performance - Java benchmarking - why is the second loop faster?

Question

I'm curious about this.

I wanted to check which function was faster, so I create a little code and I executed a lot of times.

public static void main(String[] args) {

        long ts;
        String c = "sgfrt34tdfg34";

        ts = System.currentTimeMillis();
        for (int k = 0; k < 10000000; k++) {
            c.getBytes();
        }
        System.out.println("t1->" + (System.currentTimeMillis() - ts));

        ts = System.currentTimeMillis();
        for (int i = 0; i < 10000000; i++) {
            Bytes.toBytes(c);
        }
        System.out.println("t2->" + (System.currentTimeMillis() - ts));

    }

The "second" loop is faster, so, I thought that Bytes class from hadoop was faster than the function from String class. Then, I changed the order of the loops and then c.getBytes() got faster. I executed many times, and my conclusion was, I don't know why, but something happen in my VM after the first code execute so that the results become faster for the second loop.

Answer 1

This is a classic java benchmarking issue. Hotspot/JIT/etc will compile your code as you use it, so it gets faster during the run.

Run around the loop at least 3000 times (10000 on a server or on 64 bit) first - then do your measurements.