Dolphin Wulfkit

[Greg Lindahl]

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
unfortunate.

greg