[GitHub] [flink] StefanRRichter commented on a change in pull request #8431: [FLINK-12480] Introduce mailbox to StreamTask main-loop

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StefanRRichter commented on a change in pull request #8431: [FLINK-12480] Introduce mailbox to StreamTask main-loop
URL: https://github.com/apache/flink/pull/8431#discussion_r283683226
 
 

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 File path: flink-streaming-java/src/main/java/org/apache/flink/streaming/runtime/tasks/mailbox/MailboxImpl.java
 ##########
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+/*
+ * Licensed to the Apache Software Foundation (ASF) under one
+ * or more contributor license agreements.  See the NOTICE file
+ * distributed with this work for additional information
+ * regarding copyright ownership.  The ASF 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 org.apache.flink.streaming.runtime.tasks.mailbox;
+
+import org.apache.flink.util.Preconditions;
+
+import javax.annotation.Nonnull;
+import javax.annotation.concurrent.GuardedBy;
+
+import java.util.concurrent.locks.Condition;
+import java.util.concurrent.locks.ReentrantLock;
+
+/**
+ * Implementation of {@link Mailbox} inspired by {@link java.util.concurrent.ArrayBlockingQueue} and tailored towards
+ * our use case with multiple writers, single reader and volatile reads instead of lock & read on {@link #count}.
+ */
+public class MailboxImpl implements Mailbox {
+
+	/**
+	 * The enqueued letters.
+	 */
+	@GuardedBy("lock")
+	private final Runnable[] ringBuffer;
+
+	/**
+	 * Lock for all concurrent ops.
+	 */
+	private final ReentrantLock lock;
+
+	/**
+	 * Condition that is triggered when the buffer is no longer empty.
+	 */
+	@GuardedBy("lock")
+	private final Condition notEmpty;
+
+	/**
+	 * Condition that is triggered when the buffer is no longer full.
+	 */
+	@GuardedBy("lock")
+	private final Condition notFull;
+
+	/**
+	 * Index of the ring buffer head.
+	 */
+	@GuardedBy("lock")
+	private int headIndex;
+
+	/**
+	 * Index of the ring buffer tail.
+	 */
+	@GuardedBy("lock")
+	private int tailIndex;
+
+	/**
+	 * Number of letters in the mailbox.
+	 */
+	@GuardedBy("lock")
+	private volatile int count;
+
+	/**
+	 * A mask to wrap around the indexes of the ring buffer. We use this to avoid ifs or modulo ops.
+	 */
+	private final int moduloMask;
+
+	public MailboxImpl() {
+		this(6); // 2^6 = 64
+	}
+
+	public MailboxImpl(int capacityPow2) {
+		final int capacity = 1 << capacityPow2;
+		Preconditions.checkState(capacity > 0);
+		this.moduloMask = capacity - 1;
+		this.ringBuffer = new Runnable[capacity];
+		this.lock = new ReentrantLock();
+		this.notEmpty = lock.newCondition();
+		this.notFull = lock.newCondition();
+	}
+
+	@Override
+	public boolean hasMail() {
+		return count > 0;
+	}
+
+	@Override
+	public Runnable tryTakeMail() {
+		final ReentrantLock lock = this.lock;
+		lock.lock();
+		try {
+			return isEmpty() ? null : takeInternal();
+		} finally {
+			lock.unlock();
+		}
+	}
+
+	@Nonnull
+	@Override
+	public Runnable takeMail() throws InterruptedException {
+		final ReentrantLock lock = this.lock;
+		lock.lockInterruptibly();
+		try {
+			while (isEmpty()) {
+				notEmpty.await();
+			}
+			return takeInternal();
+		} finally {
+			lock.unlock();
+		}
+	}
+
+	@Override
+	public void waitUntilHasMail() throws InterruptedException {
+		final ReentrantLock lock = this.lock;
+		lock.lockInterruptibly();
+		try {
+			while (isEmpty()) {
+				notEmpty.await();
+			}
+		} finally {
+			lock.unlock();
+		}
+	}
+
+	//------------------------------------------------------------------------------------------------------------------
+
+	@Override
+	public boolean tryPutMail(@Nonnull Runnable letter) {
+		final ReentrantLock lock = this.lock;
+		lock.lock();
+		try {
+			if (isFull()) {
+				return false;
+			} else {
+				putInternal(letter);
+				return true;
+			}
+		} finally {
+			lock.unlock();
+		}
+	}
+
+	@Override
+	public void putMail(@Nonnull Runnable letter) throws InterruptedException {
+		final ReentrantLock lock = this.lock;
+		lock.lockInterruptibly();
+		try {
+			while (isFull()) {
+				notFull.await();
+			}
+			putInternal(letter);
+		} finally {
+			lock.unlock();
+		}
+	}
+
+	@Override
+	public void waitUntilHasCapacity() throws InterruptedException {
+		final ReentrantLock lock = this.lock;
+		lock.lockInterruptibly();
+		try {
+			while (isFull()) {
+				notFull.await();
+			}
+		} finally {
+			lock.unlock();
+		}
+	}
+
+	//------------------------------------------------------------------------------------------------------------------
+
+	private void putInternal(Runnable letter) {
+		assert lock.isHeldByCurrentThread();
+		this.ringBuffer[tailIndex] = letter;
+		tailIndex = increaseIndexWithWrapAround(tailIndex);
+		++count;
+		notEmpty.signal();
+	}
+
+	private Runnable takeInternal() {
+		assert lock.isHeldByCurrentThread();
+		final Runnable[] buffer = this.ringBuffer;
+		Runnable letter = buffer[headIndex];
+		buffer[headIndex] = null;
+		headIndex = increaseIndexWithWrapAround(headIndex);
+		--count;
+		notFull.signal();
+		return letter;
+	}
+
+	private int increaseIndexWithWrapAround(int old) {
+		return (old + 1) & moduloMask;
+	}
+
+	private boolean isFull() {
+		return count >= ringBuffer.length;
 
 Review comment:
   No, it can't. Just a habbit to include this case still if there is no extra cost to it.

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