Avoid use of global AtomicLong for ScheduledFutureTask ids (#9599)

Motivation

Currently a static AtomicLong is used to allocate a unique id whenever a
task is scheduled to any event loop. This could be a source of
contention if delayed tasks are scheduled at a high frequency and can be
easily avoided by having a non-volatile id counter per queue.

Modifications

- Replace static AtomicLong ScheduledFutureTask#nextTaskId with a long
field in AbstractScheduledExecutorService
- Set ScheduledFutureTask#id based on this when adding the task to the
queue (in event loop) instead of at construction time
- Add simple benchmark

Result

Less contention / cache-miss possibility when scheduling future tasks

Before:

Benchmark      (num)   Mode  Cnt    Score    Error  Units
scheduleLots  100000  thrpt   20  346.008 ± 21.931  ops/s

Benchmark      (num)   Mode  Cnt    Score    Error  Units
scheduleLots  100000  thrpt   20  654.824 ± 22.064  ops/s

common/src/main/java/io/netty/util/concurrent/AbstractScheduledEventExecutor.java

@@ -39,6 +39,8 @@ public abstract class AbstractScheduledEventExecutor extends AbstractEventExecut
 
     PriorityQueue<ScheduledFutureTask<?>> scheduledTaskQueue;
 
+    long nextTaskId;
+
     protected AbstractScheduledEventExecutor() {
     }
 
@@ -241,12 +243,12 @@ public abstract class AbstractScheduledEventExecutor extends AbstractEventExecut
 
     private <V> ScheduledFuture<V> schedule(final ScheduledFutureTask<V> task) {
         if (inEventLoop()) {
-            scheduledTaskQueue().add(task);
+            scheduledTaskQueue().add(task.setId(nextTaskId++));
         } else {
             executeScheduledRunnable(new Runnable() {
                 @Override
                 public void run() {
-                    scheduledTaskQueue().add(task);
+                    scheduledTaskQueue().add(task.setId(nextTaskId++));
                 }
             }, true, task.deadlineNanos());
         }

common/src/main/java/io/netty/util/concurrent/ScheduledFutureTask.java

@@ -23,11 +23,9 @@ import java.util.Queue;
 import java.util.concurrent.Callable;
 import java.util.concurrent.Delayed;
 import java.util.concurrent.TimeUnit;
-import java.util.concurrent.atomic.AtomicLong;
 
 @SuppressWarnings("ComparableImplementedButEqualsNotOverridden")
 final class ScheduledFutureTask<V> extends PromiseTask<V> implements ScheduledFuture<V>, PriorityQueueNode {
-    private static final AtomicLong nextTaskId = new AtomicLong();
     private static final long START_TIME = System.nanoTime();
 
     static long nanoTime() {
@@ -44,7 +42,9 @@ final class ScheduledFutureTask<V> extends PromiseTask<V> implements ScheduledFu
         return START_TIME;
     }
 
-    private final long id = nextTaskId.getAndIncrement();
+    // set once when added to priority queue
+    private long id;
+
     private long deadlineNanos;
     /* 0 - no repeat, >0 - repeat at fixed rate, <0 - repeat with fixed delay */
     private final long periodNanos;
@@ -79,6 +79,11 @@ final class ScheduledFutureTask<V> extends PromiseTask<V> implements ScheduledFu
         periodNanos = 0;
     }
 
+    ScheduledFutureTask<V> setId(long id) {
+        this.id = id;
+        return this;
+    }
+
     @Override
     protected EventExecutor executor() {
         return super.executor();
@@ -182,9 +187,7 @@ final class ScheduledFutureTask<V> extends PromiseTask<V> implements ScheduledFu
         StringBuilder buf = super.toStringBuilder();
         buf.setCharAt(buf.length() - 1, ',');
 
-        return buf.append(" id: ")
-                  .append(id)
-                  .append(", deadline: ")
+        return buf.append(" deadline: ")
                   .append(deadlineNanos)
                   .append(", period: ")
                   .append(periodNanos)

microbench/src/main/java/io/netty/util/concurrent/ScheduleFutureTaskBenchmark.java

@@ -0,0 +1,88 @@
+/*
+ * Copyright 2019 The Netty Project
+ *
+ * The Netty Project licenses this file to you under the Apache License,
+ * version 2.0 (the "License"); you may not use this file except in compliance
+ * with the License. You may obtain a copy of the License at:
+ *
+ *   http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
+ * WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
+ * License for the specific language governing permissions and limitations
+ * under the License.
+ */
+package io.netty.util.concurrent;
+
+import java.util.concurrent.Callable;
+import java.util.concurrent.TimeUnit;
+
+import org.openjdk.jmh.annotations.Benchmark;
+import org.openjdk.jmh.annotations.Level;
+import org.openjdk.jmh.annotations.Measurement;
+import org.openjdk.jmh.annotations.Param;
+import org.openjdk.jmh.annotations.Scope;
+import org.openjdk.jmh.annotations.Setup;
+import org.openjdk.jmh.annotations.State;
+import org.openjdk.jmh.annotations.TearDown;
+import org.openjdk.jmh.annotations.Threads;
+import org.openjdk.jmh.annotations.Warmup;
+
+import io.netty.channel.DefaultEventLoop;
+import io.netty.microbench.util.AbstractMicrobenchmark;
+
+@Warmup(iterations = 5, time = 1, timeUnit = TimeUnit.SECONDS)
+@Measurement(iterations = 10, time = 1, timeUnit = TimeUnit.SECONDS)
+@State(Scope.Benchmark)
+public class ScheduleFutureTaskBenchmark extends AbstractMicrobenchmark {
+
+    static final Callable<Void> NO_OP = new Callable<Void>() {
+        @Override
+        public Void call() throws Exception {
+            return null;
+        }
+    };
+
+    @State(Scope.Thread)
+    public static class ThreadState {
+
+        @Param({ "100000" })
+        int num;
+
+        AbstractScheduledEventExecutor eventLoop;
+
+        @Setup(Level.Trial)
+        public void reset() {
+            eventLoop = new DefaultEventLoop();
+        }
+
+        @Setup(Level.Invocation)
+        public void clear() {
+            eventLoop.submit(new Runnable() {
+                @Override
+                public void run() {
+                    eventLoop.cancelScheduledTasks();
+                }
+            }).awaitUninterruptibly();
+        }
+
+        @TearDown(Level.Trial)
+        public void shutdown() {
+            eventLoop.shutdownGracefully().awaitUninterruptibly();
+        }
+    }
+
+    @Benchmark
+    @Threads(3)
+    public Future<?> scheduleLots(final ThreadState threadState) {
+        return threadState.eventLoop.submit(new Runnable() {
+            @Override
+            public void run() {
+                for (int i = 1; i <= threadState.num; i++) {
+                    threadState.eventLoop.schedule(NO_OP, i, TimeUnit.HOURS);
+                }
+            }
+        }).syncUninterruptibly();
+    }
+}

microbench/src/main/java/io/netty/util/concurrent/package-info.java

@@ -0,0 +1,19 @@
+/*
+ * Copyright 2019 The Netty Project
+ *
+ * The Netty Project licenses this file to you under the Apache License,
+ * version 2.0 (the "License"); you may not use this file except in compliance
+ * with the License. You may obtain a copy of the License at:
+ *
+ *   http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
+ * WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
+ * License for the specific language governing permissions and limitations
+ * under the License.
+ */
+/**
+ * Benchmarks for {@link io.netty.util.concurrent}.
+ */
+package io.netty.util.concurrent;