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Subject: Re: [OMPI users] IMB-MPI broadcast test stalls for large core counts: debug ideas?
From: Randolph Pullen (randolph_pullen_at_[hidden])
Date: 2010-08-23 21:39:29


I have had a similar load related problem with Bcast.  I don't know what caused it though.  With this one, what about the possibility of a buffer overrun or network saturation?

--- On Tue, 24/8/10, Richard Treumann <treumann_at_[hidden]> wrote:

From: Richard Treumann <treumann_at_[hidden]>
Subject: Re: [OMPI users] IMB-MPI broadcast test stalls for large core counts: debug ideas?
To: "Open MPI Users" <users_at_[hidden]>
Received: Tuesday, 24 August, 2010, 9:39 AM

It is hard to imagine how a total data load of 41,943,040 bytes could be a problem. That is really not much data. By the time the BCAST is done, each task (except root) will have received a single half meg message form one sender. That is not much.

IMB does shift the root so some tasks may be in iteration 9 while some are still in iteration 8 or 7 but a 1/2 meg message should use rendezvous protocol so no message will be injected into the network until the destination task is ready to receive it.

Any task can be in only one MPI_Bcast at a time so the total active data cannot ever exceed the 41,943,040 bytes, no matter how fast the MPI_Bcast loop tries to iterate.

(There are MPI_Bcast algorithms that chunk the data into smaller messages but even with those algorithms, the total concurrent load will not exceed 41,943,040 bytes.)

Dick Treumann  -  MPI Team          

users-bounces_at_[hidden] wrote on 08/23/2010 05:09:56 PM:

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> On Sun, Aug 22, 2010 at 9:57 PM, Randolph Pullen <randolph_pullen_at_[hidden]

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