Speed up processing of chunk loads and generation

Credit to Spotted for the idea

A lot of the new chunk system requires constant back and forth the main thread
to handle priority scheduling and ensuring conflicting tasks do not run at the
same time.

The issue is, these queues are only checked at either:

A) Sync Chunk Loads
B) End of Tick while sleeping

This results in generating chunks sitting waiting for a full tick to
complete before it will even start the next unit of work to do.

Additionally, this also delays loading of chunks until this same timing.

We will now periodically poll the chunk task queues throughout the tick,
looking for work to do.
We do this in a fair method that considers all worlds, not just the one being
ticked, so that each world can get 1 task procesed each before the next pass.

We also cap the throughput of these task processes to 1 per world per 0.1ms or
200 max per tick, to ensure that high volume of tasks do not overload the current
tick time.

In a view distance of 15, chunk loading performance was visually faster on the client.

Flying at high speed in spectator mode was able to keep up with chunk loading (as long as they are already generated)

Spigot-Server-Patches/0462-Speed-up-processing-of-chunk-loads-and-generation.patch

@@ -0,0 +1,229 @@
+From 78cfc2701151f762cf58e4fb0390ecc680ae71df Mon Sep 17 00:00:00 2001
+From: Aikar <aikar@aikar.co>
+Date: Thu, 9 Apr 2020 00:09:26 -0400
+Subject: [PATCH] Speed up processing of chunk loads and generation
+
+Credit to Spotted for the idea
+
+A lot of the new chunk system requires constant back and forth the main thread
+to handle priority scheduling and ensuring conflicting tasks do not run at the
+same time.
+
+The issue is, these queues are only checked at either:
+
+A) Sync Chunk Loads
+B) End of Tick while sleeping
+
+This results in generating chunks sitting waiting for a full tick to
+complete before it will even start the next unit of work to do.
+
+Additionally, this also delays loading of chunks until this same timing.
+
+We will now periodically poll the chunk task queues throughout the tick,
+looking for work to do.
+We do this in a fair method that considers all worlds, not just the one being
+ticked, so that each world can get 1 task procesed each before the next pass.
+
+We also cap the throughput of these task processes to 1 per world per 0.1ms or
+200 max per tick, to ensure that high volume of tasks do not overload the current
+tick time.
+
+In a view distance of 15, chunk loading performance was visually faster on the client.
+
+Flying at high speed in spectator mode was able to keep up with chunk loading (as long as they are already generated)
+
+diff --git a/src/main/java/co/aikar/timings/MinecraftTimings.java b/src/main/java/co/aikar/timings/MinecraftTimings.java
+index 69e26a8267..434833d50e 100644
+--- a/src/main/java/co/aikar/timings/MinecraftTimings.java
++++ b/src/main/java/co/aikar/timings/MinecraftTimings.java
+@@ -13,6 +13,7 @@ import java.util.Map;
+ public final class MinecraftTimings {
+ 
+     public static final Timing serverOversleep = Timings.ofSafe("Server Oversleep");
++    public static final Timing midTickChunkTasks = Timings.ofSafe("Mid Tick Chunk Tasks");
+     public static final Timing playerListTimer = Timings.ofSafe("Player List");
+     public static final Timing commandFunctionsTimer = Timings.ofSafe("Command Functions");
+     public static final Timing connectionTimer = Timings.ofSafe("Connection Handler");
+diff --git a/src/main/java/net/minecraft/server/ChunkProviderServer.java b/src/main/java/net/minecraft/server/ChunkProviderServer.java
+index e627440c41..15450f36a4 100644
+--- a/src/main/java/net/minecraft/server/ChunkProviderServer.java
++++ b/src/main/java/net/minecraft/server/ChunkProviderServer.java
+@@ -602,6 +602,7 @@ public class ChunkProviderServer extends IChunkProvider {
+         this.world.getMethodProfiler().enter("purge");
+         this.world.timings.doChunkMap.startTiming(); // Spigot
+         this.chunkMapDistance.purgeTickets();
++        this.world.getMinecraftServer().midTickLoadChunks(); // Paper
+         this.tickDistanceManager();
+         this.world.timings.doChunkMap.stopTiming(); // Spigot
+         this.world.getMethodProfiler().exitEnter("chunks");
+@@ -611,6 +612,7 @@ public class ChunkProviderServer extends IChunkProvider {
+         this.world.timings.doChunkUnload.startTiming(); // Spigot
+         this.world.getMethodProfiler().exitEnter("unload");
+         this.playerChunkMap.unloadChunks(booleansupplier);
