How to disable compiler and JVM optimizations?

java optimization compiler-construction jvm jit
oshai picture oshai · Mar 9, 2011 · Viewed 12.2k times · Source

I have this code that is testing Calendar.getInstance().getTimeInMillis() vs System.currentTimeMilli() :

long before = getTimeInMilli();
for (int i = 0; i < TIMES_TO_ITERATE; i++)
{
  long before1 = getTimeInMilli();
  doSomeReallyHardWork();
  long after1 = getTimeInMilli();
}
long after = getTimeInMilli();
System.out.println(getClass().getSimpleName() + " total is " + (after - before));

I want to make sure no JVM or compiler optimization happens, so the test will be valid and will actually show the difference.

How to be sure?

EDIT: I changed the code example so it will be more clear. What I am checking here is how much time it takes to call getTimeInMilli() in different implementations - Calendar vs System.

Answer

ilalex picture ilalex · Mar 9, 2011

I think you need to disable JIT. Add to your run command next option:

-Djava.compiler=NONE

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