[petsc-users] Time cost by Vec Assembly

[Jed Brown]

Barry Smith <bsmith at mcs.anl.gov> writes:
>     There is still something wonky here, whether it is the MPI implementation or how PETSc handles the assembly. Without any values that need to be communicated it is unacceptably that these calls take so long. If we understood __exactly__ why the performance suddenly drops so dramatically we could perhaps fix it. I do not understand why.

I guess it's worth timing.  If they don't have MPI_Reduce_scatter_block
then it falls back to a big MPI_Allreduce.  After that, it's all
point-to-point messaging that shouldn't suck and there actually
shouldn't be anything to send or receive anyway.  The BTS implementation
should be much smarter and literally reduces to a barrier in this case.
-------------- next part --------------
A non-text attachment was scrubbed...
Name: signature.asc
Type: application/pgp-signature
Size: 800 bytes
Desc: not available
URL: <http://lists.mcs.anl.gov/pipermail/petsc-users/attachments/20161007/c67668de/attachment.pgp>