在多线程程序中获取执行时间

Question

I am trying to sort an array using multithreading in java and note its execution time. I am using System.nanoTime() for the same.

Here is a template of the code

start = System.nanoTime();
sort();   // calls different threads to completely sort the array
end = System.nanoTime()

What I want to ask is that the above is in main Thread so will it give me incorrect time results given that I did not put main.sleep() ?? or is it handled by the JVM . Also if I am in need of fast execution is there something that can improve the performance of above code in terms of time execution??

Answer 1

What I want to ask is that the above is in main Thread so will it give me incorrect time results given that I did not put main.sleep() ?? or is it handled by the JVM .

The System.nanoTime() is supposed to give you the best (most precise) available measure of elapsed time. Note:

• this is NOT a measure of the CPU time used, and
• on some versions of Java / OS platforms, System.nanoTime() uses a hardware clock that is not synchronized across different cores, and (apparently) can be rather inaccurate as a result.

Personally, I'd use System.currentTimeMillis() , and make sure that I benchmarked the sorting of a large array or collection ... and did various other things to make sure that my benchmark was valid.

( EDIT - If you want to find out how much CPU time was used by each thread, take a look the ThreadMXBean API.)

Also if I am in need of fast execution is there something that can improve the performance of above code in terms of time execution??

That is far too general a question to answer, except to say "probably yes" ... or "profile it".

Answer 2

You can use the JVisualVM profiler (or another profiler) to find how fast each of your methods executes.

All you need is a pause at the beginning to allow you to attach it, and a pause at the end so you can view and/or save the results.