ThreadPoolExecutor more threads=slower android

Question

   BlockingQueue<Runnable> queue = new LinkedBlockingQueue<Runnable>();
                ThreadPoolExecutor ex = new ThreadPoolExecutor(1,1, 1L, TimeUnit.MINUTES, queue);

                final HashMap<String,Response> responses = new  HashMap<String,Response>();

                ex.execute(new Runnable() {
                    @Override
                    public void run() {
                        android.os.Process.setThreadPriority(Process.THREAD_PRIORITY_MORE_FAVORABLE);
                        Response response = getService().getProducts();
                        responses.put("1",response);


                    }
                });
....
....

...


 ex.shutdown();
 ex.awaitTermination(1000, TimeUnit.DAYS);
 long  time2 = System.currentTimeMillis();

I make many requests that I want to be notified when finished.

Testing on android 2.3 gingerbead on old device with ThreadPoolExecutor with 1 thread takes 1-1.2 seconds. Using 4 threads takes lot more 4-5 seconds!.

Testing on galaxy tab 3 it takes with one thread 100 milliseconds with 4 it takes 60 ms.

Why does this happen ??

Answer 1

No matter how many threads you create, you only have so many physical cores in your processor.

The older device probably has one core, so with more threads it constantly has to jump between the threads to process them.

The new device has multiple cores so can spread the threads out over them and run them in parallel.

In general for intensive tasks you should never start more threads than you have cores. For tasks that only run occasionally more threads can be ok but something like a ScheduledExecutor with a thread pool behind it would still be better.