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
IBM Systems & Technology Group
Dept X2ZA / MS P963 -- 2455 South Road -- Poughkeepsie, NY 12601
Tele (845) 433-7846 Fax (845) 433-8363
firstname.lastname@example.org wrote on 08/23/2010 05:09:56
> [image removed]
> Re: [OMPI users] IMB-MPI broadcast test stalls for large core
> counts: debug ideas?
> Rahul Nabar
> Open MPI Users
> 08/23/2010 05:11 PM
> Sent by:
> Please respond to Open MPI Users
> On Sun, Aug 22, 2010 at 9:57 PM, Randolph Pullen
> > wrote:
> Its a long shot but could it be related to the total data volume ?
> ie 524288 * 80 = 41943040 bytes active in the cluster
> Can you exceed this 41943040 data volume with a smaller message
> repeated more often or a larger one less often?
> Not so far, so your diagnosis could be right. The failures have been
> at the following data volumes:
> Unfortunately, I'm not sure I can change the repeat rate with the
> OFED/MPI tests. Can I do that? Didn't see a suitable flag.
> In any case, assuming it is related to the total data volume what
> could be causing such a failure?
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