ScheduledExecutorService consumes 100% CPU when corePoolSize = 0

Question

I've ran into an interesting issue in production.

I've had the following ScheduledThreadPool allocation code:

ScheduledExecutorService executorService =
            Executors.newScheduledThreadPool(Runtime.getRuntime().availableProcessors() - 1);

The thread pool was processing some tasks from the queue periodically. And everything was working fine till the moment when the service was deployed on a single core environment. Apparently, the line above converted to:

ScheduledExecutorService executorService = Executors.newScheduledThreadPool(0);

Since then, the JVM process CPU utilization was constantly around 100%. The moment I've changed Runtime.getRuntime().availableProcessors() - 1 to constant 1 the issue have gone.

It took sometime to find out the root cause, but still I don't know the reason behind it. ScheduledExecutorService JavaDoc states:

/**
 * Creates a thread pool that can schedule commands to run after a
 * given delay, or to execute periodically.
 * @param corePoolSize the number of threads to keep in the pool,
 * even if they are idle
 * @return a newly created scheduled thread pool
 * @throws IllegalArgumentException if {@code corePoolSize < 0}
 */
public static ScheduledExecutorService newScheduledThreadPool(int corePoolSize) {
    return new ScheduledThreadPoolExecutor(corePoolSize);
}

Basically, 0 (zero) is a valid argument for the thread pool instantiation, but it works super weird with this value.

Can please someone explain why so?

Simple and verifiable test case

import java.util.Queue;
import java.util.concurrent.*;

public class Test {

    public static void main(String[] args) throws InterruptedException {
        MessageTaskExecutor asyncEmailGatewayTaskExecutor = new MessageTaskExecutor();

        // Infinitely add new tasks to the queue every second
        for (int i = 1; ; i++) {
            System.out.println(String.format("Adding message #%s to the queue", i));

            asyncEmailGatewayTaskExecutor.putMessageIntoQueue(i);

            Thread.sleep(1_000);
        }
    }

    static class MessageTaskExecutor {

        static final int INITIAL_DELAY_SECONDS = 1;
        static final int PROCESSING_RATE_MILLISECONDS = 5_000;

        final Queue<Runnable> messageQueue = new ArrayBlockingQueue<>(1_000_000);
        final ScheduledExecutorService executorService = Executors.newScheduledThreadPool(0);

        MessageTaskExecutor() {
            // Scavenging Message Tasks Queue every 'PROCESSING_RATE_MILLISECONDS'. Initial delay is fixed for 'INITIAL_DELAY_SECONDS'
            executorService.schedule(this::processEmailTasks, INITIAL_DELAY_SECONDS, TimeUnit.SECONDS);
        }

        void putMessageIntoQueue(int messageId) {
            Runnable messageTask = () -> System.out.println(String.format("Message #%s is getting processed!", messageId));

            messageQueue.offer(messageTask);
        }

        void processEmailTasks() {
            System.out.println(String.format("There are %s messages in the queue. Processing the messages...", messageQueue.size()));

            // Processing messages queue
            while (!messageQueue.isEmpty()) {
                executorService.submit(messageQueue.poll()); // Submitting task to executor service
            }

            // Re-scheduling processing job
            executorService.schedule(this::processEmailTasks, PROCESSING_RATE_MILLISECONDS, TimeUnit.MILLISECONDS);
        }
    }
}

This code allocates ~30 MB and JVM process consumes ~100% CPU on a single-core virtual machine (tested on Win 7/CentOS 7). JDK 1.8.0.181.

By changing field:

final ScheduledExecutorService executorService = Executors.newScheduledThreadPool(0);

to:

final ScheduledExecutorService executorService = Executors.newScheduledThreadPool(1);

CPU consumption goes down to normal 3-5%.

Answer 1

This is a known bug: JDK-8129861. It has been fixed in JDK 9.

The workaround is to set core pool size at least 1:

int corePoolSize = Math.max(Runtime.getRuntime().availableProcessors() - 1, 1);