChunksLength = (upperChunkZ - lowerChunkZ + 1);
+
+        final ObjectRBTreeSet<NextTickListEntry<T>>[] containingChunks = new ObjectRBTreeSet[xChunksLength * zChunksLength];
+
+        final int offset = (xChunksLength * -lowerChunkZ - lowerChunkX);
+        int totalEntries = 0;
+        for (int currChunkX = lowerChunkX; currChunkX <= upperChunkX; ++currChunkX) {
+            for (int currChunkZ = lowerChunkZ; currChunkZ <= upperChunkZ; ++currChunkZ) {
+                // todo optimize
+                //final int index = (currChunkX - lowerChunkX) + xChunksLength * (currChunkZ - lowerChunkZ);
+                final int index = offset + currChunkX + xChunksLength * currChunkZ;
+                final ObjectRBTreeSet<NextTickListEntry<T>> set = containingChunks[index] = this.entriesByChunk.get(MCUtil.getCoordinateKey(currChunkX, currChunkZ));
+                if (set != null) {
+                    totalEntries += set.size();
+                }
+            }
+        }
+
+        final List<NextTickListEntry<T>> ret = new ArrayList<>(totalEntries);
+
+        final int matchOne = (STATE_SCHEDULED | STATE_PENDING_TICK) | (excludeTicked ? 0 : (STATE_TICKING | STATE_TICKED));
+
+        MCUtil.mergeSortedSets((NextTickListEntry<T> entry) -> {
+            if (!isBlockInSortof(structureboundingbox, entry.getPosition())) {
+                return;
+            }
+            final int tickState = entry.tickState;
+            if ((tickState & matchOne) == 0) {
+                return;
+            }
+
+            ret.add(entry);
+            return;
+        }, TickListServerInterval.ENTRY_COMPARATOR, containingChunks);
+
+        if (removeReturned) {
+            for (NextTickListEntry<T> entry : ret) {
+                this.removeEntry(entry);
+            }
+        }
+
+        return ret;
+    }
+
+    @Override
+    public void copy(StructureBoundingBox structureboundingbox, BlockPosition blockposition) {
+        // start copy from TickListServer // TODO check on update
+        List<NextTickListEntry<T>> list = this.getEntriesInBoundingBox(structureboundingbox, false, false);
+        Iterator<NextTickListEntry<T>> iterator = list.iterator();
+
+        while (iterator.hasNext()) {
+            NextTickListEntry<T> nextticklistentry = iterator.next();
+
+            if (structureboundingbox.hasPoint( nextticklistentry.getPosition())) {
+                BlockPosition blockposition1 = nextticklistentry.getPosition().add(blockposition);
+                T t0 = nextticklistentry.getData();
+
+                this.schedule(new NextTickListEntry<>(blockposition1, t0, nextticklistentry.getTargetTick(), nextticklistentry.getPriority()));
+            }
+        }
+        // end copy from TickListServer
+    }
+
+    @Override
+    public List<NextTickListEntry<T>> getEntriesInChunk(ChunkCoordIntPair chunkPos, boolean removeReturned, boolean excludeTicked) {
+        // Vanilla DOES get the entries 2 blocks out of the chunk too, but that doesn't matter since we ignore chunks
+        // not at ticking status, and ticking status requires neighbours loaded
+        // so with this method we will reduce scheduler churning
+        final int matchOne = (STATE_SCHEDULED | STATE_PENDING_TICK) | (excludeTicked ? 0 : (STATE_TICKING | STATE_TICKED));
+
+        final ObjectRBTreeSet<NextTickListEntry<T>> entries = this.entriesByChunk.get(MCUtil.getCoordinateKey(chunkPos));
+
+        if (entries == null) {
+            return Collections.emptyList();
+        }
+
+        final List<NextTickListEntry<T>> ret = new ArrayList<>(entries.size());
+
+        for (NextTickListEntry<T> entry : entries) {
+            if ((entry.tickState & matchOne) == 0) {
+                continue;
+            }
+            ret.add(entry);
+        }
+
+        if (removeReturned) {
+            for (NextTickListEntry<T> entry : ret) {
+                this.removeEntry(entry);
+            }
+        }
+
+        return ret;
+    }
+
+    @Override
+    public NBTTagList serialize(ChunkCoordIntPair chunkcoordintpair) {
+        // start copy from TickListServer  // TODO check on update
