[Openmp-dev] Using taskyield to emulate a background service

[Jim Cownie via Openmp-dev]

> 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 <https://ieeexplore.ieee.org/document/8425570>)
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 at lists.llvm.org>
> To: openmp-dev <openmp-dev at lists.llvm.org>
> Subject: [Openmp-dev] Using taskyield to emulate a background service
> Message-ID: <b35cf7d0-751d-a45e-4112-0eca25ef1ad8 at 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 at hlrs.de
> 



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




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