Implement Chunk Priority / Urgency System for World Gen

Mark chunks that are blocking main thread for world generation as urgent

Implements a general priority system so that chunks that are sorted in
the generator queues can prioritize certain chunks over another.

Urgent chunks will jump to the front of the line, ensuring that a
sync chunk load on an ungenerated chunk does not lag the server for
a long period of time if the servers generator queues are filled with
lots of chunks already.

This massively reduces the lag spikes from sync chunk gens.

This is also a precursor to my next improvement to prioritize chunks
in front of the player (Frustum Priorization)

Spigot-Server-Patches/0466-Implement-Chunk-Priority-Urgency-System-for-World-Ge.patch

@@ -0,0 +1,284 @@
+From e5f47c1c4318542b46e6297c40cd2b5ac306ffb9 Mon Sep 17 00:00:00 2001
+From: Aikar <aikar@aikar.co>
+Date: Sat, 11 Apr 2020 03:56:07 -0400
+Subject: [PATCH] Implement Chunk Priority / Urgency System for World Gen
+
+Mark chunks that are blocking main thread for world generation as urgent
+
+Implements a general priority system so that chunks that are sorted in
+the generator queues can prioritize certain chunks over another.
+
+Urgent chunks will jump to the front of the line, ensuring that a
+sync chunk load on an ungenerated chunk does not lag the server for
+a long period of time if the servers generator queues are filled with
+lots of chunks already.
+
+This massively reduces the lag spikes from sync chunk gens.
+
+diff --git a/src/main/java/net/minecraft/server/ChunkProviderServer.java b/src/main/java/net/minecraft/server/ChunkProviderServer.java
+index bacfc4cba6..f741a034e8 100644
+--- a/src/main/java/net/minecraft/server/ChunkProviderServer.java
++++ b/src/main/java/net/minecraft/server/ChunkProviderServer.java
+@@ -227,6 +227,7 @@ public class ChunkProviderServer extends IChunkProvider {
+     }
+ 
+     private long asyncLoadSeqCounter;
++    public static boolean IS_CHUNK_LOAD_BLOCKING_MAIN = false;
+ 
+     public void getChunkAtAsynchronously(int x, int z, boolean gen, java.util.function.Consumer<Chunk> onComplete) {
+         if (Thread.currentThread() != this.serverThread) {
+@@ -384,10 +385,18 @@ public class ChunkProviderServer extends IChunkProvider {
+             }
+ 
+             gameprofilerfiller.c("getChunkCacheMiss");
++            // Paper start - Chunk Load/Gen Priority
++            boolean prevBlocking = IS_CHUNK_LOAD_BLOCKING_MAIN;
++            IS_CHUNK_LOAD_BLOCKING_MAIN = true;
++            // Paper end
+             CompletableFuture<Either<IChunkAccess, PlayerChunk.Failure>> completablefuture = this.getChunkFutureMainThread(i, j, chunkstatus, flag);
+ 
+             if (!completablefuture.isDone()) { // Paper
+                 // Paper start - async chunk io/loading
++                PlayerChunk playerChunk = this.getChunk(ChunkCoordIntPair.pair(x, z));
++                if (playerChunk != null) {
++                    playerChunk.markChunkUrgent(chunkstatus);
++                }
+                 this.world.asyncChunkTaskManager.raisePriority(x, z, com.destroystokyo.paper.io.PrioritizedTaskQueue.HIGHEST_PRIORITY);
+                 com.destroystokyo.paper.io.chunk.ChunkTaskManager.pushChunkWait(this.world, x, z);
+                 // Paper end
+@@ -397,6 +406,11 @@ public class ChunkProviderServer extends IChunkProvider {
+                 com.destroystokyo.paper.io.chunk.ChunkTaskManager.popChunkWait(); // Paper - async chunk debug
+                 this.world.timings.chunkAwait.stopTiming(); // Paper
+             } // Paper
++            PlayerChunk playerChunk = this.getChunk(ChunkCoordIntPair.pair(x, z));
++            if (playerChunk != null) {
++                playerChunk.clearChunkUrgent();
++            }
++            IS_CHUNK_LOAD_BLOCKING_MAIN = prevBlocking;// Paper
+             ichunkaccess = (IChunkAccess) ((Either) completablefuture.join()).map((ichunkaccess1) -> {
+                 return ichunkaccess1;
+             }, (playerchunk_failure) -> {
+diff --git a/src/main/java/net/minecraft/server/PlayerChunk.java b/src/main/java/net/minecraft/server/PlayerChunk.java
