Question

I wrote a simple code that uses multiple threads to calculate number of primes from 1 to N.

public static void main (String[] args) throws InterruptedException
{
    Date start;
    start = new Date();
    long startms = start.getTime();
    int number_primes = 0, number_threads =0;
    number_primes = Integer.parseInt(args[0]);
    number_threads = Integer.parseInt(args[1]);

    MakeThread[] mt = new MakeThread[number_threads];

    for(int i=1;i<=number_threads;i++)
    {
        mt[i-1] = new MakeThread(i,(i-1)*(number_primes/number_threads),i*(number_primes/number_threads));
        mt[i-1].start();
    }

    for(int i=1;i<number_threads;i++)
    {
        mt[i-1].join();
    }

    Date end = new Date();
    long endms = end.getTime();

    System.out.println("Time taken = "+(endms-startms));
}

}

As show in above, I want the final time taken to be displayed (just to measure performance for different inputs). However I noticed that when I enter a really big value of N and assign only 1 or 2 threads, the scheduler seems to override the join functionality (i.e the last print statement is displayed before other threads end). Is the kernel allowed to do this? Or do I have some bug in my code?

P.S: I have only shown a part of my code. I have a similar System.out.println at the end of the function that the newly forked threads call.

Answer 1

Your loop is the problem.

for(int i=1;i<number_threads;i++)
{
    mt[i-1].join();
}

Either you change the condition to <= or you make a less cryptic loop like this:

for(int i=0; i < number_threads;i++){
    mt[i].join();
}

Or a for each loop:

for(MakeThread thread : mt)
  thread.join();