Re: ScheduledExecutorService very inaccurate?

Kevin McMurtrie <>
Thu, 29 Jul 2010 21:26:12 -0700
You're allocating memory, allocating threads, and probably loading
classes between your clock starting and the executor's clock starting.

Sun's Delayed interface is also badly flawed. It's a Comparable
relative to the current time so comparing X to Y produces a different
result than comparing Y to X. The time shifting scrambles the
PriorityQueue to some degree, especially on GC pauses. In your specific
case below, Sun has a workaround for the problem by casting the other
Comparable to an internal implementation class to compare absolute
times. There are other uses of ScheduledThreadPoolExecutor where it
can't do the cast and it makes a mess.

In article <op.vgl6betlw1eelo@msrvcn04.belkin>,
 "Chris Seidel" <> wrote:


i'm using a ScheduledExecutorService to schedule a task. The problem is,
that the task gets execute about 10 - 20 percent too late, e.g. when I
schdule it with 20 s delay, it gets executed after 24 s.

Is this "normal"?

Here is the testcode:


import static junit.framework.Assert.assertEquals;

import java.util.concurrent.Executors;
import java.util.concurrent.ScheduledExecutorService;
import java.util.concurrent.TimeUnit;

import org.junit.Test;

public class SchedulerTest {
    private ScheduledExecutorService s;
    private long executedAtMillis;

    public void test1() {
        s = Executors.newScheduledThreadPool(1);
        final long currentTimeMillis = System.currentTimeMillis();
        long delay = 20000L;
        s.schedule(new Runnable() {
            public void run() {
                executedAtMillis = System.currentTimeMillis();
        }, delay, TimeUnit.MILLISECONDS);
        try {
            Thread.sleep(delay * 2);
        } catch (InterruptedException e) {
        assertEquals(delay, executedAtMillis - currentTimeMillis);

When I use java.util.Timer everything is ok:

    public void test2() {
        Timer t = new Timer();
        final long currentTimeMillis = System.currentTimeMillis();
        long delay = 20000L;

        t.schedule(new TimerTask() {
            public void run() {
                executedAtMillis = System.currentTimeMillis();
        }, delay);
        assertEquals(delay, executedAtMillis - currentTimeMillis);

Thank you.

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