Starting an OpenMP parallel section is extremely slow on a hyper-threaded Nehalem

[Edwin Bennink]

Dear gcc list,


I noticed that starting an OpenMP parallel section takes a significant 
amount of time on Nehalem cpu's with hyper-threading enabled.
The differences with HTT turned on and off are really huge:

 - HTT disabled: about 100.000 parallel sections per second
 - HTT enabled: about 15 parallel sections per second

Is this a known problem? It has apparently something to do with setting 
the cpu affinity; when I set the GOMP_CPU_AFFINITY environment variable 
to "0-7", then it is almost as fast as with HTT disabled...


This is the code I used to test it. Simply compile it with -fopenmp. I 
used 100.000 iterations instead of 100 to time it with HTT disabled.

========================

int main () {
    int i;

    for (i = 0; i < 100; i++) {
#pragma omp parallel
        {
        }
    }
}

========================


System specs:
OS: Ubuntu 9.10, amd64 (2.6.31-19-generic)
gcc: version 4.4.1 (Ubuntu 4.4.1-4ubuntu9)
cpu: Intel(R) Core(TM) i7 CPU         920  @ 2.67GHz


Cheers,
Edwin