generate CPU load in Java

Question

I am conducting some throughput testing. My application has to

1. read from JMS
2. do some processing
3. write to JMS

My goal here is to simulate #2, 'some processing'. That is, introduce a delay and occupy the CPU for a given time (e.g. 500ms) before forwarding the event.

The naive approach would be to Thread.sleep(500). This would introduce the right delay in execution, but would not exercise the CPU.

Calculating Fibonacci numbers is one option. Has anyone used any interesting techniques just to keep CPU(s) busy for a given time?

Ideal characteristics would be:

• Performs a variety of instructions, rather than (for example) just spinning on a loop
• Not something the HotSpot VM is going to optimise away to nothing
• Has an easy way to adjust the processing period up or down (time to complete will clearly vary given the hardware)

Answer 1

You could try something simple like

private static void spin(int milliseconds) {
    long sleepTime = milliseconds*1000000L; // convert to nanoseconds
    long startTime = System.nanoTime();
    while ((System.nanoTime() - startTime) < sleepTime) {}
}

Test:

public static void main(String[] args) {
    final int NUM_TESTS = 1000;
    long start = System.nanoTime();
    for (int i = 0; i < NUM_TESTS; i++) {
        spin(500);
    }
    System.out.println("Took " + (System.nanoTime()-start)/1000000 +
        "ms (expected " + (NUM_TESTS*500) + ")");
}

My output:

$ java SpinTest
Took 500023ms (expected 500000)

So the loop didn't get optimized away (and yeah, I spiked my CPU to 100% for eight minutes just to test this :)).

Answer 2

Encrypt a string (in a loop) by calling Cipher.update(). Encryption algorithms are by definition very difficult to optimize. The only problem is that there is some non-trivial setup you need to perform. I'm marking this answer as community wiki, so that somebody who's written it recently can fill it in.