Question

Does anyone know good way to profile a sorting algorithm in java(sequential and fork join) ?
because the running time is too short (sorting list size 5000..), System.nanoTime() seems not work properly.

I plan to run same test case many times (1000) and get rid of first 100 results (avoid HotSpot compiler problem) and do an average of running time using System.nanoTime().
Any suggestion on this ?

Thanks a lot!

Can I do this way ?

double count = 0;
double start, end;
for(int r = 0; r < warmup; r++) {
    // do test
}
for(int t = 0; t < runs; t++){
    start = System.nanoTime();
    // do test
    end = System.nanoTime();
    count += start - end;
}
double avg = count/avg

Answer 1

I can assure you that nanoTime() does work and if you want to avoid all hotspot warmup you need to run it 10K times. You should find that a sort of 5K element is fairly quick and even 1K test is not very much. You need to write a test which produces reproduce-able results. If you haven’t, that is down to you to fix the test because it not very good.

I suggest you try it and see what results you get.

On an old computer, a sort of 5K random int values takes about 500 us. Note: sorting a sorted array will not give you same result. (so you cannot sort the same array each time)

A simple way to run a test a certain number of times ignoring the first N runs is to do.

long start = 0;
for(int r = -warmup; r < runs; r++) {
    if (r == 0) start = System.nanoTime();
    // do test
}
long avg = (System.nanoTime() - start)/runs;