+index 9f8818c2d4..a9a2ce3d3f 100644
+--- a/src/main/java/net/minecraft/server/PlayerChunk.java
++++ b/src/main/java/net/minecraft/server/PlayerChunk.java
+@@ -43,6 +43,111 @@ public class PlayerChunk {
+     long lastAutoSaveTime; // Paper - incremental autosave
+     long inactiveTimeStart; // Paper - incremental autosave
+ 
++    // Paper start - Chunk gen/load priority system
++    volatile int chunkPriority = 0;
++    volatile boolean isUrgent = false;
++    final java.util.List<PlayerChunk> urgentNeighbors = new java.util.ArrayList<>();
++    volatile PlayerChunk rootUrgentOriginator;
++    volatile PlayerChunk urgentOriginator;
++    public void onNeighborRequest(PlayerChunk neighbor, ChunkStatus status) {
++        if (isUrgent && !neighbor.isUrgent && !java.util.Objects.equals(neighbor, rootUrgentOriginator) && !java.util.Objects.equals(neighbor, urgentOriginator)) {
++            synchronized (this.urgentNeighbors) {
++                if (!neighbor.isUrgent) {
++                    neighbor.markChunkUrgent(status, this.rootUrgentOriginator, this);
++                    this.urgentNeighbors.add(neighbor);
++                }
++            }
++        }
++    }
++
++    public void onNeighborsDone() {
++        List<PlayerChunk> urgentNeighbors;
++        synchronized (this.urgentNeighbors) {
++            urgentNeighbors = new java.util.ArrayList<>(this.urgentNeighbors);
++            this.urgentNeighbors.clear();
++        }
++        for (PlayerChunk urgentNeighbor : urgentNeighbors) {
++            if (urgentNeighbor != null) {
++                urgentNeighbor.clearChunkUrgent(this);
++            }
++        }
++    }
++
++    public void clearChunkUrgent() {
++        clearChunkUrgent(this);
++    }
++    public void clearChunkUrgent(PlayerChunk requester) {
++        if (this.isUrgent && java.util.Objects.equals(requester, this.urgentOriginator)) {
++            this.isUrgent = false;
++            this.urgentOriginator = null;
++            this.rootUrgentOriginator = null;
++            this.onNeighborsDone();
++        }
++    }
++
++    public void markChunkUrgent(ChunkStatus targetStatus) {
++        this.markChunkUrgent(targetStatus, this , this);
++    }
++    public void markChunkUrgent(ChunkStatus targetStatus, PlayerChunk rootUrgentOriginator, PlayerChunk urgentOriginator) {
++        if (!this.isUrgent) {
++            this.rootUrgentOriginator = rootUrgentOriginator;
++            this.urgentOriginator = urgentOriginator;
++            this.isUrgent = true;
++            int x = location.x;
++            int z = location.z;
++            IChunkAccess chunk = getAvailableChunkNow();
++            final ChunkStatus chunkCurrentStatus = chunk == null ? null : chunk.getChunkStatus();
++            final ChunkStatus completedStatus = this.getChunkHolderStatus();
++            final ChunkStatus nextStatus = getNextStatus(completedStatus != null ? completedStatus : ChunkStatus.EMPTY);
++
++            if (chunkCurrentStatus == null || completedStatus == null) {
++                this.chunkMap.world.asyncChunkTaskManager.raisePriority(x, z, com.destroystokyo.paper.io.PrioritizedTaskQueue.HIGHEST_PRIORITY);
++                // next status is empty, empty has no neighbours needing loading
++                return;
++            }
++
++            if (!targetStatus.isAtLeastStatus(nextStatus)) {
++                // we don't want a status greater-than the one we already have, don't prioritise these loads - they will get in the way
++                return;
++            }
++
++            // at this point we want a chunk that has a status higher than the one we have already completed
++
++            // does the next status need neighbours at all?
++            final int requiredNeighbours = nextStatus.getNeighborRadius();
++            if (requiredNeighbours <= 0) {
++                // no it doesn't, we're done here. we've already prioritised this chunk, no neighbours need prioritising
++                return;
++            }
++
