Very strange performance issue

Question

Hi!
I've got a very very strange performance problem and I have no more ideas what it might be:
The following code fragment contains the useless line int count=0;
However, deleting the line reduces(!) the execution speed of the program by ~30%. This is not only weird because the line is absolutely useless, but also because the whole function only takes <1% total execution time in the program.
    public void simuliere_bis_soll_erreicht()
        int count=0;  //????
        while(welt.soll_simulationszeit>welt.ist_simulationszeit && simu_active)
            simuliere_einen_schritt();
    }The problem occurs both under Java 1.5 and 1.4.2, using the HotSpot-Client.
Cleaning and rebuilding the project does not help.
It occurs on different computers in the same way, too.
Thank you very much in advance! :-)
Mirko Seithe

Answer 1

well, this is what you get:
1.) run totally interpreted since compilation threshhold is reached (1500 (client jvm) - 10000 (server-jvm)
2.) start background compilation, run further in interpreted mode till compilation finishes (depends on code, but about 1000-10000)
3.) run compiled code
so this is what you get:
~20.000 invokations interpreted (about 70-90%n of total time)
~80.000 invokations compiled (30-10% of total time)
maybe your int takes much longer to optimize, so the loop executes much longer in interpreted code.
I would not bet on such microbenchmarks, since they do not tell you real stories under real circumstances. Believe me, I am the programmer working on performance optimizations in the company I am employed.
lg Clemens