lindahl at keyresearch.com
Sat May 4 11:02:10 PDT 2002
On Sat, May 04, 2002 at 02:43:52PM +0200, Steffen Persvold wrote:
> Well ScaMPI does "grouping" too I guess. Examples :
> # /opt/scali/bin/mpimon someapp -- node1 node2 nnode1 node2
> Here rank 0 and 2 would run on node1 but when they are communicating
> with eachother shm is used, same with rank 1 and 3 on node2.
This is "round robin", not grouping. The question was about your MPI's
default policy. The correct default policy is to put ranks 0 and 1 on
the same node, not 0 and 2, because that keeps nearest neighbor codes
happy even if they don't call the MPI cartesian routines...
And then if you do point-to-point benchmarking, it's important to make
sure you know what you're measuring.
> AFAIK the PMB benchmark uses rank 0 and 1 when doing ping-ping and
> ping-pong tests,
So a "round robin" default breaks PMB if you aren't careful. Which is
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