Why POSIX Threads are Slower Than OpenMP

9
votes

I'm running a completely parallel matrix multiplication program on a Mac Pro with a Xeon processor. I create 8 threads (as many threads as cores), and there are no shared writing issues (no writing to the same locations). For some reason, my use of pthread_create and pthread_join is about twice as slow as using #pragma openmp.

There are no other differences in anything... same compile options, same number of threads in both cases, same code (except the pragma/pthread portions obviously), etc.

And the loops are very big -- I'm not parallelizing small loops.

(I can't really post the code because it's school work.)

Why might this be happening? Doesn't OpenMP use POSIX threads itself? How can it be faster?

1
cpthreadsopenmp
Do they both use the same amount of cumulative CPU time? – Gabe
Have you verified that OpenMP is using the same number of threads as your manual version? – Gabe
What happens if you only use 7 threads on each? – Jess
@Jess: Brilliant question!! I tried it, it was faster... it turned out I was creating 8 threads, but I already had a master thread, for a total of 9, which is one more than the number of cores! (Wow, haha...) Feel free to put that as the answer so I'll accept it. :) – user541686

1 Answers

6
votes

(edited) What is your main thread doing? Without seeing your code, I was guessing that the main thread is actually barely running, but still eating up clock-cycles while the pthreads finish, then it starts again and continues. Each time its given cycles there is overhead to pausing/continuing the other threads.

In OpenMP, the main thread probably goes to sleep, and waits for a wake-up event when the parallel regions finish.