[petsc-dev] Question about MPICH device we use

[Satish Balay]

On Thu, 23 Jul 2020, Jeff Hammond wrote:

> Open-MPI refuses to let users over subscribe without an extra flag to
> mpirun.

Yes - and when using this flag - it lets the run through - but there is still performance degradation in oversubscribe mode.

> I think Intel MPI has an option for blocking poll that supports
> oversubscription “nicely”.

What option is this? Is it compile time option or something for mpiexec?

Satish

> MPICH might have a “no local” option that
> disables shared memory, in which case nemesis over libfabric with the
> sockets or TCP provider _might_ do the right thing. But you should ask
> MPICH people for details.
> 
> Jeff
> 
> On Thu, Jul 23, 2020 at 12:40 PM Jed Brown <jed at jedbrown.org> 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 <petsc-dev at mcs.anl.gov> 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
> > >>
> > >>
> >
>