Do not overload I/O threads with chunk data while flush saving

If the chunk count is high, then the memory used by the
chunks adds up and could cause problems. By flushing
every so many chunks, the server will not become
stressed for memory. It will also not increase the total
time to save, as flush saving performs a full flush at
the end anyways.

patches/server/0827-Do-not-overload-I-O-threads-with-chunk-data-while-fl.patch

@@ -0,0 +1,42 @@
+From 0000000000000000000000000000000000000000 Mon Sep 17 00:00:00 2001
+From: Spottedleaf <Spottedleaf@users.noreply.github.com>
+Date: Sat, 16 Oct 2021 01:36:00 -0700
+Subject: [PATCH] Do not overload I/O threads with chunk data while flush
+ saving
+
+If the chunk count is high, then the memory used by the
+chunks adds up and could cause problems. By flushing
+every so many chunks, the server will not become
+stressed for memory. It will also not increase the total
+time to save, as flush saving performs a full flush at
+the end anyways.
+
+diff --git a/src/main/java/net/minecraft/server/level/ChunkMap.java b/src/main/java/net/minecraft/server/level/ChunkMap.java
+index 4b1797659f6bb5913b105641ecaac38954ce8bc0..65090afbb510231c05db3132ba416502d188f1c0 100644
+--- a/src/main/java/net/minecraft/server/level/ChunkMap.java
++++ b/src/main/java/net/minecraft/server/level/ChunkMap.java
+@@ -950,6 +950,16 @@ public class ChunkMap extends ChunkStorage implements ChunkHolder.PlayerProvider
+     // Paper end
+ 
+     protected void saveAllChunks(boolean flush) {
++        // Paper start - do not overload I/O threads with too much work when saving
++        int[] saved = new int[1];
++        int maxAsyncSaves = 50;
++        Runnable onChunkSave = () -> {
++            if (++saved[0] >= maxAsyncSaves) {
++                saved[0] = 0;
++                com.destroystokyo.paper.io.PaperFileIOThread.Holder.INSTANCE.flush();
++            }
++        };
++        // Paper end - do not overload I/O threads with too much work when saving
+         if (flush) {
+             List<ChunkHolder> list = (List) this.updatingChunks.getVisibleValuesCopy().stream().filter(ChunkHolder::wasAccessibleSinceLastSave).peek(ChunkHolder::refreshAccessibility).collect(Collectors.toList()); // Paper
+             MutableBoolean mutableboolean = new MutableBoolean();
+@@ -972,6 +982,7 @@ public class ChunkMap extends ChunkStorage implements ChunkHolder.PlayerProvider
+                 }).filter((ichunkaccess) -> {
+                     return ichunkaccess instanceof ImposterProtoChunk || ichunkaccess instanceof LevelChunk;
+                 }).filter(this::save).forEach((ichunkaccess) -> {
++                    onChunkSave.run(); // Paper - do not overload I/O threads with too much work when saving
+                     mutableboolean.setTrue();
+                 });
+             } while (mutableboolean.isTrue());