Calculate maximum number of thread that can be used for better performance in a Threadpool

Question

Recently I face a interview where interviewer asked me about what will be maximum thread you can assign to thread pool. I answered him it will be depend upon hardware combination. Also I can manually test execution by increasing threads in thread-pool.

He seem to be not happy with that.

can anyone tell how can we decide what will be the maximum threads we should use for better performance. any guide link will be appreciated(In core java application)

Answer 1

Can anyone tell how can we decide what will be the maximum threads we should use for better performance - it's definitely not maximum amount of threads.

For the best performance, amount of threads should be equal to number of processor cores (don't forget to use -XmsYYYYM and -XmxYYYYM, without them you can face situation when your processor not assigning threads to the cores).

About maximum threads your answer was correct, it depends on hardware and OS. On linux it can be checked:

cat /proc/sys/kernel/threads-max

edited.

You can create thread pool with Integer.MAX_VALUE

But you limited with amount of maximum thread usage. On my laptop. Command "cat /proc/sys/kernel/threads-max" shows me 126987.

Code that I ran :

package com.stackoverflow.test;

import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;

public class TestMaxAmountOfThreads {
  public static void main(String[] args) {
    ExecutorService serivce = Executors.newFixedThreadPool(Integer.MAX_VALUE);
    for (int i = 0; i < Integer.MAX_VALUE; i++) {
      serivce.submit(new Runnable() {
        public void run() {
          try {
            Thread.sleep(Integer.MAX_VALUE);
          } catch (InterruptedException e) {
          }
        }
      });
      System.out.println(i);
    }
  }
}

Output:

31850

Exception in thread "main" java.lang.OutOfMemoryError: unable to create new native thread at java.lang.Thread.start0(Native Method) at java.lang.Thread.start(Thread.java:714) at java.util.concurrent.ThreadPoolExecutor.addWorker(ThreadPoolExecutor.java:949) at java.util.concurrent.ThreadPoolExecutor.execute(ThreadPoolExecutor.java:1360) at java.util.concurrent.AbstractExecutorService.submit(AbstractExecutorService.java:110) at com.stackoverflow.test.TestMaxAmountOfThreads.main(TestMaxAmountOfThreads.java:10)

So I can use only 31850 threads, without jvm tuning.