java threading execution time question

[centenial]

Hey everyone,

Let me state up front that this is a homework assignment.

I'm supposed to take a simple task (flipping a coin n number of times) and use threading to increase the execution speed of the program. The basic idea is that to flip a coin 1,000,000 times, you could launch n threads and have it execute approx. n times as fast.

I've been working on it for a few days. The code is complete in that the task gets divided up and runs on multiple threads. However, the execution time doesn't nearly reflect what I would expect it to be.

For example, flipping 10,000,000 "coins" takes:

~700ms using 1 thread.
~980ms using 2 threads.
~920ms using 4 threads.

I would expect that the execution time would decrease as I add more threads. This time difference is magnified as I increase the number of coin flips. For example, flipping 100,000,000 "coins" takes:

~7 seconds using 1 thread.
~10 seconds using 2 threads.

Does this make sense?

I appreciate your time and any thoughts you might have. Thanks!

[Norm]

See posts at threading execution time question - Java Forums

[copeg]

You are calling a static function Math.random within several threads. This function is most-likely thread-safe although I don't have time right not to verify this. So basically, when more than one thread is created, when each thread tries this function - if another thread is using it - it will have to wait until the lock is released. Suggestion, create an instance variable of Random in your coin flip class. This should result in expected behavior if the above is correct

[helloworld922]

It's also possible that you don't have a multi-core cpu. Only cpu's with multiple cores actually gain the speed boost for multi-threaded programs (note: there may still be other benefits of creating a multi-threaded application even though it will be run on a single core cpu). Otherwise you're just wasting time creating the threads, then your OS has to manage which thread gets CPU time, resulting in some slow down.

[centenial]

You are calling a static function Math.random within several threads. This function is most-likely thread-safe although I don't have time right not to verify this. So basically, when more than one thread is created, when each thread tries this function - if another thread is using it - it will have to wait until the lock is released. Suggestion, create an instance variable of Random in your coin flip class. This should result in expected behavior if the above is correct

You're a lifesaver! Math.random() was the problem. I changed my program to use the nextBoolean() method in the random class, and I now see approx. a 2 times speed up when using 2 threads vs. 1. Thanks so much for your comments and advice.