[mpiwg-sessions] Fwd: World/session model

HOLMES Daniel d.holmes at epcc.ed.ac.uk
Fri Dec 18 10:15:30 CST 2020

Hi Victor,

Thanks for the comments. We’ll discuss them within the WG in the new year.

Dr Daniel Holmes PhD
Architect (HPC Research)
d.holmes at epcc.ed.ac.uk<mailto:d.holmes at epcc.ed.ac.uk>
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Begin forwarded message:

From: Victor Eijkhout <eijkhout at tacc.utexas.edu<mailto:eijkhout at tacc.utexas.edu>>
Subject: Re: World/session model
Date: 18 December 2020 at 15:58:20 GMT
To: HOLMES Daniel <d.holmes at epcc.ed.ac.uk<mailto:d.holmes at epcc.ed.ac.uk>>

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My cc to wg-sessions was rejected. I’m resending this in case it didn’t reach you.

If a program uses both the Sessions model and the World model

Thanks. At first I thought the two models were mutually exclusive but I have since found statements that they can be used together.

But then I find statements such as

7.2.4 When using the Sessions Model (Section 11.3) for initialization of MPI
re- sources, MPI_COMM_WORLD and MPI_COMM_SELF are not valid for use as
a communica- tor.

….a little very confusing. To me this sentence says that if I do anything with sessions, the use of MPI_COMM_WORLD is 100 percent ruled out. You mean to say: is ruled out for session-related activities. It certainly confused me.

Likewise, in 11.3 there is a very absolute sounding statement “MPI_COMM_WORLD is not valid for use as a communicator.” I’m sure that this sentence comes with an implicit qualification that it applies to using the Session model, but I’m not quite sure how.

While I’m nitpicking, 11.2.3 has an ambiguous sentence:

“For a library using got the World Model, it needs to know if MPI has been initialized using MPI_INIT  or MPI_INIT_THREAD”.

The wrong interpretation is “needs to know which of MPI_INIT / MPI_INIT_THREAD” has been used.

The correct interpretation (which really took me a minute) is “needs to know if MPI has been initialized using _either_of_ MPI_INIT / MPI_INIT_THREAD”

(Oh, and I think “For a library, it needs to know” is not grammatical. Either “A library needs to know” or “For a library, it may be necessary to know”)


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