[mpich-discuss] Hydra: prefer localhost instead of first in host file

[Torquil Macdonald Sørensen]

Hi!

I'm in an environment with multiple computers sharing the same filesystem.

I'm using the latest Mpich2 with hydra and a host file that looks like

hostA:4
hostB:4
hostC:4

The host file is the same on all the computers, because it is on a shared 
filesystem.

Sometimes I want to run with 12 processes and in that case this works fine.

However, sometime I want only 4 processes, and I would like them to run on the 
computer I'm currently logged into (not always hostA). E.g. I want "mpiexec -n 4 
prog.exe" to start 4 processes on hostB if I run this command from hostB.

That way, I can e.g. run three different simulations, each of 4 processes, each 
on its own computer, just by executing "mpiexec -n 4 prog.exe" on each 
individual computer.

With earlier versions of Mpich2 this seemed to work as I wished (not using Hydra).

Is there a way of making Mpich2 prefer to start the first 4 process on the local 
machine instead of starting them on the computer listed first in the host file?

I'v been reading the Hydra documentation, obviously without finding out if this 
can be accomplished.

Best regards
Torquil Sørensen