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Subject: Re: [OMPI users] OpenMPI MPI_Bcast Algorithms
From: neeraj_at_[hidden]
Date: 2009-04-28 06:35:22


Hi Axida,

        There are two ways you can call MPI_collectives.
        1) Automatic decision by OpenMPI which in turn call tuned
collectives
        2) Forced decision, where you can override OpenMPI to call certain
algorithms available for collective say MPI_Bcast.

        The logic for 1. can be found in function
ompi_coll_tuned_bcast_intra_dec_fixed at (file
:ompi/mca/coll/tuned/coll_tuned_decision_fixed.c , line 221, version
openmpi-1.3.2).
        By default, openmpi switches algo based on message size,
communicator size which they have put based on some heuristics which may
differ from one network topology to another.

        In that case, i would suggest you to run benchmark programs with
2. option and fine tune the MPI Collectives suited for your cluster
architecture.
Regards

Neeraj Chourasia
Member of Technical Staff
Computational Research Laboratories Limited
(A wholly Owned Subsidiary of TATA SONS Ltd)
P: +91.9225520634

shan axida <axida2009_at_[hidden]>
Sent by: users-bounces_at_[hidden]
04/28/2009 03:25 PM
Please respond to
Open MPI Users <users_at_[hidden]>

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Subject
[OMPI users] OpenMPI MPI_Bcast Algorithms

Hi all,
I think there are several algorithms used in MPI_Bcast.
I am wondering how are they decided to be excuted ?
I mean, How to decide which algorithm will be used?
Is it depending on the message size or something ?
Would some people help me?

Thank you!

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