Bug#953116: [petsc-maint] 32 bit vs 64 bit PETSc

[Drew Parsons]

On 2020-05-23 23:45, Satish Balay wrote:
> 
> One more issue: Most externalpackages don't support 64bit-indices.
> 
> Note: OpenBLAS supports 64bit indices. MKL has bunch of packages built 
> as ILP64
> 
> [MPICH/OpenMPI - as far as I know is LP64]
> 
> 
> The primary reason PETSc defaults to 32bit indices is - this is the
> compiler default on LP64 systems.
> 
> If debian is building ILP64 system [with compilers defaulting to
> 64-bit integers] - that would mean all packages would be ILP64
> [obviously most packages are not tested in this mode - so might break]
> 
> --with-64-bit-indices is the option for 64-bit-indices on LP64 systems
> which most packages don't support - hence the usage of PetscBLASInt
> PetscMPIInt etc.
> 
> If I remember correctly - the 'full 64-bit' mode relies on fortran 
> compiler
> option '-i8' - which is basally equivalent to ILP64 - and this mode 
> only works
> with ILP64 MPI, BLAS etc from Intel-MPI/MKL


If I understand correctly, the Debian systems are LP64 (so gcc defaults 
to int=int32_t).
Our user who started these discussions with Bug#953116 reports that 
--with-64-bit-indices is working fine for his local build. But he may 
not have tested using MUMPS in 64-bit PETSc.

> We haven't tried using MUMPS in this mode with PETSc

This will be the interesting test. I'll start with the 64-bit build of 
MUMPS and see how tests hold up.

Drew