[petsc-users] MatCreateMPIAIJ with zero size on some ranks

[Thomas Witkowski]

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 
> <mailto: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.

Thomas
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