[Mpi-forum] Threading guarantees of MPI_User_function

[Jeff Hammond]

It sounds like we need to add some text to the standard about this. At the very least, an advice to users about the thread context seems prudent.

Can you add a ticket about this? Collectives WG seems like the right place...

Jeff

Sent from my iPhone

> On May 29, 2017, at 9:18 AM, "Marc.PERACHE at CEA.FR" <Marc.PERACHE at CEA.FR> wrote:
> 
> I’m sorry but the MPI_User_function is not always invoked by the thread that invokes the MPI function that uses it. When you have progression thread, the MPI_User_function may be called by this additional thread.
>  
> Marc
>  
> De : mpi-forum-bounces at lists.mpi-forum.org [mailto:mpi-forum-bounces at lists.mpi-forum.org] De la part de William Gropp
> Envoyé : vendredi 26 mai 2017 01:58
> À : Main MPI Forum mailing list
> Objet : Re: [Mpi-forum] Threading guarantees of MPI_User_function
>  
> Just to be clear on this, the MPI standard generally requires the user to enforce any synchronization needed when using threads with MPI, so Jeff’s answer is aligned with the standard.
>  
> Bill
>  
> William Gropp
> Interim Director and Chief Scientist, NCSA
> Thomas M. Siebel Chair in Computer Science
> University of Illinois Urbana-Champaign
>  
>  
>  
> 
>  
> On May 25, 2017, at 1:49 PM, Phil Ruffwind <rf at rufflewind.com> wrote:
>  
> On Thu, May 25, 2017, at 12:55, Jeff Hammond wrote:
> 
> I expect that MPI_User_function is invoked by the thread that invokes the
> MPI function that uses it, and that you need a mutex to touch shared
> state
> associated with MPI_User_function when running with MPI_THREAD_MULTIPLE,
> but not otherwise.
> 
> I would hope that MPI implementations do not serialize calls to
> MPI_User_function
> in MPI_THREAD_MULTIPLE mode, because there are many cases where
> MPI_User_function
> is trivially reentrant and I would like simultaneous progress if I call
> MPI_Allreduce concurrently from 2+ threads (with different communicators,
> of course) with the same MPI_User_function.
> 
> Thanks for clarifying.  In that case, I'll assume synchronization is
> necessary.
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