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Subject: Re: [OMPI users] Hybrid OpenMPI / OpenMP run pins OpenMP threads to a single core
From: Ralph Castain (rhc_at_[hidden])
Date: 2010-07-29 10:46:23


If you check, I expect you will find that your threads and processes are not bound to a core, but are now constrained to stay within a socket.

This means that if you run more threads than cores in a socket, you will see threads idled due to contention.

On Jul 29, 2010, at 8:29 AM, David Akin wrote:

> Adding -bysocket -bind-to-socket worked! Now to figure out why that
> is? I also assumed it was my code. You can try my simple example code
> below.
>
> On Thu, Jul 29, 2010 at 8:49 AM, Ralph Castain <rhc_at_[hidden]> wrote:
>>
>> On Jul 29, 2010, at 5:09 AM, Terry Dontje wrote:
>>
>> Ralph Castain wrote:
>>
>> How are you running it when the threads are all on one core?
>>
>> If you are specifying --bind-to-core, then of course all the threads will be
>> on one core since we bind the process (not the thread). If you are
>> specifying -mca mpi_paffinity_alone 1, then the same behavior results.
>>
>> Generally, if you want to bind threads, the only way to do it is with a rank
>> file. We -might- figure out a way to provide an interface for thread-level
>> binding, but I'm not sure about that right now. As things stand, OMPI has no
>> visibility into the fact that your app spawned threads.
>>
>>
>>
>>
>> Huh??? That's not completely correct. If you have a multiple socket
>> machine you could to -bind-to-socket -bysocket and spread the processes that
>> way. Also couldn't you use the -cpus-per-proc with -bind-to-core to get a
>> process to bind to a non-socket amount of cpus?
>>
>> Yes, you could do bind-to-socket, though that still constrains the threads
>> to only that one socket. What was asked about here was the ability to
>> bind-to-core at the thread level, and that is something OMPI doesn't
>> support.
>>
>>
>> This is all documented in the mpirun manpage.
>>
>> That being said, I also am confused, like Ralph, as to why no options is
>> causing your code bind. Maybe add a --report-bindings to your mpirun line
>> to see what OMPI thinks it is doing in this regard?
>>
>> This is a good suggestion - I'm beginning to believe that the binding is
>> happening in the user's app and not OMPI.
>>
>>
>> --td
>>
>> --td
>>
>> On Jul 28, 2010, at 5:47 PM, David Akin wrote:
>>
>>
>>
>> All,
>> I'm trying to get the OpenMP portion of the code below to run
>> multicore on a couple of 8 core nodes.
>>
>> Good news: multiple threads are being spawned on each node in the run.
>> Bad news: each of the threads only runs on a single core, leaving 7
>> cores basically idle.
>> Sorta good news: if I provide a rank file I get the threads running on
>> different cores within each node (PITA.
>>
>> Here's the first lines of output.
>>
>> /usr/mpi/gcc/openmpi-1.4-qlc/bin/mpirun -host c005,c006 -np 2 -rf
>> rank.file -x OMP_NUM_THREADS=4 hybrid4.gcc
>>
>> Hello from thread 2 out of 4 from process 1 out of 2 on c006.local
>> another parallel region: name:c006.local MPI_RANK_ID=1 OMP_THREAD_ID=2
>> Hello from thread 3 out of 4 from process 1 out of 2 on c006.local
>> another parallel region: name:c006.local MPI_RANK_ID=1 OMP_THREAD_ID=3
>> Hello from thread 1 out of 4 from process 1 out of 2 on c006.local
>> another parallel region: name:c006.local MPI_RANK_ID=1 OMP_THREAD_ID=1
>> Hello from thread 1 out of 4 from process 0 out of 2 on c005.local
>> another parallel region: name:c005.local MPI_RANK_ID=0 OMP_THREAD_ID=1
>> Hello from thread 3 out of 4 from process 0 out of 2 on c005.local
>> Hello from thread 2 out of 4 from process 0 out of 2 on c005.local
>> another parallel region: name:c005.local MPI_RANK_ID=0 OMP_THREAD_ID=3
>> another parallel region: name:c005.local MPI_RANK_ID=0 OMP_THREAD_ID=2
>> Hello from thread 0 out of 4 from process 0 out of 2 on c005.local
>> another parallel region: name:c005.local MPI_RANK_ID=0 OMP_THREAD_ID=0
>> Hello from thread 0 out of 4 from process 1 out of 2 on c006.local
>> another parallel region: name:c006.local MPI_RANK_ID=1 OMP_THREAD_ID=0
>> another parallel region: name:c005.local MPI_RANK_ID=0 OMP_THREAD_ID=3
>> another parallel region: name:c005.local MPI_RANK_ID=0 OMP_THREAD_ID=2
>> another parallel region: name:c005.local MPI_RANK_ID=0 OMP_THREAD_ID=0
>> another parallel region: name:c006.local MPI_RANK_ID=1 OMP_THREAD_ID=3
>> another parallel region: name:c005.local MPI_RANK_ID=0 OMP_THREAD_ID=3
>> another parallel region: name:c005.local MPI_RANK_ID=0 OMP_THREAD_ID=2
>> another parallel region: name:c006.local MPI_RANK_ID=1 OMP_THREAD_ID=0
>> another parallel region: name:c006.local MPI_RANK_ID=1 OMP_THREAD_ID=1
>> .
>> .
>> .
>>
>> Here's the simple code:
>> #include <stdio.h>
>> #include "mpi.h"
>> #include <omp.h>
>>
>> int main(int argc, char *argv[]) {
>> int numprocs, rank, namelen;
>> char processor_name[MPI_MAX_PROCESSOR_NAME];
>> int iam = 0, np = 1;
>> char name[MPI_MAX_PROCESSOR_NAME]; /* MPI_MAX_PROCESSOR_NAME ==
>> 128 */
>> int O_ID; /* OpenMP thread ID
>> */
>> int M_ID; /* MPI rank ID
>> */
>> int rtn_val;
>>
>> MPI_Init(&argc, &argv);
>> MPI_Comm_size(MPI_COMM_WORLD, &numprocs);
>> MPI_Comm_rank(MPI_COMM_WORLD, &rank);
>> MPI_Get_processor_name(processor_name, &namelen);
>>
>> #pragma omp parallel default(shared) private(iam, np,O_ID)
>> {
>> np = omp_get_num_threads();
>> iam = omp_get_thread_num();
>> printf("Hello from thread %d out of %d from process %d out of %d on
>> %s\n",
>> iam, np, rank, numprocs, processor_name);
>> int i=0;
>> int j=0;
>> double counter=0;
>> for(i =0;i<99999999;i++)
>> {
>> O_ID = omp_get_thread_num(); /* get OpenMP
>> thread ID */
>> MPI_Get_processor_name(name,&namelen);
>> rtn_val = MPI_Comm_rank(MPI_COMM_WORLD,&M_ID);
>> printf("another parallel region: name:%s
>> MPI_RANK_ID=%d OMP_THREAD_ID=%d\n", name,M_ID,O_ID);
>> for(j = 0;j<999999999;j++)
>> {
>> counter=counter+i;
>> }
>> }
>>
>> }
>>
>> MPI_Finalize();
>>
>> }
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>> --
>> <Mail Attachment.gif>
>> Terry D. Dontje | Principal Software Engineer
>> Developer Tools Engineering | +1.650.633.7054
>> Oracle - Performance Technologies
>> 95 Network Drive, Burlington, MA 01803
>> Email terry.dontje_at_[hidden]
>>
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