Bug#953116: 32 bit vs 64 bit PETSc

[Drew Parsons]

Hi, the Debian project is discussing whether we should start providing a 
64 bit build of PETSc (which means we'd have to upgrade our entire 
computational library stack, starting from BLAS and going through MPI, 
MUMPS, etc).

A default PETSc build uses 32 bit addressing to index vectors and 
matrices.  64 bit addressing can be switched on by configuring with 
--with-64-bit-indices=1, allowing much larger systems to be handled.

My question for petsc-maint is, is there a reason why 64 bit indexing is 
not already activated by default on 64-bit systems?  Certainly C 
pointers and type int would already be 64 bit on these systems.

Is it a question of performance?  Is 32 bit indexing executed faster (in 
the sense of 2 operations per clock cycle), such that 64-bit addressing 
is accompanied with a drop in performance? In that case we'd only want 
to use 64-bit PETSc if the system being modelled is large enough to 
actually need it. Or is there a different reason that 64 bit indexing is 
not switched on by default?

Drew