joachim at sonne.lfbs.rwth-aachen.de
Sat May 4 01:10:16 PDT 2002
> If a benchmark measures process-to-process bandwidth and latency
> between only 2 processes, it matters whether or not they are on the
> same node (shmem) or not. It's not a matter of round robin vs. groups.
I need to refer to the disclaimer again: most interesting is not the p2p-
performance, as we all know, but the scaling behaviour. If you look at
the other PMB result Patrick has posted, it's obvious that these values
are very heavily dependand on the optimized setup of the hard- and software.
The scaling behaviour usually stays the same.
> BTW, groups _are_ strongly preferred, but that's not the issue. Why?
> If you have a program that does nearest neighbor communication, where
> are your neighbors? With round robin you are accessing your array in
> the wrong order.
Agreed - SCI-MPICH (i.e.) does "grouping" no matter in which order the
nodes are specified because it always makes sense (counter example?).
Don't know about the details of the ScaMPI mapping
algorithm - I'm not Scali...
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