[petsc-dev] DMGetLocalToGlobalMapping(), can it be collective

[Jed Brown]

I would like to make DMDAGetISLocalToGlobalMapping() DM-level functionality
since it is an essential ingredient in the MatGetLocalSubMatrix-based
assembly, but it's not an essential part of how every conceivable DM
functions.  I would rather allocate that memory lazily so that not every
user is required to pay for it (e.g. when they don't even have a matrix),
but creating an ISLocalToGlobalMapping is a collective operation
(somewhat strangely, because only the rarely used inversion of the map is a
collective operation).  This means that either DMGetLocalToGlobalMapping
becomes a collective operation, or everyone needs to pay for it to exist?
 Thoughts?

Jed
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