+        List<NextTickListEntry<T>> list = this.getEntriesInChunk(chunkcoordintpair, false, true);
+
+        return TickListServer.serialize(this.getMinecraftKeyFrom, list, this.world.getTime());
+        // end copy from TickListServer
+    }
+
+    @Override
+    public int getTotalScheduledEntries() {
+        // good thing this is only used in debug reports // TODO check on update
+        int ret = 0;
+
+        for (NextTickListEntry<T> entry : this.longScheduled) {
+            if (entry.tickState == STATE_SCHEDULED) {
+                ++ret;
+            }
+        }
+
+        for (Iterator<Long2ObjectMap.Entry<ArrayList<NextTickListEntry<T>>>> iterator = this.pendingChunkTickLoad.long2ObjectEntrySet().iterator(); iterator.hasNext();) {
+            ArrayList<NextTickListEntry<T>> list = iterator.next().getValue();
+
+            for (NextTickListEntry<T> entry : list) {
+                if (entry.tickState == STATE_SCHEDULED) {
+                    ++ret;
+                }
+            }
+        }
+
+        for (TickListServerInterval<T> interval : this.shortScheduled) {
+            for (Iterable<NextTickListEntry<T>> set : interval.byPriority) {
+                for (NextTickListEntry<T> entry : set) {
+                    if (entry.tickState == STATE_SCHEDULED) {
+                        ++ret;
+                    }
+                }
+            }
+        }
+
+        return ret;
+    }
+}
diff --git a/src/main/java/com/destroystokyo/paper/server/ticklist/TickListServerInterval.java b/src/main/java/com/destroystokyo/paper/server/ticklist/TickListServerInterval.java
new file mode 100644
index 0000000000000000000000000000000000000000..13cf1a55a9babbffccbca1acfbe1c496e8925521
--- /dev/null
+++ b/src/main/java/com/destroystokyo/paper/server/ticklist/TickListServerInterval.java
@@ -0,0 +1,41 @@
+package com.destroystokyo.paper.server.ticklist;
+
+import com.destroystokyo.paper.util.set.LinkedSortedSet;
+import net.minecraft.server.NextTickListEntry;
+import net.minecraft.server.TickListPriority;
+import java.util.Comparator;
+
+// represents a set of entries to tick at a specified time
+public final class TickListServerInterval<T> {
+
+    public static final int TOTAL_PRIORITIES = TickListPriority.values().length;
+    public static final Comparator<NextTickListEntry<?>> ENTRY_COMPARATOR_BY_ID = (entry1, entry2) -> {
+        return Long.compare(entry1.getId(), entry2.getId());
+    };
+    public static final Comparator<NextTickListEntry<?>> ENTRY_COMPARATOR = (Comparator)NextTickListEntry.comparator();
+
+    // we do not record the interval, this class is meant to be used on a ring buffer
+
+    // inlined enum map for TickListPriority
+    public final LinkedSortedSet<NextTickListEntry<T>>[] byPriority = new LinkedSortedSet[TOTAL_PRIORITIES];
+
+    {
+        for (int i = 0, len = this.byPriority.length; i < len; ++i) {
+            this.byPriority[i] = new LinkedSortedSet<>(ENTRY_COMPARATOR_BY_ID);
+        }
+    }
+
+    public void addEntryLast(final NextTickListEntry<T> entry) {
+        this.byPriority[entry.getPriority().ordinal()].addLast(entry);
+    }
+
+    public void addEntryFirst(final NextTickListEntry<T> entry) {
+        this.byPriority[entry.getPriority().ordinal()].addFirst(entry);
+    }
+
+    public void clear() {
+        for (int i = 0, len = this.byPriority.length; i < len; ++i) {
+            this.byPriority[i].clear(); // O(1) clear
+        }
+    }
+}
diff --git a/src/main/java/com/destroystokyo/paper/util/set/LinkedSortedSet.java b/src/main/java/com/destroystokyo/paper/util/set/LinkedSortedSet.java
new file mode 100644
index 0000000000000000000000000000000000000000..118988c39e58f28e8a2851792b9c014f341f06fc
--- /dev/null
+++ b/src/main/java/com/destroystokyo/paper/util/set/LinkedSortedSet.java
@@ -0,0 +1,142 @@
+package com.destroystokyo.paper.util.set;
+
+import java.util.Comparator;
+import java.util.Iterator;
+import java.util.NoSuchElementException;
+
+public final class LinkedSortedSet<E> implements Iterable<E> {
+
+    public final Comparator<? super E> comparator;
+
+    protected Link<E> head;
+    protected Link<E> tail;
+
+    public LinkedSortedSet() {
+        this((Comparator)Comparator.naturalOrder());
