java - Executing 2 or more Runnables causes performance issues -

i need factor 64-bit number (n = pq). implemented method searches consequentially numbers in range of [1; sqrt(n)].

it took 27 secs execute on android 1,2 ghz processor (unfortunately, don't know number of cpu cores). decided make parallel. well, 2 runnables giving me results in 51 secs , 3 — in 83.

my program nothing calling method in oncreate.

final static private int workers_count = 3;  final static public int[] pqfactor(final long pq) {     stopfactorflag = false;      long blocksize = (long)math.ceil(math.sqrt(pq) / workers_count);     executorservice executor = executors.newfixedthreadpool(workers_count);      (int workeridx = 0; workeridx < workers_count; ++workeridx) {         runnable worker = new factortask(pq, workeridx * blocksize, (workeridx + 1) * blocksize);         executor.execute(worker);     }      executor.shutdown();     try {         executor.awaittermination(5, timeunit.minutes);     } catch (interruptedexception e) {         e.printstacktrace();     }      return result; }   private static boolean stopfactorflag; private static int p, q;  static private class factortask implements runnable {     final private long pq;     private long leftborder;     private long rightborder;     public long pinternal;     public long qinternal;      /* constructor there */      @override     public void run() {         (qinternal = rightborder; !stopfactorflag && qinternal > leftborder && qinternal > 1l; qinternal -= 2l) {             if (pq % qinternal == 0l) {                 pinternal = pq / qinternal;                 p = (int)pinternal;                 q = (int)qinternal;                 stopfactorflag = true;                 break;             }         }     } } 

p. s. not homework, need this. maybe other way.

executing 2 or more runnables causes performance issues

this looks me android device has either 1 or 2 cores , adding threads problem not going make run faster because have exhausted cpu resources. i'd recommend looking device specs determine how many cores has.

if run code under 4 core macbook pro:

• 2 threads in ~6secs
• 3 threads in ~4secs
• 4 threads in ~3.5secs

this seems me reasonably linear (taking account startup/shutdown overhead) , indicates me not code holding back.

btw, stopfactorflag should volatile. don't see how creating result array i'm worried race conditions there.