[petsc-users] Choosing VecScatter Method in Matrix-Vector Product

[Jed Brown]

Victor Eijkhout <eijkhout at tacc.utexas.edu> writes:

> On , 2020Jan22, at 09:11, Felix Huber <st107539 at stud.uni-stuttgart.de<mailto:st107539 at stud.uni-stuttgart.de>> wrote:
>
> weak scaling behaviour in a CG solver
>
> Norms and inner products have Log(P) complexity, so you’ll never get perfect weak scaling.

Allreduce is nearly constant time with hardware collectives on nice
networks.  The increased cost frequently observed is due to load
imbalance causing different processes to enter at different times.

https://www.mcs.anl.gov/~fischer/gop/