Scheduler optimization

paper-server/src/main/java/io/papermc/paper/util/concurrent/ScheduledTask.java

@@ -0,0 +1,5 @@
+package io.papermc.paper.util.concurrent;
+
+public interface ScheduledTask {
+    long getNextRun();
+}

paper-server/src/main/java/io/papermc/paper/util/concurrent/TimingWheel.java

@@ -0,0 +1,147 @@
+package io.papermc.paper.util.concurrent;
+
+import org.jetbrains.annotations.NotNull;
+import java.util.ArrayDeque;
+import java.util.ArrayList;
+import java.util.Collection;
+import java.util.Collections;
+import java.util.Iterator;
+import java.util.List;
+import java.util.NoSuchElementException;
+import java.util.concurrent.ConcurrentLinkedQueue;
+import java.util.function.Predicate;
+
+/**
+ * This class schedules tasks in ticks and executes them efficiently using a circular array (the wheel).
+ * Each slot in the wheel represents a specific tick modulo the wheel size.
+ * Tasks are placed into slots based on their target execution tick.
+ * On each tick, the wheel checks the current slot and runs any tasks whose execute tick has been reached.
+ *
+ * O(1) task scheduling and retrieval within a single wheel rotation.
+ * We are using power of 2 for faster operations than modulo.
+ *
+ */
+public class TimingWheel<T extends ScheduledTask> implements Iterable<T> {
+    private final int wheelSize;
+    private final long mask;
+    private final ArrayDeque<T>[] wheel;
+
+    @SuppressWarnings("unchecked")
+    public TimingWheel(int exponent) {
+        this.wheelSize = 1 << exponent;
+        this.mask = wheelSize - 1L;
+
+        this.wheel = (ArrayDeque<T>[]) new ArrayDeque[wheelSize];
+        for (int i = 0; i < wheelSize; i++) {
+            wheel[i] = new ArrayDeque<>();
+        }
+    }
+
+    public void add(T task) {
+        int slot = (int) (task.getNextRun() & mask);
+        wheel[slot].add(task);
+    }
+
+    public void addAll(Collection<? extends T> tasks) {
+        for (T task : tasks) {
+            this.add(task);
+        }
+    }
+
+    public @NotNull List<T> popValid(long currentTick) {
+        int slot = (int) (currentTick & mask);
+        ArrayDeque<T> bucket = wheel[slot];
+        if (bucket.isEmpty()) return Collections.emptyList();
+
+        Iterator<T> iter = bucket.iterator();
+        List<T> list = new ArrayList<>();
+        while (iter.hasNext()) {
+            T task = iter.next();
+
+            if (task.getNextRun() <= currentTick) {
+                iter.remove();
+                list.add(task);
+            }
+        }
+
+        return list;
+    }
+
+    public boolean isReady(long currentTick) {
+        int slot = (int) (currentTick & mask);
+        ArrayDeque<T> bucket = wheel[slot];
+        if (bucket.isEmpty()) return false;
+
+        for (final T task : bucket) {
+            if (task.getNextRun() <= currentTick) {
+                return true;
+            }
+        }
+
+        return false;
+    }
+
+    public void removeIf(Predicate<T> apply) {
+        Iterator<T> itr = iterator();
+        while (itr.hasNext()) {
+            T next = itr.next();
+            if (apply.test(next)) {
+                itr.remove();
+            }
+        }
+    }
+
+    @SuppressWarnings("unchecked")
+    private class Itr implements Iterator<T> {
+        private int index = 0;
+        private Iterator<T> current = Collections.emptyIterator();
+        private Iterator<T> lastIterator = null;
+
+        @Override
+        public boolean hasNext() {
+            if (current.hasNext()) {
+                return true;
+            }
+
+            for (int i = index; i < wheelSize; i++) {
+                if (!wheel[i].isEmpty()) {
+                    return true;
+                }
+            }
+
+            return false;
+        }
+
+        @Override
+        public T next() {
+            while (true) {
+                if (current.hasNext()) {
+                    lastIterator = current;
+                    return current.next();
+                }
+
+                if (index >= wheelSize) {
+                    throw new NoSuchElementException();
+                }
+
+                current = wheel[index++].iterator();
+            }
+        }
+
+        @Override
+        public void remove() {
+            if (lastIterator == null) {
+                throw new NoSuchElementException();
+            }
+
+            lastIterator.remove();
+            lastIterator = null;
+        }
+    }
+
+
+    @Override
+    public @NotNull Iterator<T> iterator() {
+        return new Itr();
+    }
+}

paper-server/src/main/java/org/bukkit/craftbukkit/scheduler/CraftAsyncScheduler.java

