RFR: 8354076: LinkedBlockingDeque offer() creates nodes even if capacity has been reached

[kabutz]

On Tue, 8 Apr 2025 18:39:30 GMT, kabutz <duke at openjdk.org> wrote:

> In the JavaDoc of LinkedBlockingDeque, it states: "Linked nodes are dynamically created upon each insertion unless this would bring the deque above capacity." However, in the current implementation, nodes are always created, even if the deque is full. This is because count is non-volatile, and we only check inside the linkFirst/Last() methods whether the queue is full. At this point we have already locked and have created the Node. Instead, the count could be volatile, and we could check before locking.
> 
> In the current version, calling offer() on a full LinkedBlockingDeque creates unnecessary objects and contention. Similarly for poll() and peek(), we could exit prior to locking by checking the count field.
> 
> Our suggestion is to make count volatile, and then exiting early from poll() and offer()

My code is similar to how LinkedBlockingQueue works.

>From LinkedBlockingQueue:


    public boolean offer(E e) {
        if (e == null) throw new NullPointerException();
        final AtomicInteger count = this.count;
        if (count.get() == capacity)
            return false;
        final int c;
        final Node<E> node = new Node<E>(e);
        final ReentrantLock putLock = this.putLock;
        putLock.lock();
        try {
            if (count.get() == capacity)
                return false;
            enqueue(node);
            c = count.getAndIncrement();
            if (c + 1 < capacity)
                notFull.signal();
        } finally {
            putLock.unlock();
        }
        if (c == 0)
            signalNotEmpty();
        return true;
    }


However, I'm not sure whether it is a good idea to make this change, since making count volatile in LBD might impact performance for the most common use case (unbounded deque). For example:


import java.util.*;
import java.util.concurrent.*;
import java.util.stream.*;

public class NodeCreationBeforeLockLBD {
    /*
      This tests how quickly we can offer and poll on an "unbounded" deque,
      in order to see whether it is better to create the node before the lock.
     */

    public static void main(String... args) {
        for (int i = 0; i < 3; i++) {
            test(new LinkedBlockingDeque<>());
        }
    }

    private static void test(Queue<Integer> q) {
        System.out.println(q.getClass().getSimpleName());
        long time = System.nanoTime();
        try {
            IntStream.range(0, 10_000_000)
                    .parallel()
                    .forEach(i -> {
                        for (int j = 0; j < 5; j++) q.offer(j);
                        for (int j = 0; j < 5; j++) q.poll();
                    });
            System.out.println("q = " + q);
        } finally {
            time = System.nanoTime() - time;
            System.out.printf("time = %dms%n", (time / 1_000_000));
        }
    }
}


Output with Java 25+17:


$ java -showversion NodeCreationBeforeLockLBD.java
openjdk version "25-ea" 2025-09-16
OpenJDK Runtime Environment (build 25-ea+17-1966)
OpenJDK 64-Bit Server VM (build 25-ea+17-1966, mixed mode, sharing)
LinkedBlockingDeque
q = []
time = 4790ms
LinkedBlockingDeque
q = []
time = 4411ms
LinkedBlockingDeque
q = []
time = 4588ms


Output with these changes:


$ java -showversion NodeCreationBeforeLockLBD.java
openjdk version "25-internal" 2025-09-16
OpenJDK Runtime Environment (build 25-internal-adhoc.kabutz.jdk)
OpenJDK 64-Bit Server VM (build 25-internal-adhoc.kabutz.jdk, mixed mode)
LinkedBlockingDeque
q = []
time = 6436ms
LinkedBlockingDeque
q = []
time = 5871ms
LinkedBlockingDeque
q = []
time = 5982ms

-------------

PR Comment: https://git.openjdk.org/jdk/pull/24521#issuecomment-2803938784