What really is to “warm up” threads on multithreading processing?

问题

I’m dealing with multithreading in Java and, as someone pointed out to me, I noticed that threads warm up, it is, they get faster as they are repeatedly executed. I would like to understand why this happens and if it is related to Java itself or whether it is a common behavior of every multithreaded program.

The code (by Peter Lawrey) that exemplifies it is the following:

for (int i = 0; i < 20; i++) {
    ExecutorService es = Executors.newFixedThreadPool(1);
    final double[] d = new double[4 * 1024];
    Arrays.fill(d, 1);
    final double[] d2 = new double[4 * 1024];
    es.submit(new Runnable() {
    @Override
    public void run() {
        // nothing.
    }
    }).get();
    long start = System.nanoTime();
    es.submit(new Runnable() {
    @Override
    public void run() {
        synchronized (d) {
            System.arraycopy(d, 0, d2, 0, d.length);
        }
    }
    });
    es.shutdown();
    es.awaitTermination(10, TimeUnit.SECONDS);
    // get a the values in d2.
    for (double x : d2) ;
    long time = System.nanoTime() - start;
    System.out.printf("Time to pass %,d doubles to another thread and back was %,d ns.%n", d.length, time);
}

Results:

Time to pass 4,096 doubles to another thread and back was 1,098,045 ns.
Time to pass 4,096 doubles to another thread and back was 171,949 ns.
 ... deleted ...
Time to pass 4,096 doubles to another thread and back was 50,566 ns.
Time to pass 4,096 doubles to another thread and back was 49,937 ns.

I.e. it gets faster and stabilises around 50 ns. Why is that?

If I run this code (20 repetitions), then execute something else (lets say postprocessing of the previous results and preparation for another mulithreading round) and later execute the same Runnable on the same ThreadPool for another 20 repetitions, it will be warmed up already, in any case?

On my program, I execute the Runnable in just one thread (actually one per processing core I have, its a CPU-intensive program), then some other serial processing alternately for many times. It doesn’t seem to get faster as the program goes. Maybe I could find a way to warm it up…

回答1:

It isn't the threads that are warming up so much as the JVM.

The JVM has what's called JIT (Just In Time) compiling. As the program is running, it analyzes what's happening in the program and optimizes it on the fly. It does this by taking the byte code that the JVM runs and converting it to native code that runs faster. It can do this in a way that is optimal for your current situation, as it does this by analyzing the actual runtime behavior. This can (not always) result in great optimization. Even more so than some programs that are compiled to native code without such knowledge.

You can read a bit more at http://en.wikipedia.org/wiki/Just-in-time_compilation

You could get a similar effect on any program as code is loaded into the CPU caches, but I believe this will be a smaller difference.

回答2:

The only reasons I see that a thread execution can end up being faster are:

The memory manager can reuse already allocated object space (e.g., to let heap allocations fill up the available memory until the max memory is reached - the Xmx property)
The working set is available in the hardware cache
Repeating operations might create operations the compiler can easier reorder to optimize execution

来源：https://stackoverflow.com/questions/5198575/what-really-is-to-warm-up-threads-on-multithreading-processing

标签

java

multithreading

warm-up