OpenMP and MPI hybrid program

Question

I have a machine with 8 processors. I want to alternate using OpenMP and MPI on my code like this:

OpenMP phase:

• ranks 1-7 wait on a MPI_Barrier
• rank 0 uses all 8 processors with OpenMP

MPI phase:

• rank 0 reaches barrier and all ranks use one processor each

So far, I've done:

• set I_MPI_WAIT_MODE 1 so that ranks 1-7 don't use the CPU while on the barrier.
• set omp_set_num_threads(8) on rank 0 so that it launches 8 OpenMP threads.

It all worked. Rank 0 did launch 8 threads, but all are confined to one processor. On the OpenMP phase I get 8 threads from rank 0 running on one processor and all other processors are idle.

How do I tell MPI to allow rank 0 to use the other processors? I am using Intel MPI, but could switch to OpenMPI or MPICH if needed.

Answer 1

The following code shows an example on how to save the CPU affinity mask before the OpenMP part, alter it to allow all CPUs for the duration of the parallel region and then restore the previous CPU affinity mask. The code is Linux specific and it makes no sense if you do not enable process pinning by the MPI library - activated by passing --bind-to-core or --bind-to-socket to mpiexec in Open MPI; deactivated by setting I_MPI_PIN to disable in Intel MPI (the default on 4.x is to pin processes).

#define _GNU_SOURCE

#include <sched.h>

...

cpu_set_t *oldmask, *mask;
size_t size;
int nrcpus = 256; // 256 cores should be more than enough
int i;

// Save the old affinity mask
oldmask = CPU_ALLOC(nrcpus);
size = CPU_ALLOC_SIZE(nrcpus);
CPU_ZERO_S(size, oldmask);
if (sched_getaffinity(0, size, oldmask) == -1) { error }

// Temporary allow running on all processors
mask = CPU_ALLOC(nrcpus);
for (i = 0; i < nrcpus; i++)
   CPU_SET_S(i, size, mask);
if (sched_setaffinity(0, size, mask) == -1) { error }

#pragma omp parallel
{
}

CPU_FREE(mask);

// Restore the saved affinity mask
if (sched_setaffinity(0, size, oldmask) == -1) { error }

CPU_FREE(oldmask);

...

You can also tweak the pinning arguments of the OpenMP run-time. For GCC/libgomp the affinity is controlled by the GOMP_CPU_AFFINITY environment variable, while for Intel compilers it is KMP_AFFINITY. You can still use the code above if the OpenMP run-time intersects the supplied affinity mask with that of the process.

Just for the sake of completeness - saving, setting and restoring the affinity mask on Windows:

#include <windows.h>

...

HANDLE hCurrentProc, hDupCurrentProc;
DWORD_PTR dwpSysAffinityMask, dwpProcAffinityMask;

// Obtain a usable handle of the current process
hCurrentProc = GetCurrentProcess();
DuplicateHandle(hCurrentProc, hCurrentProc, hCurrentProc,
                &hDupCurrentProc, 0, FALSE, DUPLICATE_SAME_ACCESS);

// Get the old affinity mask
GetProcessAffinityMask(hDupCurrentProc,
                       &dwpProcAffinityMask, &dwpSysAffinityMask);

// Temporary allow running on all CPUs in the system affinity mask
SetProcessAffinityMask(hDupCurrentProc, &dwpSysAffinityMask);

#pragma omp parallel
{
}

// Restore the old affinity mask
SetProcessAffinityMask(hDupCurrentProc, &dwpProcAffinityMask);

CloseHandle(hDupCurrentProc);

...

Should work with a single processor group (up to 64 logical processors).