How to disable compiler and JVM optimizations?

Question

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

long before = getTimeInMil         


        

Answer 1

Sorry, but what you are trying to do makes little sense.

If you turn off JIT compilation, then you are only going to measure how long it takes to call that method with JIT compilation turned off. This is not useful information ... because it tells you little if anything about what will happen when JIT compilation is turned on.

The times between JIT on and off can be different by a huge factor. You are unlikely to want to run anything in production with JIT turned off.

A better approach would be to do this:

long before1 = getTimeInMilli();
for (int i = 0; i < TIMES_TO_ITERATE; i++) {
    doSomeReallyHardWork();
}
long after1 = getTimeInMilli();

... and / or use the nanosecond clock.

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If you are trying to measure the time taken to call the two versions of getTimeInMillis(), then I don't understand the point of your call to doSomeReallyHardWork(). A more senible benchmark would be this:

public long test() {
    long before1 = getTimeInMilli();
    long sum = 0;
    for (int i = 0; i < TIMES_TO_ITERATE; i++) {
        sum += getTimeInMilli();
    }
    long after1 = getTimeInMilli();
    System.out.println("Took " + (after - before) + " milliseconds");
    return sum;
}

... and call that a number of times, until the times printed stabilize.

Either way, my main point still stands, turning of JIT compilation and / or optimization would mean that you were measuring something that is not useful to know, and not what you are really trying to find out. (Unless, that is, you are intending to run your application in production with JIT turned off ... which I find hard to believe ...)