[petsc-dev] [petsc-users] MatMultTranspose memory usage

[Jed Brown]

"Smith, Barry F. via petsc-users" <petsc-users at mcs.anl.gov> writes:

>    The reason this worked for 4 processes is that the largest count in that case was roughly 6,653,750,976/4 which does fit into an int. PETSc only needs to know the number of nonzeros on each process, it doesn't need to know the amount across all the processors. In other words you may want to use a different PETSC_ARCH (different configuration) for small number of processors and large number depending on how large your problem is. Or you can always use 64 bit integers at a little performance and memory cost.

We could consider always using 64-bit ints for quantities like row
starts, keeping column indices (the "heavy" part) in 32-bit.  This may
become a more frequent issue with fatter nodes and many GPUs potentially
being driven by a single MPI rank.