multi-threading with gcc43 openmp on dual core

Kris Kennaway kris at
Wed Oct 17 10:31:07 PDT 2007

B. Estrade wrote:
> I am messing around with gcc43's openmp capabilities on an IBM Thinkcentre, which has a dual core Intel vPro.  I am running FreeBSD 6.2 with SMP.
> At this stage in the game I am trying to push around the limits of the systems capabilities, and I was wondering if FreeBSD attempts to offload some of the threads to the 2nd core.  I admittedly don't know the chip architecure or how FreeBSD handles man threads in SMP, but I'd like too, which is why I am playing with this.
> As the number of threads increases, one core seems to be oversubscribed according to top (i.e., wcpu > 100%) yet the system is said to be 50% idle (+/- a few points). The load average also seems to not get much higher than 1.0. While I am happy with the load handling, I am wondering:
> 1. why are threads not evenly distributed b/w CPU0 and CPU1
> 2. is there a way to explicitly facilitate this thread balancing (don't know if this is related to CPU affinity)
> 3. what is a good way to push the system? 
> The program I am running spawns an increasing number of threads from 1 to OMP_NUM_THREADS, where each thread has its own loop that simply assigns the sum of j and i to k. My OpenMP-foo is not advanced, but it's steadily getting there. 
> Here is the code I am using right now:
> #include <stdio.h>
> int main (int argc, char *argv[]) {
>  int i,j,k,m,n;
>  int MAX=500;
>  n=omp_get_max_threads();
>  for (m=1;m<=n;m++) {
>    k=0;
>    omp_set_num_threads(m);
>    #pragma omp parallel private(i,j) shared(MAX) reduction(+:k)
>    { for (i=0;i<MAX;i++) {
>        for (j=0;j<MAX;j++) {
>          k = j+i;
>        }
>        #pragma omp barrier
>      }
>    }
>    printf("%8d threads: %d\n",m,k);
>  }
>  return 0;
> }
> It is compiled and executed with the following command:
> % gcc43 -fopenmp test.c && export OMP_NUM_THREADS=1500 && ./a.out
> Any thoughts?
> Once I get familiar with what 6.x is doing, I plan on putting 7.0 on this system to see how it compares. Ultimately, I'd like to a straightforward code I can use to evaluate how a particular SMP system running FreeBSD scales.  I know it is not a "real" workload, but it would give /me some information that I'd find useful.
> TIA && Cheers,
> Brett

FreeBSD uses all CPUs for running processes and threads.  This relies on 
your program being sufficiently parallel though.  I don't know enough 
about openmp but maybe it is not able to parallelize your workload very 


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