# Thursday, 08 June 2006
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More AtomicInteger Performance

In my previous entry on AtomicInteger I showed timings on my single CPU ThinkPad which has a 1.7 GHz Pentium M. Today I investigated the performance on my iMac which has a 1.83 GHz Core Duo and like the ThinkPad runs Windows XP SP2.

I've also modified AtomicInteger to take advantage of Interlocked.Increment, Decrement and Exchange. Instead of following the reference implementation which builds everything on top of the compareAndSet operation.

Running the same test produces the following numbers:

  Time (ms)
JDK 1.5 HotSpot Server VM


JDK 1.5 HotSpot Client VM






Note that the test is significantly slower than on the Pentium M. This is most likely because of the bus locking overhead.

It gets even more interesting when we modify the test to have two threads that concurrently increment the same field:

  Time (ms)
JDK 1.5 HotSpot Server VM


JDK 1.5 HotSpot Client VM






The first thing to note is the fact that the test now takes an order of magnitude more time. Presumably, this is caused by the communication overhead between the two cores.

The second is that here IKVM significantly outperforms HotSpot Server. The reason for this is simple: IKVM uses Interlocked.Increment which ultimately uses the XADD x86 instruction that atomically performs the load/increment/store. By contrast, the AtomicInteger reference implementation uses the following loop to increment the value:

public final int incrementAndGet() {
    for (;;) {
        int current = get();
        int next = current + 1;
        if (compareAndSet(current, next))
            return next;

When another thread updates the field between the get and compareAndSet, the compareAndSet will fail and the loop will have to run for another iteration. When multiple CPUs are continuously trying to increment the value this is likely to happen relatively frequently.

Standard disclaimer: As always these microbenchmarks are designed to magnify a particular effect. Be careful about drawing overly large conclusions from them.

Thursday, 08 June 2006 21:08:01 (W. Europe Daylight Time, UTC+02:00)  #    Comments [2]
Tuesday, 26 May 2009 21:21:04 (W. Europe Daylight Time, UTC+02:00)
Hi Jeroen --

I enjoyed your blog article. Would the Java microbenchmarks you used to test AtomicInteger.incrementAndGet performance happen to be available somewhere?

-- Tom
Wednesday, 27 May 2009 07:10:13 (W. Europe Daylight Time, UTC+02:00)
The source is in the previous article linked to at the top of this one.
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