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Subject: [OMPI devel] PML selection logic
From: Ralph H Castain (rhc_at_[hidden])
Date: 2008-06-23 12:36:39


Yo all

I've been doing further research into the modex and came across something I
don't fully understand. It seems we have each process insert into the modex
the name of the PML module that it selected. Once the modex has exchanged
that info, it then loops across all procs in the job to check their
selection, and aborts if any proc picked a different PML module.

All well and good...assuming that procs actually -can- choose different PML
modules and hence create an "abort" scenario. However, if I look inside the
PML's at their selection logic, I find that a proc can ONLY pick a module
other than ob1 if:

1. the user specifies the module to use via -mca pml xyz or by using a
module specific mca param to adjust its priority. In this case, since the
mca param is propagated, ALL procs have no choice but to pick that same
module, so that can't cause us to abort (we will have already returned an
error and aborted if the specified module can't run).

2. the pml/cm module detects that an MTL module was selected, and that it is
other than "psm". In this case, the CM module will be selected because its
default priority is higher than that of OB1.

In looking deeper into the MTL selection logic, it appears to me that you
either have the required capability or you don't. I can see that in some
environments (e.g., rsh across unmanaged collections of machines), it might
be possible for someone to launch across a set of machines where some do and
some don't have the required support. However, in all other cases, this will
be homogeneous across the system.

Given this analysis (and someone more familiar with the PML should feel free
to confirm or correct it), it seems to me that this could be streamlined via
one or more means:

1. at the most, we could have rank=0 add the PML module name to the modex,
and other procs simply check it against their own and return an error if
they differ. This accomplishes the identical functionality to what we have
today, but with much less info in the modex.

2. we could eliminate this info from the modex altogether by requiring the
user to specify the PML module if they want something other than the default
OB1. In this case, there can be no confusion over what each proc is to use.
The CM module will attempt to init the MTL - if it cannot do so, then the
job will return the correct error and tell the user that CM/MTL support is
unavailable.

3. we could again eliminate the info by not inserting it into the modex if
(a) the default PML module is selected, or (b) the user specified the PML
module to be used. In the first case, each proc can simply check to see if
they picked the default - if not, then we can insert the info to indicate
the difference. Thus, in the "standard" case, no info will be inserted.

In the second case, we will already get an error if the specified PML module
could not be used. Hence, the modex check provides no additional info or
value.

I understand the motivation to support automation. However, in this case,
the automation actually doesn't seem to buy us very much, and it isn't
coming "free". So perhaps some change in how this is done would be in order?

Ralph