Sorry, I had a mistake in calculation.
Not 131072 (double) but 131072 KB.
It means around 128 MB.
From: Jeff Squyres <jsquyres_at_[hidden]>
To: Open MPI Users <users_at_[hidden]>
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:
> 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!
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