+    }
+
+    public LinkedSortedSet(final Comparator<? super E> comparator) {
+        this.comparator = comparator;
+    }
+
+    public void clear() {
+        this.head = this.tail = null;
+    }
+
+    @Override
+    public Iterator<E> iterator() {
+        return new Iterator<E>() {
+
+            Link<E> next = LinkedSortedSet.this.head;
+
+            @Override
+            public boolean hasNext() {
+                return this.next != null;
+            }
+
+            @Override
+            public E next() {
+                final Link<E> next = this.next;
+                if (next == null) {
+                    throw new NoSuchElementException();
+                }
+                this.next = next.next;
+                return next.element;
+            }
+        };
+    }
+
+    public boolean addLast(final E element) {
+        final Comparator<? super E> comparator = this.comparator;
+
+        Link<E> curr = this.tail;
+        if (curr != null) {
+            int compare;
+
+            while ((compare = comparator.compare(element, curr.element)) < 0) {
+                Link<E> prev = curr;
+                curr = curr.prev;
+                if (curr != null) {
+                    continue;
+                }
+                this.head = prev.prev = new Link<>(element, null, prev);
+                return true;
+            }
+
+            if (compare != 0) {
+                // insert after curr
+                final Link<E> next = curr.next;
+                final Link<E> insert = new Link<>(element, curr, next);
+                curr.next = insert;
+
+                if (next == null) {
+                    this.tail = insert;
+                } else {
+                    next.prev = insert;
+                }
+                return true;
+            }
+
+            return false;
+        } else {
+            this.head = this.tail = new Link<>(element);
+            return true;
+        }
+    }
+
+    public boolean addFirst(final E element) {
+        final Comparator<? super E> comparator = this.comparator;
+
+        Link<E> curr = this.head;
+        if (curr != null) {
+            int compare;
+
+            while ((compare = comparator.compare(element, curr.element)) > 0) {
+                Link<E> prev = curr;
+                curr = curr.next;
+                if (curr != null) {
+                    continue;
+                }
+                this.tail = prev.next = new Link<>(element, prev, null);
+                return true;
+            }
+
+            if (compare != 0) {
+                // insert before curr
+                final Link<E> prev = curr.prev;
+                final Link<E> insert = new Link<>(element, prev, curr);
+                curr.prev = insert;
+
+                if (prev == null) {
+                    this.head = insert;
+                } else {
+                    prev.next = insert;
+                }
+                return true;
+            }
+
+            return false;
+        } else {
+            this.head = this.tail = new Link<>(element);
+            return true;
+        }
+    }
+
+    protected static final class Link<E> {
+        public E element;
+        public Link<E> prev;
+        public Link<E> next;
+
+        public Link() {}
+
+        public Link(final E element) {
+            this.element = element;
+        }
+
+        public Link(final E element, final Link<E> prev, final Link<E> next) {
+            this.element = element;
+            this.prev = prev;
+            this.next = next;
+        }
+    }
+}
diff --git a/src/main/java/net/minecraft/server/BlockPosition.java b/src/main/java/net/minecraft/server/BlockPosition.java
index 063f8eb08635aaa44803f2a67d118805294ae938..58ba1209155e05e802171800f4420e3ae419950f 100644
--- a/src/main/java/net/minecraft/server/BlockPosition.java
+++ b/src/main/java/net/minecraft/server/BlockPosition.java
@@ -126,6 +126,7 @@ public class BlockPosition extends BaseBlockPosition implements MinecraftSeriali
         return i == 0 && j == 0 && k == 0 ? this : new BlockPosition(this.getX() + i, this.getY() + j, this.getZ() + k);
     }
 
