Sorry, I had a mistake in calculation.
Not 131072 (double) but 131072 KB.
It means around 128 MB.
 


From: Jeff Squyres <jsquyres@cisco.com>
To: Open MPI Users <users@open-mpi.org>
Sent: Thursday, April 23, 2009 8:23:52 PM
Subject: Re: [OMPI users] MPI_Bcast from OpenMPI

Very strange; 6 seconds for a 1MB broadcast over 64 processes is *way* too long.  Even 2.5 sec at 2MB seems too long -- what is your network speed?  I'm not entirely sure what you mean by "4 link" on your graph.

Without more information, I would first check your hardware setup to see if there's some kind of network buffering / congestion issue occurring.  Here's a total guess: your ethernet switch(es) are low quality (from an HPC perspective, at least) such that you're incurring congestion and/or retransmission at that size for some reason.

You could also be running up against memory bus congestion (I assume you mean 4 cores per node; are they NUMA or UMA?).  But that wouldn't account for the huge spike at 1MB.


On Apr 23, 2009, at 1:32 AM, shan axida wrote:

> Hi,
> One more question:
> I have executed the MPI_Bcast() in 64 processes in 16 nodes Ethernet multiple links cluster.
> The result is shown in the file attached on this E-mail.
> What is going on at 131072 double message size?
> I have executed it many times but the result is still the same.
>
> THANK YOU!
>
>
>
>
>
>
>
> <openmpi.pdf>_______________________________________________
> users mailing list
> users@open-mpi.org
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--Jeff Squyres
Cisco Systems

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