++            // even though we might want a higher status than targetFinalStatus, we cannot queue neighbours for it - we
++            // instead use the current chunk status in progress (nextCompletedStatus) to ensure we aren't waiting on
++            // unprioritised logic for the next status to complete
++
++            for (int cx = -requiredNeighbours; cx <= requiredNeighbours; ++cx) {
++                for (int cz = -requiredNeighbours; cz <= requiredNeighbours; ++cz) {
++                    if (cx == 0 && cz == 0) {
++                        continue;
++                    }
++                    PlayerChunk neighbor = this.chunkMap.getUpdatingChunk(ChunkCoordIntPair.asLong(x + cz, z + cx));
++                    if (neighbor == null) {
++                        continue;
++                    }
++
++                    IChunkAccess neighborChunk = neighbor.getAvailableChunkNow();
++                    ChunkStatus neededStatus = this.chunkMap.getNeededStatusByRadius(nextStatus, Math.max(Math.abs(cx), Math.abs(cz)));
++                    ChunkStatus neighborCurrentStatus = neighborChunk != null ? neighborChunk.getChunkStatus() : ChunkStatus.EMPTY;
++                    if (nextStatus == ChunkStatus.LIGHT || !neighborCurrentStatus.isAtLeastStatus(neededStatus)) {
++                        // we don't need to gen neighbours if our current chunk's status has already gone through the gen
++                        // light is always an exception, no matter what if we go through light we need its neighbours - the light engine requires them
++                        this.onNeighborRequest(neighbor, neededStatus);
++                    }
++                }
++            }
++        }
++    }
++    // Paper end
++
+     public PlayerChunk(ChunkCoordIntPair chunkcoordintpair, int i, LightEngine lightengine, PlayerChunk.c playerchunk_c, PlayerChunk.d playerchunk_d) {
+         this.statusFutures = new AtomicReferenceArray(PlayerChunk.CHUNK_STATUSES.size());
+         this.fullChunkFuture = PlayerChunk.UNLOADED_CHUNK_FUTURE;
+@@ -139,6 +244,12 @@ public class PlayerChunk {
+         }
+         return null;
+     }
++    public static ChunkStatus getNextStatus(ChunkStatus status) {
++        if (status == ChunkStatus.FULL) {
++            return status;
++        }
++        return CHUNK_STATUSES.get(status.getStatusIndex() + 1);
++    }
+     // Paper end
+ 
+     public CompletableFuture<Either<IChunkAccess, PlayerChunk.Failure>> getStatusFutureUnchecked(ChunkStatus chunkstatus) {
+@@ -351,7 +462,7 @@ public class PlayerChunk {
+     }
+ 
+     public int k() {
+-        return this.n;
++        return Math.max(1, this.n - this.chunkPriority - (isUrgent ? 20 : 0)); // Paper - allow modifying priority, subtracts 20 if urgent
+     }
+ 
+     private void d(int i) {
+diff --git a/src/main/java/net/minecraft/server/PlayerChunkMap.java b/src/main/java/net/minecraft/server/PlayerChunkMap.java
+index 92c9ab43d7..c38d31fafe 100644
+--- a/src/main/java/net/minecraft/server/PlayerChunkMap.java
++++ b/src/main/java/net/minecraft/server/PlayerChunkMap.java
+@@ -324,6 +324,7 @@ public class PlayerChunkMap extends IChunkLoader implements PlayerChunk.d {
+         List<CompletableFuture<Either<IChunkAccess, PlayerChunk.Failure>>> list = Lists.newArrayList();
+         int j = chunkcoordintpair.x;
+         int k = chunkcoordintpair.z;
++        PlayerChunk requestingNeighbor = this.requestingNeighbor; // Paper
+ 
+         for (int l = -i; l <= i; ++l) {
+             for (int i1 = -i; i1 <= i; ++i1) {
+@@ -341,6 +342,7 @@ public class PlayerChunkMap extends IChunkLoader implements PlayerChunk.d {
+                 }
+ 
+                 ChunkStatus chunkstatus = (ChunkStatus) intfunction.apply(j1);
++                if (requestingNeighbor != null) requestingNeighbor.onNeighborRequest(playerchunk, chunkstatus); // Paper
+                 CompletableFuture<Either<IChunkAccess, PlayerChunk.Failure>> completablefuture = playerchunk.a(chunkstatus, this);
+ 
+                 list.add(completablefuture);
+@@ -799,23 +801,28 @@ public class PlayerChunkMap extends IChunkLoader implements PlayerChunk.d {
+         };
