[mpi3-coll] Non-blocking Collectives Proposal Draft

[Supalov, Alexander]

Dear Rich,

Thank you. Note that the first operation is an Ibarrier. According to
Torsten's explanation, process 0 will block in the Wait, which it may
well do if it is Wait in process 1 that actually completes its Ibarrier.

Now, how can process 0 make progress in its Wait when there's no
matching Wait on the other side yet? Do we assume that any messages that
may have been sent by process 0's IBarrier will be handled as unexpected
messages by process 1 that is busy doing its (also unmatched) part of
the Bcast?

Likewise, do we assume that all messages that process 1 has to send in
its Ibarrier can leave process 1 before it enters its Wait? Aren't we
assuming a bit too much? In other words, aren't we assuming that all
message sends and receives will be started in Ibarrier, and will be able
to proceed even though the Wait is some blocking collective ops away?

Given the way most MPIs implement nonblocking pt2pt ops now, something
looks fishy here. In any case, I'd like to see a detailed explanation
how things will work, and moreover, a proof that this does not preclude
some algorithms - both in the sense of the pt2pt operations deployed,
and in the sense of collective exchange pattern, for the applicable
message sizes - from being used.

Best regards.

Alexander

-----Original Message-----
From: mpi3-coll-bounces at lists.mpi-forum.org
[mailto:mpi3-coll-bounces at lists.mpi-forum.org] On Behalf Of Graham,
Richard L.
Sent: Friday, October 17, 2008 11:57 AM
To: mpi3-coll at lists.mpi-forum.org
Subject: Re: [mpi3-coll] Non-blocking Collectives Proposal Draft

To me this does not look like a deadlock, proc o will make progress in
the wait, and just like with point-to-point commuications process 1 will
make general mpi progress while in the bcast.  This includes progress on
the ibcast so that process 0 can comete it's ibcast and move on to the
bcast.

Rich

----- Original Message -----
From: mpi3-coll-bounces at lists.mpi-forum.org
<mpi3-coll-bounces at lists.mpi-forum.org>
To: MPI-3 Collective Subgroup Discussions
<mpi3-coll at lists.mpi-forum.org>
Sent: Fri Oct 17 05:14:04 2008
Subject: Re: [mpi3-coll] Non-blocking Collectives Proposal Draft

Dear Torsten,

Thanks. I'm still unsure I understand how the example would work if
processes blocked in the Wait calls as you said below. In this case
process 0 would have no chance to reach the Bcast, while process 1 would
be inside it and never reached the Wait.

Here again is the original diagram:

Process 0			Process 1

MPI_Ibarrier(req)		MPI_Ibarrier(req)
MPI_Wait(req)*
MPI Bcast			MPI_Bcast*
				MPI_Wait(req)

The stars mark the point where the respective processes would block, as
far as I understand. Looks like deadlock?

Apart from this, stating that the nonblocking collectives can block in
Wait presumes a certain implementation. What if Ibarrier was able to do
all right away, and Wait were just a no-op?

I think mixing blocking and nonblocking collectives in the proposed way
should either be very carefully considered, or simply disallowed.

Best regards.

Alexander

-----Original Message-----
From: mpi3-coll-bounces at lists.mpi-forum.org
[mailto:mpi3-coll-bounces at lists.mpi-forum.org] On Behalf Of Torsten
Hoefler
Sent: Thursday, October 16, 2008 11:15 PM
To: MPI-3 Collective Subgroup Discussions
Subject: Re: [mpi3-coll] Non-blocking Collectives Proposal Draft

Hello Alexander,

> Thank you. The pt2pt progress comment below was taken into account
when
> you removed the nonblocking collective progress description.
ah, ok - so I think a reference to the chapter should be sufficient.

> As for the example, if it is legal, we should probably count the
> blocking operations as well when we decide how many simultaneous ops
are
> to be supported.
yes, maybe. I am not sure but this could be an issue. It's on the agenda
https://svn.mpi-forum.org/trac/mpi-forum-web/wiki/forum201008

> Why "if" above: I'm not sure what a delayed nonblocking barrier would
> mean for process 1. Won't one process possibly block somewhere? Can
you
> explain this please?
the barriers will only block in the respective MPI_Wait() (or a tight
MPI_Test() loop). But the operation can be initiated as soon as the
critical region is left (and there mihgt be some other independent
computation to do).

> The format matter is complicated. I think it is worth discussing at
the
> Forum as we're likely to have more and more proposals approaching the
> final stage, when editing them in PDF format will become an issue.
yes, it's on the WG agenda - maybe we should discuss this in the whole
forum?

Best,
  Torsten

-- 
 bash$ :(){ :|:&};: --------------------- http://www.unixer.de/ -----
Torsten Hoefler       | Postdoctoral Researcher
Open Systems Lab      | Indiana University    
150 S. Woodlawn Ave.  | Bloomington, IN, 474045, USA
Lindley Hall Room 135 | +01 (812) 855-3608
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