Using taskyield to emulate a background service

Runtimes OpenMP
1

I am trying to emulate recurring service calls in OpenMP (similar to
what OmpSs-2 offers) in order to progress outstanding MPI operations.

You may want to check out our IPDPS paper (Parallelizing MPI Using Tasks for Hybrid Programming Models)
on using OpenMP tasks in an MPI implementation, though I guess you may not be down inside the MPI library.

Date: Wed, 15 Jul 2020 11:33:57 +0200
From: Joseph Schuchart via Openmp-dev <openmp-dev@lists.llvm.org>
To: openmp-dev <openmp-dev@lists.llvm.org>
Subject: [Openmp-dev] Using taskyield to emulate a background service
Message-ID: <b35cf7d0-751d-a45e-4112-0eca25ef1ad8@hlrs.de>
Content-Type: text/plain; charset=utf-8; format=flowed

Dear devs,

I am trying to emulate recurring service calls in OpenMP (similar to
what OmpSs-2 offers) in order to progress outstanding MPI operations. My
attempt is to have one task executed by some thread that invokes the
progress service and calls taskyield to participate in the execution of
tasks generated by any of the other threads. The task’s code is similar
to this snippet:

#pragma omp task shared(do_progress)
{
while(do_progress) {
call_progress();
#pragma omp taskyield
}
}
}

The variable do_progress is a volatile flag that is unset by another
thread once progress is not needed anymore.

What I found is that the thread executing this service task does not
participating in the execution of available tasks from the other
threads. In other words, the yield does nothing. However, if I set
KMP_TASK_STEALING_CONSTRAINT=0 then the thread does participate in the
execution of tasks. All tasks are tied (because of issues with untied
tasks).

Can someone tell me why taskyield does not steal tasks by default? Is
that related to the the fact that all tasks are tied?

Many thanks!

Cheers
Joseph

–
Dipl.-Inf. Joseph Schuchart
High Performance Computing Center Stuttgart (HLRS)
Nobelstr. 19
D-70569 Stuttgart

Tel.: +49(0)711-68565890
Fax: +49(0)711-6856832
E-Mail: schuchart@hlrs.de

– Jim
James Cownie <jcownie@gmail.com>
Mob: +44 780 637 7146