+    public final BlockPosition add(BaseBlockPosition baseblockposition) { return this.a(baseblockposition); } // Paper - OBFHELPER
     public BlockPosition a(BaseBlockPosition baseblockposition) {
         return this.b(baseblockposition.getX(), baseblockposition.getY(), baseblockposition.getZ());
     }
diff --git a/src/main/java/net/minecraft/server/ChunkProviderServer.java b/src/main/java/net/minecraft/server/ChunkProviderServer.java
index e67e00653575c3e57fe16980d2ff074d8413a95e..ca1b5b3b094b847f96742d8466fc42c5bdedbff5 100644
--- a/src/main/java/net/minecraft/server/ChunkProviderServer.java
+++ b/src/main/java/net/minecraft/server/ChunkProviderServer.java
@@ -200,6 +200,13 @@ public class ChunkProviderServer extends IChunkProvider {
     }
     // Paper end
 
+    // Paper start - rewrite ticklistserver
+    public final boolean isTickingReadyMainThread(BlockPosition pos) {
+        PlayerChunk chunk = this.playerChunkMap.getUpdatingChunk(MCUtil.getCoordinateKey(pos));
+        return chunk != null && chunk.isTickingReady();
+    }
+    // Paper end - rewrite ticklistserver
+
 
     public ChunkProviderServer(WorldServer worldserver, File file, DataFixer datafixer, DefinedStructureManager definedstructuremanager, Executor executor, ChunkGenerator<?> chunkgenerator, int i, WorldLoadListener worldloadlistener, Supplier<WorldPersistentData> supplier) {
         this.world = worldserver;
diff --git a/src/main/java/net/minecraft/server/NextTickListEntry.java b/src/main/java/net/minecraft/server/NextTickListEntry.java
index 33cfeabdee03195a294f303f28044a313cb1c4ed..2287e47d1b891135a5f2579ec324c70589141192 100644
--- a/src/main/java/net/minecraft/server/NextTickListEntry.java
+++ b/src/main/java/net/minecraft/server/NextTickListEntry.java
@@ -5,11 +5,13 @@ import java.util.Comparator;
 public class NextTickListEntry<T> {
 
     private static final java.util.concurrent.atomic.AtomicLong COUNTER = new java.util.concurrent.atomic.AtomicLong(); // Paper - async chunk loading
-    private final T e;
-    public final BlockPosition a;
-    public final long b;
-    public final TickListPriority c;
-    private final long f;
+    private final T e; public final T getData() { return this.e; } // Paper - OBFHELPER
+    public final BlockPosition a; public final BlockPosition getPosition() { return this.a; } // Paper - OBFHELPER
+    public final long b; public final long getTargetTick() { return this.b; } // Paper - OBFHELPER
+    public final TickListPriority c; public final TickListPriority getPriority() { return this.c; } // Paper - OBFHELPER
+    private final long f; public final long getId() { return this.f; } // Paper - OBFHELPER
+    private final int hash; // Paper
+    public int tickState; // Paper
 
     public NextTickListEntry(BlockPosition blockposition, T t0) {
         this(blockposition, t0, 0L, TickListPriority.NORMAL);
@@ -21,6 +23,7 @@ public class NextTickListEntry<T> {
         this.e = t0;
         this.b = i;
         this.c = ticklistpriority;
+        this.hash = this.computeHash(); // Paper
     }
 
     public boolean equals(Object object) {
@@ -33,19 +36,31 @@ public class NextTickListEntry<T> {
         }
     }
 
+    // Paper start - optimize hashcode
+    @Override
     public int hashCode() {
+        return this.hash;
+    }
+    public final int computeHash() {
+        // Paper end - optimize hashcode
         return this.a.hashCode();
     }
 
-    public static <T> Comparator<Object> a() { // Paper - decompile fix
-        return Comparator.comparingLong((nextticklistentry) -> {
-            return ((NextTickListEntry<T>) nextticklistentry).b; // Paper - decompile fix
-        }).thenComparing((nextticklistentry) -> {
-            return ((NextTickListEntry<T>) nextticklistentry).c; // Paper - decompile fix
-        }).thenComparingLong((nextticklistentry) -> {
-            return ((NextTickListEntry<T>) nextticklistentry).f; // Paper - decompile fix
-        });
+    // Paper start - let's not use more functional code for no reason.
+    public static <T> Comparator<Object> comparator() { return NextTickListEntry.a(); } // Paper - OBFHELPER
+    public static <T> Comparator<Object> a() {
+        return (Comparator)(Comparator<NextTickListEntry>)(NextTickListEntry nextticklistentry, NextTickListEntry nextticklistentry1) -> {
+            int i = Long.compare(nextticklistentry.getTargetTick(), nextticklistentry1.getTargetTick());
+
+            if (i != 0) {
+                return i;
+            } else {
+                i = nextticklistentry.getPriority().compareTo(nextticklistentry1.getPriority());
+                return i != 0 ? i : Long.compare(nextticklistentry.getId(), nextticklistentry1.getId());
+            }
+        };
     }
+    // Paper end - let's not use more functional code for no reason.
 