++        this.world.getMinecraftServer().midTickLoadChunks(); // Paper
+         this.world.timings.doChunkUnload.stopTiming(); // Spigot
+         this.world.getMethodProfiler().exit();
+         this.clearCache();
+@@ -669,7 +671,7 @@ public class ChunkProviderServer extends IChunkProvider {
+                 entityPlayer.playerNaturallySpawnedEvent.callEvent();
+             };
+             // Paper end
+-            this.playerChunkMap.forEachVisibleChunk((playerchunk) -> { // Paper - safe iterator incase chunk loads, also no wrapping
++            final int[] chunksTicked = {0}; this.playerChunkMap.forEachVisibleChunk((playerchunk) -> { // Paper - safe iterator incase chunk loads, also no wrapping
+                 Optional<Chunk> optional = ((Either) playerchunk.b().getNow(PlayerChunk.UNLOADED_CHUNK)).left();
+ 
+                 if (optional.isPresent()) {
+@@ -752,6 +754,7 @@ public class ChunkProviderServer extends IChunkProvider {
+                         this.world.timings.chunkTicks.startTiming(); // Spigot // Paper
+                         this.world.a(chunk, k);
+                         this.world.timings.chunkTicks.stopTiming(); // Spigot // Paper
++                        if (chunksTicked[0]++ % 10 == 0) this.world.getMinecraftServer().midTickLoadChunks(); // Paper
+                     }
+                 }
+             });
+@@ -893,6 +896,39 @@ public class ChunkProviderServer extends IChunkProvider {
+             super.executeTask(runnable);
+         }
+ 
++        // Paper start
++        private long lastChunkTask = 0;
++        public void midTickLoadChunks() {
++            MinecraftServer server = ChunkProviderServer.this.world.getMinecraftServer();
++            try (co.aikar.timings.Timing ignored = co.aikar.timings.MinecraftTimings.midTickChunkTasks.startTiming()) {
++                while (server.canSleepForTick()) {
++                    try {
++                        // always try to load chunks as long as we aren't falling behind, restrain generation/other updates only.
++                        boolean execChunkTask = com.destroystokyo.paper.io.chunk.ChunkTaskManager.pollChunkWaitQueue() || ChunkProviderServer.this.world.asyncChunkTaskManager.pollNextChunkTask(); // Paper
++                        ChunkProviderServer.this.tickDistanceManager();
++                        if (execChunkTask) {
++                            continue;
++                        }
++                        long now = System.nanoTime();
++                        // cap to 1 task every 0.1ms per world and max 200 tasks per tick.
++                        // Anything that doesn't make it past this can load during sleep
++                        // we do not want to use this.executeNext as that also processes chunk loads and might count against task counter.
++                        // We also have already ticked the distance manager above too.
++                        if (server.chunksTasksRan < 200 && now - lastChunkTask > 100000 && super.executeNext()) {
++                            ChunkProviderServer.this.lightEngine.queueUpdate();
++                            server.chunksTasksRan++;
++                            lastChunkTask = now;
++                        }
++                        break;
++                    } finally {
++                        // from below: process pending Chunk loadCallback() and unloadCallback() after each run task
++                        playerChunkMap.callbackExecutor.run();
++                    }
++                }
++            }
++        }
++        // Paper end
++
+         @Override
+         protected boolean executeNext() {
+         // CraftBukkit start - process pending Chunk loadCallback() and unloadCallback() after each run task
+diff --git a/src/main/java/net/minecraft/server/MinecraftServer.java b/src/main/java/net/minecraft/server/MinecraftServer.java
+index 06c395000f..936434110c 100644
+--- a/src/main/java/net/minecraft/server/MinecraftServer.java
++++ b/src/main/java/net/minecraft/server/MinecraftServer.java
+@@ -919,6 +919,7 @@ public abstract class MinecraftServer extends IAsyncTaskHandlerReentrant<TickTas
+                         // Paper end
+                         tickSection = curTime;
+                     }
++                    chunksTasksRan = 0; // Paper
+                     // Spigot end
+ 
+                     //MinecraftServer.currentTick = (int) (System.currentTimeMillis() / 50); // CraftBukkit // Paper - don't overwrite current tick time
+@@ -989,7 +990,7 @@ public abstract class MinecraftServer extends IAsyncTaskHandlerReentrant<TickTas
