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My kingdom for a good timer!

Reach submillisecond timing precision in Java

By Vladimir Roubtsov, JavaWorld.com, 01/10/03

January 10, 2003

QIs the resolution of System.currentTimeMillis() enough for profiling Java code?

AEvaluating the actual resolution is easy enough. The following code snippet, 

    public static void main (String [] args)
    {
        // JIT/hotspot warmup:
        for (int r = 0; r < 3000; ++ r) System.currentTimeMillis ();
        long time = System.currentTimeMillis (), time_prev = time;
        for (int i = 0; i < 5; ++ i)
        {
            // Busy wait until system time changes: 
            while (time == time_prev)
                time = System.currentTimeMillis ();
            System.out.println ("delta = " + (time - time_prev) + " ms");
            time_prev = time;
        }
    }


when run in JDK 1.4.1 on a Windows 2000 system, produces the following:

delta = 10 ms
delta = 10 ms
delta = 10 ms
delta = 10 ms
delta = 10 ms


You will discover that results vary based on your platform. Java developers on Linux enjoy 1-millisecond (ms) resolution, while Windows 98 users suffer with 50-ms resolution. In most cases, the actual resolution has nothing to do with the fact that System.currenTimeMillis()'s return value is current time in milliseconds.

Java developer forums are full of arguments about whether a particular language trick offers performance advantages, with developers presenting "proofs" based on System.currentTimeMilllis() returning "0 ms versus 10 ms" for some new way of implementing a particular code paradigm. You should see now that this is naive at best. On the system used above, System.currentTimeMilllis() is suitable only for profiling relatively long-lasting (100 ms and longer) operations. If you cannot loop over code being profiled a sufficient number of times to get the cumulative execution time in this range, you need another approach.

Occasionally, you can find alternative suggestions based on Object.wait(long) or Thread.sleep(long) methods. Unfortunately, these are not good alternatives either, as the following code demonstrates: 

    public static void main (String [] args) throws Exception
    {
        final DecimalFormat format = new DecimalFormat ();
        format.setMinimumFractionDigits (3);
        format.setMaximumFractionDigits (3);
        // Create an ITimer using the Factory class:
        final ITimer timer = TimerFactory.newTimer ();
        
        // JIT/hotspot warmup:
        for (int i = 0; i < 3000; ++ i)
        {
            timer.start ();
            timer.stop ();
            timer.getDuration ();
            timer.reset ();
        }
        final Object lock = new Object (); // used by monitor.wait() below
        
        for (int i = 0; i < 5; ++ i)
        {
            timer.start ();
           
            // Uncomment various lines below to see the resolution
            // offered by other Java time-related methods:
            
            synchronized (lock) { lock.wait (1); }
            //Thread.currentThread ().sleep (1);
            //Thread.currentThread ().sleep (0, 500);
            //Thread.currentThread ().join (1);
            timer.stop ();
            System.out.println ("duration = "
                + format.format (timer.getDuration ()) + " ms");            
            timer.reset ();
        }
    }


On the same system used for the first experiment, this results in:

duration = 5.678 ms
duration = 7.313 ms
duration = 14.805 ms
duration = 15.078 ms
duration = 15.105 ms


(You might be wondering about the mysterious ITimer timer object above. I will explain this shortly. For now, realize that I, of course, could not use System.currenTimeMillis() to measure resolution of anything that might be better than that method.)

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