Given an index set
IS on each processor, generates a large index set (same on each processor) by concatenating together each processors index set.
is - the distributed index set
isout - the concatenated index set (same on all processors)
ISAllGather() is clearly not scalable for large index sets.
IS created on each processor must be created with a common
PETSC_COMM_WORLD). If the index sets were created
PETSC_COMM_SELF, this routine will not work as expected, since
each process will generate its own new
IS that consists only of