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Subject: [OMPI devel] PLM consistency: priority
From: Ralph H Castain (rhc_at_[hidden])
Date: 2008-07-11 10:56:18

Okay, another fun one. Some of the PLM modules use MCA params to adjust
their relative selection priority. This can lead to very unexpected behavior
as which module gets selected will depend on the priorities of the other
selectable modules - which changes from release to release as people
independently make adjustments and/or new modules are added.

Fortunately, this doesn't bite us too often since many environments only
support one module, and since there is nothing to tell the user that the plm
module whose priority they raised actually -didn't- get used!

However, in the interest of "least astonishment", some of us working on the
RTE had changed our coding approach to avoid this confusion. Given that we
have this nice mca component select logic that takes the specified module -
i.e., "-mca plm foo" always yields foo if it can run, or errors out if it
can't - then the safest course is to remove MCA params that adjust module
priorities and have the user simply tell us which module they want us to

Do we want to make this consistent, at least in the PLM? Or do you want to
leave the user guessing? :-)