On Wed, Apr 25, 2012 at 7:42 AM, Thomas Witkowski < thomas.witkowski at tu-dresden.de> wrote:
> Am 25.04.2012 13:21, schrieb Jed Brown: > > On Wed, Apr 25, 2012 at 02:21, Thomas Witkowski < > thomas.witkowski at tu-dresden.de> wrote: > >> 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? > > > 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. > > What is the circumstance where this occurs for you? > > Also some coarse space method. When not all ranks contribute to the coarse > space, a matrix defined on the coarse space leads to local empty matrices > on these ranks. So the question is if this leads to technical troubles in > PETSc. The share of nodes participating in the coarse space may vary. > No Matt > > Thomas > -- What most experimenters take for granted before they begin their experiments is infinitely more interesting than any results to which their experiments lead. -- Norbert Wiener -------------- next part -------------- An HTML attachment was scrubbed... URL: <http://lists.mcs.anl.gov/pipermail/petsc-users/attachments/20120425/ef392618/attachment.htm>