@@ -75,17 +75,20 @@ public void mainThreadHeartbeat() {
 
     private synchronized void runTasks(int currentTick) {
         parsePending();
-        while (!this.pending.isEmpty() && this.pending.peek().getNextRun() <= currentTick) {
-            CraftTask task = this.pending.remove();
+        // Paper start - Timing Wheel
+        List<CraftTask> tasks = this.pending.popValid(currentTick);
+        for (CraftTask task : tasks) {
+            // Paper end - Timing Wheel
             if (executeTask(task)) {
                 final long period = task.getPeriod();
                 if (period > 0) {
                     task.setNextRun(currentTick + period);
                     temp.add(task);
                 }
             }
-            parsePending();
         }
+        parsePending();
+
         this.pending.addAll(temp);
         temp.clear();
     }

paper-server/src/main/java/org/bukkit/craftbukkit/scheduler/CraftScheduler.java

@@ -75,16 +75,7 @@ public class CraftScheduler implements BukkitScheduler {
     /**
      * Main thread logic only
      */
-    final PriorityQueue<CraftTask> pending = new PriorityQueue<CraftTask>(10, // Paper
-            new Comparator<CraftTask>() {
-                @Override
-                public int compare(final CraftTask o1, final CraftTask o2) {
-                    int value = Long.compare(o1.getNextRun(), o2.getNextRun());
-
-                    // If the tasks should run on the same tick they should be run FIFO
-                    return value != 0 ? value : Long.compare(o1.getCreatedAt(), o2.getCreatedAt());
-                }
-            });
+    final io.papermc.paper.util.concurrent.TimingWheel<CraftTask> pending = new io.papermc.paper.util.concurrent.TimingWheel<>(12); // Paper - Timing wheel
     /**
      * Main thread logic only
      */
@@ -459,8 +450,10 @@ public void mainThreadHeartbeat() {
         // Paper end
         final List<CraftTask> temp = this.temp;
         this.parsePending();
-        while (this.isReady(this.currentTick)) {
-            final CraftTask task = this.pending.remove();
+        // Paper start - Timing Wheel
+        final List<CraftTask> tasks = this.pending.popValid(this.currentTick);
+        for (CraftTask task : tasks) {
+            // Paper end - Timing Wheel
             if (task.getPeriod() < CraftTask.NO_REPEATING) {
                 if (task.isSync()) {
                     this.runners.remove(task.getTaskId(), task);
@@ -564,7 +557,8 @@ void parsePending() { // Paper
     }
 
     private boolean isReady(final int currentTick) {
-        return !this.pending.isEmpty() && this.pending.peek().getNextRun() <= currentTick;
+        // return !this.pending.isEmpty() && this.pending.peek().getNextRun() <= currentTick;
+        return this.pending.isReady(currentTick); // Paper - Timing wheel
     }
 
     @Override

paper-server/src/main/java/org/bukkit/craftbukkit/scheduler/CraftTask.java

@@ -6,7 +6,8 @@
 import org.bukkit.plugin.Plugin;
 import org.bukkit.scheduler.BukkitTask;
 
-public class CraftTask implements BukkitTask, Runnable { // Spigot
+// Paper - Timing Wheel
+public class CraftTask implements BukkitTask, Runnable, io.papermc.paper.util.concurrent.ScheduledTask { // Spigot
 
     private volatile CraftTask next = null;
     public static final int ERROR = 0;
@@ -93,7 +94,7 @@ void setPeriod(long period) {
         this.period = period;
     }
 
-    long getNextRun() {
+    public long getNextRun() { // Paper - Timing Wheel
         return this.nextRun;
     }