Behavior of omp_get_max_threads in parallel regions

Question

I compile this bit of code on Snow Leopard and linux and I get different results. On Snow leopard, the first call of omp_get_max_threads returns 2, which is my number of cores, while the second returns 1. On linux, both calls return 4, which is my number of cores. I think Linux has the correct behavior, am I right? Are both correct and I just have a misunderstanding of this function?

#include <stdio.h>
#include <omp.h>

int main() {
  printf(" In a serial region; max threads are : %d\n", omp_get_max_threads());
#pragma omp parallel
{
  #pragma omp master
  printf(" In a parallel region; max threads are : %d\n", omp_get_max_threads());
}
}

Mac output:

 In a serial region; max threads are : 2
 In a parallel region; max threads are : 1

Linux output:

 In a serial region; max threads are : 4
 In a parallel region; max threads are : 4

Answer 1

this call is well specified in the openmp spec. linux has the correct behavior here. with that being said, you are in a master region which is effectively serial and fhe the main thread, so the num threads call is explainable. if you arent tied to pure c I would encourage you to look at the c++ tbb library and particularly the ppl subset, you will find more generality and composability like for nested parallelism. I'm on myphone so I apologize for typos here.

Answer 2

With Apple-supplied gcc 4.2 [gcc version 4.2.1 (Apple Inc. build 5566)] on Leopard, I get the same results as you (except that my MacBook has fewer cores).

In a serial region; max threads are : 2
In a parallel region; max threads are : 1

Ditto for 4.3.4 from MacPorts.

However, with gcc 4.4.2 and 4.5.0 20091231 (experimental) from MacPorts, on the same computer I get:

In a serial region; max threads are : 2
In a parallel region; max threads are : 2

It looks like this isn't a Mac versus Linux issue, but due to the gcc version.

P.S. OpenMP can do nested parallelism.

Answer 3

Just a reminder, there is a forum devoted just to OpenMP, and read by the developers of OpenMP as well as OpenMP experts world wide. It's over at the official OpenMP website: http://openmp.org/forum

Great place to ask questions like this, and to find a lot of other resources at openmp.org

Answer 4

Weird. I always get the expected behaviour (using 4.2.1, build 5646 dot 1) with OS X 10.6.2:

On my Mac Pro

 In a serial region; max threads are : 8
 In a parallel region; max threads are : 8

and on my iMac

In a serial region; max threads are : 2
In a parallel region; max threads are : 2

Must be something else going on here. Compiling with just?

gcc fname.c -fopenmp