[petsc-users] coding VecMDot_Seq as gemv

[Kokron, Daniel S. (ARC-606.2)[CSRA, LLC]]

I am looking into ways to improve performance of the VecMDot_Seq routine.  I am focusing on the variant that gets called when PETSC_THREADCOMM_ACTIVE and PETSC_USE_FORTRAN_KERNEL_MDOT are NOT defined.

My current version of PETSc is 3.4.5 due solely to user requirement.  I am linking against MKL.

I tried and failed to implement VecMDot_Seq as a call to cblas_dgemv in ~/mpi/pvec2.c

cblas_dgemv(CblasRowMajor, CblasNoTrans, nv, n, 1., b, n, xbase, 1, 0., work, 1);

I could not figure out a way to extract the vectors from 'Vec y[]' and store them as rows of an allocated array.

This user post starts off with a similar request (how to construct a matrix from many vectors)
https://lists.mcs.anl.gov/pipermail/petsc-users/2015-August/026848.html

I understand that this sort of memory shuffling is expensive.  I was just hoping to prove the point to myself that it's possible.

The action performed by VecMDot_Seq is the same as matrix-vector multiplication, so I was wondering why it wasn't implemented as a call ?gemv?

Daniel Kokron
NASA Ames (ARC-TN)
SciCon group
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