+ 
+         CompletableFuture<NBTTagCompound> chunkSaveFuture = this.world.asyncChunkTaskManager.getChunkSaveFuture(chunkcoordintpair.x, chunkcoordintpair.z);
++        PlayerChunk playerChunk = getUpdatingChunk(chunkcoordintpair.pair());
++        boolean isBlockingMain = playerChunk != null && playerChunk.isUrgent;
++        int priority = isBlockingMain ? com.destroystokyo.paper.io.PrioritizedTaskQueue.HIGHEST_PRIORITY : com.destroystokyo.paper.io.PrioritizedTaskQueue.HIGH_PRIORITY;
+         if (chunkSaveFuture != null) {
+-            this.world.asyncChunkTaskManager.scheduleChunkLoad(chunkcoordintpair.x, chunkcoordintpair.z,
+-                com.destroystokyo.paper.io.PrioritizedTaskQueue.HIGH_PRIORITY, chunkHolderConsumer, false, chunkSaveFuture);
+-            this.world.asyncChunkTaskManager.raisePriority(chunkcoordintpair.x, chunkcoordintpair.z, com.destroystokyo.paper.io.PrioritizedTaskQueue.HIGH_PRIORITY);
++            this.world.asyncChunkTaskManager.scheduleChunkLoad(chunkcoordintpair.x, chunkcoordintpair.z, priority, chunkHolderConsumer, isBlockingMain, chunkSaveFuture);
+         } else {
+-            this.world.asyncChunkTaskManager.scheduleChunkLoad(chunkcoordintpair.x, chunkcoordintpair.z,
+-                com.destroystokyo.paper.io.PrioritizedTaskQueue.NORMAL_PRIORITY, chunkHolderConsumer, false);
++            this.world.asyncChunkTaskManager.scheduleChunkLoad(chunkcoordintpair.x, chunkcoordintpair.z, priority, chunkHolderConsumer, isBlockingMain);
+         }
++        this.world.asyncChunkTaskManager.raisePriority(chunkcoordintpair.x, chunkcoordintpair.z, priority);
+         return ret;
+         // Paper end
+     }
+ 
++    private PlayerChunk requestingNeighbor; // Paper
+     private CompletableFuture<Either<IChunkAccess, PlayerChunk.Failure>> b(PlayerChunk playerchunk, ChunkStatus chunkstatus) {
+         ChunkCoordIntPair chunkcoordintpair = playerchunk.i();
++        PlayerChunk prevNeighbor = requestingNeighbor; // Paper
++        this.requestingNeighbor = playerchunk; // Paper
+         CompletableFuture<Either<List<IChunkAccess>, PlayerChunk.Failure>> completablefuture = this.a(chunkcoordintpair, chunkstatus.f(), (i) -> {
+             return this.a(chunkstatus, i);
+         });
++        this.requestingNeighbor = prevNeighbor; // Paper
+ 
+         this.world.getMethodProfiler().c(() -> {
+             return "chunkGenerate " + chunkstatus.d();
+@@ -843,6 +850,7 @@ public class PlayerChunkMap extends IChunkLoader implements PlayerChunk.d {
+                 return CompletableFuture.completedFuture(Either.right(playerchunk_failure));
+             });
+         }, (runnable) -> {
++            playerchunk.onNeighborsDone(); // Paper
+             this.mailboxWorldGen.a(ChunkTaskQueueSorter.a(playerchunk, runnable)); // CraftBukkit - decompile error
+         });
+     }
+@@ -855,6 +863,7 @@ public class PlayerChunkMap extends IChunkLoader implements PlayerChunk.d {
+         }));
+     }
+ 
++    public ChunkStatus getNeededStatusByRadius(ChunkStatus chunkstatus, int i) { return a(chunkstatus, i); } // Paper - OBFHELPER
+     private ChunkStatus a(ChunkStatus chunkstatus, int i) {
+         ChunkStatus chunkstatus1;
+ 
+@@ -979,9 +988,12 @@ public class PlayerChunkMap extends IChunkLoader implements PlayerChunk.d {
+ 
+     public CompletableFuture<Either<Chunk, PlayerChunk.Failure>> a(PlayerChunk playerchunk) {
+         ChunkCoordIntPair chunkcoordintpair = playerchunk.i();
++        PlayerChunk prevNeighbor = this.requestingNeighbor; // Paper
++        this.requestingNeighbor = playerchunk; // Paper
+         CompletableFuture<Either<List<IChunkAccess>, PlayerChunk.Failure>> completablefuture = this.a(chunkcoordintpair, 1, (i) -> {
+             return ChunkStatus.FULL;
+         });
++        this.requestingNeighbor = prevNeighbor; // Paper
+         CompletableFuture<Either<Chunk, PlayerChunk.Failure>> completablefuture1 = completablefuture.thenApplyAsync((either) -> {
+             return either.flatMap((list) -> {
+                 Chunk chunk = (Chunk) list.get(list.size() / 2);
+-- 
+2.25.1
+