Thanks everyone for the lively discussion and pointing me to the relevant parts of the standard. We scanned the document to find the relevant text but could not find it. Based on the comments, I think we are good with our approach. The clarification for session init already answered our potential follow-up question :) As a background: we are currently integrating session support in MUST, so that all tool-internal communication executes in a separate session. As part of this effort, we move to MPMD mode and stop launching application code at the tool processes, which execute their run-loop in MPI_Init called from the application and never return to the application. To avoid deadlocking, if the application calls MPI_Init, we need to propagate this information to the tool processes and execute MPI_Init there as well. So we were wondering whether we should select the threading level appropriate for the tool processes or whether we need to replicate the call from the application processes. Our take-away is that we are fine with just calling MPI_Init, as the tool processes at the moment are purely single-threaded and won't use the world model anyways. Best Joachim Am 01.08.23 um 12:39 schrieb Holmes, Daniel John via mpiwg-hybridpm:

It is hinted at in the normative sentence: "Different processes in MPI_COMM_WORLD may require different levels of thread support." (MPI-4.1 section 11.2.1, page 492 lines 3-4)

We must read "may require different" as "are permitted to set \mpiarg{required} to different"

Note also that the thread support level can be requested via MPI_SESSION_INIT, which is a local procedure -- implying that the values given as INFO will apply only to the local process in the scope of the session.

Best wishes, Dan.

-----Original Message----- From: mpiwg-hybridpm <[email protected]> On Behalf Of Joseph Schuchart via mpiwg-hybridpm Sent: Tuesday, August 1, 2023 11:24 AM To: [email protected] Cc: Joseph Schuchart <[email protected]> Subject: Re: [mpiwg-hybridpm] Can MPI processes request different threading levels?

Funny that this is inside an advice to implementors (which users like Joachim are not expected to read) when it really should be part of the normative text.

Cheers Joseph

On 8/1/23 11:09, Jeff Hammond via mpiwg-hybridpm wrote:

...

It is addressed here (in the text for MPI_INIT_THREAD):

Advice to implementors. If provided is not MPI_THREAD_SINGLE then the MPI library should not invoke C or Fortran library calls that are not thread safe, e.g., in an environment where malloc is not thread safe, then malloc should not be used by the MPI library.

Some implementors may want to use different MPI libraries for different levels of thread support. They can do so using dynamic linking and selecting which library will be linked when MPI_INIT_THREAD is invoked. If this is not possible, then optimizations for lower levels of thread support will occur only when the level of thread support required is specified at link time.

*Note that required need not be the same value on all processes of MPI_COMM_WORLD.* (End of advice to implementors.)

Jeff

On Tue, Aug 1, 2023 at 11:47 AM Joachim Jenke via mpiwg-hybridpm <[email protected]> wrote:

...

Hello,

I'm not sure whether this is the right group to ask, but it is related to hybrid execution :)

Must all MPI processes collectively call MPI_Init or MPI_Init_thread - or can some processes call one and other processes call the other

function?

...

Also, can the processes request different thread-support levels?

Our use case is MPMD execution, where some processes run multi-threaded and other single-threaded.

Thanks, Joachim

-- Dr. rer. nat. Joachim Jenke

IT Center Group: High Performance Computing Division: Computational Science and Engineering RWTH Aachen University Seffenter Weg 23 D 52074  Aachen (Germany) Tel: +49 241 80- 24765 Fax: +49 241 80-624765 [email protected] www.itc.rwth-aachen.de <http://www.itc.rwth-aachen.de> _______________________________________________ mpiwg-hybridpm mailing list [email protected] https://lists.mpi-forum.org/mailman/listinfo/mpiwg-hybridpm

-- Jeff Hammond [email protected] http://jeffhammond.github.io/

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-- Dr. rer. nat. Joachim Jenke IT Center Group: High Performance Computing Division: Computational Science and Engineering RWTH Aachen University Seffenter Weg 23 D 52074 Aachen (Germany) Tel: +49 241 80- 24765 Fax: +49 241 80-624765 [email protected] www.itc.rwth-aachen.de