Multi-threading for multi-core processors

Question

I have Samsung Galaxy S3, which used its own Exynos 4 Quad processor. So I want to optimize my app, that it can use all 4 cores of processor.

So I made some tests:

1. Run task in one thread. Processing time - 8 seconds.

2. Run task in four threads. Processing time - still 8 sec.

    new Thread() {
       public void run() {
                       // do 1/4 of task
       }
   }.start();
   
   new Thread() {
       public void run() {
                       // do 1/4 of task
       }
   }.start();
   
   new Thread() {
       public void run() {
                       // do 1/4 of task
       }
   }.start();
   
   new Thread() {
       public void run() {
                       // do 1/4 of task
       }
   }.start();
   

3. Run task in ExecutorService. And processing time - still 8 sec.

   ExecutorService threadPool = null;
   threadPool = Executors.newFixedThreadPool(4);
   
   threadPool.execute(new Runnable() {
       @Override
       public void run() {
            // do 1/4 of task
                         }});
   threadPool.execute(new Runnable() {
       @Override
       public void run() {
            // do 1/4 of task
                         }});
   threadPool.execute(new Runnable() {
       @Override
       public void run() {
            // do 1/4 of task
                         }});
   threadPool.execute(new Runnable() {
       @Override
       public void run() {
            // do 1/4 of task
                         }});
   

Looks like all work done synchronously. Why its not paralleling ?

Answer 1

You can try my tests, in C or Java:

http://bigflake.com/cpu-spinner.c.txt
http://bigflake.com/MultiCore.java.txt

When MultiCore.java is run on a Nexus 4, I see:

Thread 1 finished in 1516ms (1267234688)
Thread 3 finished in 1494ms (1519485328)
Thread 0 finished in 1530ms (51099776)
Thread 2 finished in 1543ms (-1106614992)
All threads finished in 1550ms

Update: It's useful to watch the test with systrace. As part of answering a similar question I set up a page that shows the systrace output for this test. You can see how the threads are scheduled, and watch the other two cores spin up.