     public String toString() {
         return this.e + ": " + this.a + ", " + this.b + ", " + this.c + ", " + this.f;
diff --git a/src/main/java/net/minecraft/server/PlayerChunk.java b/src/main/java/net/minecraft/server/PlayerChunk.java
index bf592125f4ce5d7ea6e802e637ee3bfbe25d23aa..3d610e41969768da0d2848fa1ae195035ccfd660 100644
--- a/src/main/java/net/minecraft/server/PlayerChunk.java
+++ b/src/main/java/net/minecraft/server/PlayerChunk.java
@@ -472,7 +472,9 @@ public class PlayerChunk {
                     PlayerChunk.this.isTickingReady = true;
 
 
-
+                    // Paper start - rewrite ticklistserver
+                    PlayerChunk.this.chunkMap.world.onChunkSetTicking(PlayerChunk.this.location.x, PlayerChunk.this.location.z);
+                    // Paper end - rewrite ticklistserver
 
                 }
             });
diff --git a/src/main/java/net/minecraft/server/StructureBoundingBox.java b/src/main/java/net/minecraft/server/StructureBoundingBox.java
index dbb565e74d211ef35d5d40b3d024e2f07cef9031..185658e230688a9939c89dae9167217cbee03571 100644
--- a/src/main/java/net/minecraft/server/StructureBoundingBox.java
+++ b/src/main/java/net/minecraft/server/StructureBoundingBox.java
@@ -4,12 +4,12 @@ import com.google.common.base.MoreObjects;
 
 public class StructureBoundingBox {
 
-    public int a;
-    public int b;
-    public int c;
-    public int d;
-    public int e;
-    public int f;
+    public int a; public final int getMinX() { return this.a; } // Paper - OBFHELPER
+    public int b; public final int getMinY() { return this.b; } // Paper - OBFHELPER
+    public int c; public final int getMinZ() { return this.c; } // Paper - OBFHELPER
+    public int d; public final int getMaxX() { return this.d; } // Paper - OBFHELPER
+    public int e; public final int getMaxY() { return this.e; } // Paper - OBFHELPER
+    public int f; public final int getMaxZ() { return this.f; } // Paper - OBFHELPER
 
     public StructureBoundingBox() {}
 
@@ -84,6 +84,7 @@ public class StructureBoundingBox {
         this.e = 512;
     }
 
+    public final boolean intersects(StructureBoundingBox boundingBox) { return this.b(boundingBox); } // Paper - OBFHELPER
     public boolean b(StructureBoundingBox structureboundingbox) {
         return this.d >= structureboundingbox.a && this.a <= structureboundingbox.d && this.f >= structureboundingbox.c && this.c <= structureboundingbox.f && this.e >= structureboundingbox.b && this.b <= structureboundingbox.e;
     }
@@ -114,6 +115,7 @@ public class StructureBoundingBox {
         return new StructureBoundingBox(this.a + i, this.b + j, this.c + k, this.d + i, this.e + j, this.f + k);
     }
 
+    public final boolean hasPoint(BaseBlockPosition baseblockposition) { return this.b(baseblockposition); } // Paper - OBFHELPER
     public boolean b(BaseBlockPosition baseblockposition) {
         return baseblockposition.getX() >= this.a && baseblockposition.getX() <= this.d && baseblockposition.getZ() >= this.c && baseblockposition.getZ() <= this.f && baseblockposition.getY() >= this.b && baseblockposition.getY() <= this.e;
     }
diff --git a/src/main/java/net/minecraft/server/TickListServer.java b/src/main/java/net/minecraft/server/TickListServer.java
index f533860bbed19ff2915c90186c259b466f41ce90..3f1aa5ced697490b5481ba992cf5af5dc98b8166 100644
--- a/src/main/java/net/minecraft/server/TickListServer.java
+++ b/src/main/java/net/minecraft/server/TickListServer.java
@@ -42,6 +42,11 @@ public class TickListServer<T> implements TickList<T> {
     // Paper end
 
     public void b() {
+        // Paper start - allow overriding
+        this.tick();
+    }
+    public void tick() {
+        // Paper end
         int i = this.nextTickList.size();
 
         if (false) { // CraftBukkit
@@ -109,15 +114,30 @@ public class TickListServer<T> implements TickList<T> {
 
