Date: Wed, 17 Oct 2007 19:31:05 +0200 From: Kris Kennaway <kris@FreeBSD.org> To: "B. Estrade" <estrabd@gmail.com> Cc: freebsd-smp <freebsd-smp@freebsd.org> Subject: Re: multi-threading with gcc43 openmp on dual core Message-ID: <471646D9.5090508@FreeBSD.org> In-Reply-To: <20071017164204.GG76582@bc1.hpc.lsu.edu> References: <20071017164204.GG76582@bc1.hpc.lsu.edu>
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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
well.
Kris
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