+ 
+     }
+ 
+-    private boolean canSleepForTick() {
++    public boolean canSleepForTick() { // Paper
+         // CraftBukkit start
+         return this.forceTicks || this.isEntered() || SystemUtils.getMonotonicMillis() < (this.ac ? this.ab : this.nextTick);
+     }
+@@ -1017,6 +1018,20 @@ public abstract class MinecraftServer extends IAsyncTaskHandlerReentrant<TickTas
+         });
+     }
+ 
++    // Paper start
++    public int chunksTasksRan = 0;
++    public void midTickLoadChunks() {
++        if (!this.canSleepForTick()) {
++            return;
++        }
++
++        Iterator<WorldServer> iterator = this.getWorlds().iterator();
++        while (this.canSleepForTick() && iterator.hasNext()) {
++            iterator.next().getChunkProvider().serverThreadQueue.midTickLoadChunks();
++        }
++    }
++    // Paper end
++
+     @Override
+     protected TickTask postToMainThread(Runnable runnable) {
+         return new TickTask(this.ticks, runnable);
+diff --git a/src/main/java/net/minecraft/server/PlayerChunkMap.java b/src/main/java/net/minecraft/server/PlayerChunkMap.java
+index ce645a69bd..c21a3ea506 100644
+--- a/src/main/java/net/minecraft/server/PlayerChunkMap.java
++++ b/src/main/java/net/minecraft/server/PlayerChunkMap.java
+@@ -785,7 +785,7 @@ public class PlayerChunkMap extends IChunkLoader implements PlayerChunk.d {
+         CompletableFuture<Either<IChunkAccess, PlayerChunk.Failure>> ret = new CompletableFuture<>();
+ 
+         Consumer<ChunkRegionLoader.InProgressChunkHolder> chunkHolderConsumer = (ChunkRegionLoader.InProgressChunkHolder holder) -> {
+-            PlayerChunkMap.this.executor.addTask(() -> {
++            com.destroystokyo.paper.io.chunk.ChunkTaskManager.queueChunkWaitTask(() -> {
+                 ret.complete(syncLoadComplete.apply(holder, null));
+             });
+         };
+diff --git a/src/main/java/net/minecraft/server/WorldServer.java b/src/main/java/net/minecraft/server/WorldServer.java
+index c7ec8cbc11..43573287f2 100644
+--- a/src/main/java/net/minecraft/server/WorldServer.java
++++ b/src/main/java/net/minecraft/server/WorldServer.java
+@@ -420,6 +420,7 @@ public class WorldServer extends World {
+         }
+         timings.scheduledBlocks.stopTiming(); // Spigot
+ 
++        this.getMinecraftServer().midTickLoadChunks(); // Paper
+         gameprofilerfiller.exitEnter("raid");
+         this.timings.raids.startTiming(); // Paper - timings
+         this.persistentRaid.a();
+@@ -432,6 +433,7 @@ public class WorldServer extends World {
+         timings.doSounds.startTiming(); // Spigot
+         this.ad();
+         timings.doSounds.stopTiming(); // Spigot
++        this.getMinecraftServer().midTickLoadChunks(); // Paper
+         this.ticking = false;
+         gameprofilerfiller.exitEnter("entities");
+         boolean flag3 = true || !this.players.isEmpty() || !this.getForceLoadedChunks().isEmpty(); // CraftBukkit - this prevents entity cleanup, other issues on servers with no players
+@@ -470,7 +472,9 @@ public class WorldServer extends World {
+             org.spigotmc.ActivationRange.activateEntities(this); // Spigot
+             timings.entityTick.startTiming(); // Spigot
+             TimingHistory.entityTicks += this.globalEntityList.size(); // Paper
++            int entitiesTicked = 0; // Paper
+             while (objectiterator.hasNext()) {
++                if (entitiesTicked++ % 100 == 0) this.getMinecraftServer().midTickLoadChunks(); // Paper
+                 Entry<Entity> entry = (Entry) objectiterator.next();
+                 Entity entity1 = (Entity) entry.getValue();
+                 Entity entity2 = entity1.getVehicle();
+@@ -517,6 +521,7 @@ public class WorldServer extends World {
+             timings.entityTick.stopTiming(); // Spigot
+ 
+             this.tickingEntities = false;
++            this.getMinecraftServer().midTickLoadChunks(); // Paper
+ 
+             try (co.aikar.timings.Timing ignored = this.timings.newEntities.startTiming()) { // Paper - timings
+             while ((entity = (Entity) this.entitiesToAdd.poll()) != null) {
+@@ -527,7 +532,9 @@ public class WorldServer extends World {
+ 
+             gameprofilerfiller.exit();
+             timings.tickEntities.stopTiming(); // Spigot
++            this.getMinecraftServer().midTickLoadChunks(); // Paper
+             this.tickBlockEntities();
++            this.getMinecraftServer().midTickLoadChunks(); // Paper
+         }
+ 
+         gameprofilerfiller.exit();
+-- 
+2.25.1
+