[petsc-dev] new P^1.5 algorithm in VecAssembleBegin?

Jed Brown jed at jedbrown.org
Fri May 29 10:32:42 CDT 2015

Mark Adams <mfadams at lbl.gov> writes:

> I don't need your branch because 1) we are not doing any communication in
> this VecAssembly 

There is still synchronization cost in determining that nothing needs to
be done.  Moreover, using a size P MPI_Allreduce for that (in
PetscMaxSum) is non-scalable.  My branch uses MPI_Reduce_scatter_block
(scalable) when available (MPI-2.2).

> and I added the IGNORE stuff

So it takes no time now and everything is groovy?
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