     @Override
     public boolean b(BlockPosition blockposition, T t0) {
+        // Paper start - allow overriding
+        return this.isPendingTickThisTick(blockposition, t0);
+    }
+    public boolean isPendingTickThisTick(BlockPosition blockposition, T t0) {
+        // Paper end
         return this.g.contains(new NextTickListEntry<>(blockposition, t0));
     }
 
     @Override
     public void a(Stream<NextTickListEntry<T>> stream) {
+        // Paper start - allow overriding
+        this.scheduleAll(stream);
+    }
+    public void scheduleAll(Stream<NextTickListEntry<T>> stream) {
+        // Paper end
         stream.forEach(this::a);
     }
 
     public List<NextTickListEntry<T>> a(ChunkCoordIntPair chunkcoordintpair, boolean flag, boolean flag1) {
+        // Paper start - allow overriding
+        return this.getEntriesInChunk(chunkcoordintpair, flag, flag1);
+    }
+    public List<NextTickListEntry<T>> getEntriesInChunk(ChunkCoordIntPair chunkcoordintpair, boolean flag, boolean flag1) {
+        // Paper end
         int i = (chunkcoordintpair.x << 4) - 2;
         int j = i + 16 + 2;
         int k = (chunkcoordintpair.z << 4) - 2;
@@ -127,6 +147,11 @@ public class TickListServer<T> implements TickList<T> {
     }
 
     public List<NextTickListEntry<T>> a(StructureBoundingBox structureboundingbox, boolean flag, boolean flag1) {
+        // Paper start - allow overriding
+        return this.getEntriesInBoundingBox(structureboundingbox, flag, flag1);
+    }
+    public List<NextTickListEntry<T>> getEntriesInBoundingBox(StructureBoundingBox structureboundingbox, boolean flag, boolean flag1) {
+        // Paper end
         List<NextTickListEntry<T>> list = this.a((List) null, this.nextTickList, structureboundingbox, flag);
 
         if (flag && list != null) {
@@ -166,6 +191,11 @@ public class TickListServer<T> implements TickList<T> {
     }
 
     public void a(StructureBoundingBox structureboundingbox, BlockPosition blockposition) {
+        // Paper start - allow overriding
+        this.copy(structureboundingbox, blockposition);
+    }
+    public void copy(StructureBoundingBox structureboundingbox, BlockPosition blockposition) {
+        // Paper end
         List<NextTickListEntry<T>> list = this.a(structureboundingbox, false, false);
         Iterator iterator = list.iterator();
 
@@ -183,11 +213,17 @@ public class TickListServer<T> implements TickList<T> {
     }
 
     public NBTTagList a(ChunkCoordIntPair chunkcoordintpair) {
+        // Paper start - allow overriding
+        return this.serialize(chunkcoordintpair);
+    }
+    public NBTTagList serialize(ChunkCoordIntPair chunkcoordintpair) {
+        // Paper end
         List<NextTickListEntry<T>> list = this.a(chunkcoordintpair, false, true);
 
         return a(this.b, list, this.f.getTime());
     }
 
+    public static <T> NBTTagList serialize(Function<T, MinecraftKey> function, Iterable<NextTickListEntry<T>> iterable, long i) { return TickListServer.a(function, iterable, i); } // Paper - OBFHELPER
     public static <T> NBTTagList a(Function<T, MinecraftKey> function, Iterable<NextTickListEntry<T>> iterable, long i) {
         NBTTagList nbttaglist = new NBTTagList();
         Iterator iterator = iterable.iterator();
@@ -210,11 +246,21 @@ public class TickListServer<T> implements TickList<T> {
 
     @Override
     public boolean a(BlockPosition blockposition, T t0) {
+        // Paper start - allow overriding
+        return this.isScheduledForTick(blockposition, t0);
+    }
+    public boolean isScheduledForTick(BlockPosition blockposition, T t0) {
+        // Paper end
         return this.nextTickListHash.contains(new NextTickListEntry<>(blockposition, t0));
     }
 
