[mpiwg-sessions] MPI_Buffer_attach in the sessions model

[Rolf Rabenseifner]

Dear Dan and Tony and all,

based on your both answers, I would recommend to add an errata to 4.0, 
telling that MPI_Bsend should be used only with communicators in the world model.

Then, in the future, there is any freedom to extend MPI_Bsend somehow
the session model without the need to define any restriction or non-backward
compatible changes.

This would be a very small errata and MPI-4.1 is designed to contain such erratas about 4.0.

Kind regards
Rolf

----- Original Message -----
> From: "Dan Holmes, MPI" <danholmes at chi.scot>
> To: "MPI Sessions working group" <mpiwg-sessions at lists.mpi-forum.org>
> Cc: "Rolf Rabenseifner" <rabenseifner at hlrs.de>, "Anthony Skjellum" <skjellum at gmail.com>
> Sent: Saturday, September 17, 2022 4:23:34 PM
> Subject: Re: [mpiwg-sessions] MPI_Buffer_attach in the sessions model

> Hi Rolf & Tony,
> 
> With current text, MPI-4.0, all sessions and world model buffered send
> operations would use the same attached buffer. This is likely to be implemented
> correctly but it breaches the intent of separation of resources used by
> different sessions, so is not ideal.
> 
> A new buffer attach API could be designed to be more in keeping with the intent
> of sessions. A per-session buffer and a per-communicator buffer are both
> options to consider. There’s no GitHub issue or PR for this new API yet.
> Addressing this should fall under the remit of the sessions WG and will
> probably target MPI-5.0 at the earliest.
> 
> Thanks for raising this point.
> 
> Cheers,
> Dan.
> 
> Sent from my iPhone
> 
>> On 17 Sep 2022, at 14:18, Tony Skjellum via mpiwg-sessions
>> <mpiwg-sessions at lists.mpi-forum.org> wrote:
>> 
>> Rolf, you’ve discovered a problem with the spec… it’s global to attach a buffer,
>> so implicitly associated with ‘world model’ and the communicators derived from
>> MPI_Comm_world …
>> 
>> Each session is to be isolated resource wise … so, we have to make a new API
>> when there is no world model .
>> 
>> The API would take a session key to align buffer attachment to a session in my
>> opinion.
>> 
>> Use of the current api with a session appears erroneous. :-)
>> 
>> I am sure it would work in real implementations nonetheless.
>> 
>> Tony
>> 
>> Anthony Skjellum, PhD
>> 205-807-4968
>> 
>> 
>> On Sep 17, 2022, at 8:42 AM, Rolf Rabenseifner via mpiwg-sessions
>> <mpiwg-sessions at lists.mpi-forum.org> wrote:
>> 
>> Dear sessions-group,
>> 
>> is there any information about the usage of MPI_Buffer_attach,
>> MPI_Buffer_detach and MPI_Bsend for applications that use the
>> world model together with (several) sessions.
>> 
>> MPI-4.0, the function index entries of MPI_Buffer_attach and
>> the corresponding texts do not give an answer, or do they?
>> 
>> Is it allowed or intended that two MPI_Bsend calls with two different
>> communicators derived not from the same session or world
>> may use portions of the same buffer that was attached with MPI_Buffer_attach?
>> 
>> Is there any issue or pull request already for this problem prepared?
>> Or slides in previous meetings?
>> 
>> Best regards
>> Rolf
>> 
>> --
>> Dr. Rolf Rabenseifner . . . . . . . . . .. . . rabenseifner at hlrs.de .
>> High Performance Computing Center (HLRS) . . . ++49(0)711/685-65530 .
>> University of Stuttgart . . . . . . www.hlrs.de/people/rabenseifner .
>> Nobelstr. 19, 70569 Stuttgart, Germany
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-- 
Dr. Rolf Rabenseifner . . . . . . . . . .. . . rabenseifner at hlrs.de .
High Performance Computing Center (HLRS) . . . ++49(0)711/685-65530 .
University of Stuttgart . . . . . . www.hlrs.de/people/rabenseifner .
Nobelstr. 19, 70569 Stuttgart, Germany