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Subject: Re: [OMPI users] Question about Asynchronous collectives
From: Jeff Squyres (jsquyres_at_[hidden])
Date: 2010-09-23 09:47:03

On Sep 23, 2010, at 6:28 AM, Gabriele Fatigati wrote:

> i'm studing the interfaces of new collective routines in next MPI-3, and i've read that new collectives haven't any tag.


> So all collective operations must follow the ordering rules for collective calls.

Also correct.

> From what i understand, this means that i can't use:
> MPI_IBcast(MPI_COMM_WORLD, request_1) // first Bcast
> MPI_IBcast(MPI_COMM_WORLD, request_2) // second Bcast

No, not quite right. You can have multiple outstanding ibcast's -- they'll just be satisfied in the same order in all participating MPI processes.

> but is it possible to do this:
> MPI_IBcast(MPI_COMM_WORLD, request_1) // first Bcast
> MPI_IReducet(MPI_COMM_WORLD, request_2) // othwer collective

Correct -- this is also possible.

More generally, you can have multiple outstanding non-blocking collectives on a single communicator -- it doesn't matter if they are the same or different collective operations. They will each be unique instances and will be satisfied in order.

Jeff Squyres
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