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Subject: [OMPI devel] Change in default MCA parameter value
From: Ralph Castain (rhc_at_[hidden])
Date: 2014-03-03 11:48:56

Heads up...

 (In [30909]) Now that we are binding by default, the issue of #slots and what to do when oversubscribed has become a bit more complicated. This isn't a problem in managed environments as we are always provided an accurate assignment for the #slots, or when -host is used to define the allocation since we automatically assume one slot for every time a node is named.

The problem arises when a hostfile is used, and the user provides host names without specifying the slots= paramater. In these cases, we assign slots=1, but automatically allow oversubscription since that number isn't confirmed. We then provide a separate parameter by which the user can direct that we assign the number of slots based on the sensed hardware - e.g., by telling us to set the #slots equal to the #cores on each node. However, this has been set to "off" by default.

In order to make this a little less complex for the user, set the default such that we automatically set #slots equal to #cores (or #hwt's if use_hwthreads_as_cpus has been set) only for those cases where the user provides names in a hostfile but does not provide slot information.