The primary reason is for users - developing on laptops/desktop and doing development runs in oversubscribed mode. The choice was few percent loss in performance for sock vs exponential cost for oversubscribed usage of nemesis [so we defaulted to sock]. I think we should preserve this behavior for at-least the debug builds. [i.e switch only optimized builds to nemesis] In CI we do pay this extra cost for some of the builds [that explicitly test with wemesis, openmpi etc..] Satish On Thu, 23 Jul 2020, Jed Brown wrote:

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:

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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:

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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