Java中奇怪的代码计时行为

Question

In the following code:

long startingTime = System.nanoTime();
int max = (int) Math.pow(2, 19);
for(int i = 0; i < max; ){
    i++;
}
long timePass = System.nanoTime() - startingTime;
System.out.println("Time pass " + timePass / 1000000F);

I am trying to calculate how much time it take to perform simple actions on my machine.

All the calculations up to the power of 19 increase the time it takes to run this code, but when I went above 19(up to max int value 31) I was amazed to discover that it have no effect on the time it takes. It always shows 5 milliseconds on my machine!!!

How can this be?

Answer 1

You have just witnessed HotSpot optimizing your entire loop to oblivion. It's smart . You need to do some real action inside the loop. I recommend introducing an int accumulator var and doing some bitwise operations on it, and finally printing the result to ensure it's needed after the loop.

Answer 2

On the HotSpot JVM, -XX:CompileThreshold=10000 by default. This means a loop which iterates 10K times can trigger the whole method to be optimised. In your case you are timing how long it take to detect and compile (in the background) your method.

Answer 3

use another System.nanoTime() in the loop. no one can optimize this.

for(int i = 0; i < max; ){
i++;
dummy+=System.nanoTime();
}

dont forget to do:

System.out.println(dummy);

after the loop. ensures non-optimization