[petsc-users] Question -with-64-bit-blas-indices

[Smith, Barry F.]

> On Oct 10, 2019, at 12:38 PM, Randall Mackie via petsc-users <petsc-users at mcs.anl.gov> wrote:
> 
> With the release of PETSc 3.12, there is a new option mentioned in the list of changes:
> 
> Added --with-64-bit-blas-indices that will switch to 64 bit indices when using MKL libraries for BLAS/LAPACK and build OpenBLAS with this support
> 
> Is this option only for use when compiling PETSc with 64 bit indices and also using MKL?

   No, you can use it with 32 bit indices, though there is likely no reason to since if the integers in PETSc are 32 bit there is no chance the BLAS/LAPACK integer need to be larger than 64 bit integers.

> We’ve used 64 bit PETSc in the past in conjunction with MKL and I”m curious what if anything has changed.

   By default nothing has changed. We use to link with MKL blas/lapack that uses 32 bit integers, we will still do that by default. If you pass --with-64-bit-blas-indices then it links with 64 bit integer MKL. 

   The only reason you need it is if calls to BLAS/LAPACK have such large integers that they cannot fit in 32 bit integers for BLAS/LAPACK. You would have gotten errors if you had this problems, since you didn't get any errors you didn't need 64 bit integer BLAS/LAPACK.


> 
> Thanks,
> 
> Randy M.