multi-threading with gcc43 openmp on dual core

B. Estrade estrabd at
Wed Oct 17 09:59:31 PDT 2007

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;
 for (m=1;m<=n;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,

HPC Enablement Group
Louisiana Optical Network Initiative

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