How to disable compiler and JVM optimizations?
I think you need to disable JIT. Add to your run command next option:
-Djava.compiler=NONE
You want optimization to happen, because it will in real life - the test wouldn't be valid if the JVM didn't optimize in the same way that it would in the real situation you're interested in.
However, if you want to make sure that the JVM doesn't remove calls that it could potentially consider no-ops otherwise, one option is to use the result - so if you're calling System.currentTimeMillis()
repeatedly, you might sum all the return values and then display the sum at the end.
Note that you may still have some bias though - for example, there may be some optimization if the JVM can cheaply determine that only a tiny amount of time has passed since the last call to System.currentTimeMillis()
, so it can use a cached value. I'm not saying that's actually the case here, but it's the kind of thing you need to think about. Ultimately, benchmarks can only really test the loads you give them.
One other thing to consider: assuming you want to model a real world situation where the code is run a lot, you should run the code a lot before taking any timing - because the Hotspot JVM will optimize progressively harder, and presumably you care about the heavily-optimized version and don't want to measure the time for JITting and the "slow" versions of the code.
As Stephen mentioned, you should almost certainly take the timing outside the loop... and don't forget to actually use the results...