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