I think we should default to ch3:nemesis when --download-mpich, and only do ch3:sock when requested (which we would do in CI). Satish Balay via petsc-dev <[email protected]> writes:

Primarily because ch3:sock performance does not degrade in oversubscribe mode - which is developer friendly - i.e on your laptop.

And folks doing optimized runs should use a properly tuned MPI for their setup anyway.

In this case --download-mpich-device=ch3:nemesis is likely appropriate if using --download-mpich [and not using a separate/optimized MPI]

Having defaults that satisfy all use cases is not practical.

Satish

On Wed, 22 Jul 2020, Matthew Knepley wrote:

...

We default to ch3:sock. Scott MacLachlan just had a long thread on the Firedrake list where it ended up that reconfiguring using ch3:nemesis had a 2x performance boost on his 16-core proc, and noticeable effect on the 4 core speedup.

Why do we default to sock?

Thanks,

Matt