<div class="gmail_quote">On Wed, Apr 25, 2012 at 02:21, Thomas Witkowski <span dir="ltr"><<a href="mailto:thomas.witkowski@tu-dresden.de">thomas.witkowski@tu-dresden.de</a>></span> wrote:<br><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex">
Is it possible to create a parallel matrix (MPIAIJ) with empty size on some ranks? Even if this should work, does it make sense (with respect to runtime and efficiency) to collect all ranks having a nonempty contribution the the global matrix in an mpi group and to use this communicator for matrix create/computations?</blockquote>
</div><br><div>It depends how many ranks are empty. For coarse levels of multigrid (where this situation arises frequently), it's sometimes worthwhile to also move the data to a part of the machine that is topologically "close". Identifying "closeness" is not always easy because MPI does not necessarily expose this information in a useful fashion.</div>
<div><br></div><div>What is the circumstance where this occurs for you?</div>