     @Override
     public void a(BlockPosition blockposition, T t0, int i, TickListPriority ticklistpriority) {
+        // Paper start - allow overriding
+        this.schedule(blockposition, t0, i, ticklistpriority);
+    }
+    public void schedule(BlockPosition blockposition, T t0, int i, TickListPriority ticklistpriority) {
+        // Paper end
         if (!this.a.test(t0)) {
             this.a(new NextTickListEntry<>(blockposition, t0, (long) i + this.f.getTime(), ticklistpriority));
         }
@@ -230,6 +276,11 @@ public class TickListServer<T> implements TickList<T> {
     }
 
     public int a() {
+        // Paper start - allow overriding
+        return this.getTotalScheduledEntries();
+    }
+    public int getTotalScheduledEntries() {
+        // Paper end
         return this.nextTickListHash.size();
     }
 }
diff --git a/src/main/java/net/minecraft/server/WorldServer.java b/src/main/java/net/minecraft/server/WorldServer.java
index fcbc9f29139ce5cd3b165130006f9c326a5d9eea..5173731dc55db416a312e13afd09e4e69d829e45 100644
--- a/src/main/java/net/minecraft/server/WorldServer.java
+++ b/src/main/java/net/minecraft/server/WorldServer.java
@@ -170,6 +170,15 @@ public class WorldServer extends World {
     }
     // Paper end
 
+    // Paper start - rewrite ticklistserver
+    void onChunkSetTicking(int chunkX, int chunkZ) {
+        if (com.destroystokyo.paper.PaperConfig.useOptimizedTickList) {
+            ((com.destroystokyo.paper.server.ticklist.PaperTickList) this.nextTickListBlock).onChunkSetTicking(chunkX, chunkZ);
+            ((com.destroystokyo.paper.server.ticklist.PaperTickList) this.nextTickListFluid).onChunkSetTicking(chunkX, chunkZ);
+        }
+    }
+    // Paper end - rewrite ticklistserver
+
     // Add env and gen to constructor
     public WorldServer(MinecraftServer minecraftserver, Executor executor, WorldNBTStorage worldnbtstorage, WorldData worlddata, DimensionManager dimensionmanager, GameProfilerFiller gameprofilerfiller, WorldLoadListener worldloadlistener, org.bukkit.World.Environment env, org.bukkit.generator.ChunkGenerator gen) {
         super(worlddata, dimensionmanager, (world, worldprovider) -> {
@@ -190,12 +199,22 @@ public class WorldServer extends World {
         this.pvpMode = minecraftserver.getPVP();
         worlddata.world = this;
         // CraftBukkit end
-        this.nextTickListBlock = new TickListServer<>(this, (block) -> {
-            return block == null || block.getBlockData().isAir();
-        }, IRegistry.BLOCK::getKey, IRegistry.BLOCK::get, this::b, "Blocks"); // Paper - Timings
-        this.nextTickListFluid = new TickListServer<>(this, (fluidtype) -> {
-            return fluidtype == null || fluidtype == FluidTypes.EMPTY;
-        }, IRegistry.FLUID::getKey, IRegistry.FLUID::get, this::a, "Fluids"); // Paper - Timings
+        if (com.destroystokyo.paper.PaperConfig.useOptimizedTickList) {
+            this.nextTickListBlock = new com.destroystokyo.paper.server.ticklist.PaperTickList<>(this, (block) -> { // Paper - optimise TickListServer
+                return block == null || block.getBlockData().isAir();
+            }, IRegistry.BLOCK::getKey, IRegistry.BLOCK::get, this::b, "Blocks"); // Paper - Timings
+            this.nextTickListFluid = new com.destroystokyo.paper.server.ticklist.PaperTickList<>(this, (fluidtype) -> { // Paper - optimise TickListServer
+                return fluidtype == null || fluidtype == FluidTypes.EMPTY;
+            }, IRegistry.FLUID::getKey, IRegistry.FLUID::get, this::a, "Fluids"); // Paper - Timings
+        } else {
+            this.nextTickListBlock = new TickListServer<>(this, (block) -> { // Paper - optimise TickListServer
+                return block == null || block.getBlockData().isAir();
+            }, IRegistry.BLOCK::getKey, IRegistry.BLOCK::get, this::b, "Blocks"); // Paper - Timings
+            this.nextTickListFluid = new TickListServer<>(this, (fluidtype) -> { // Paper - optimise TickListServer
+                return fluidtype == null || fluidtype == FluidTypes.EMPTY;
+            }, IRegistry.FLUID::getKey, IRegistry.FLUID::get, this::a, "Fluids"); // Paper - Timings
+        }
+
         this.navigators = Sets.newHashSet();
         this.I = new ObjectLinkedOpenHashSet();
         this.dataManager = worldnbtstorage;
