[petsc-users] gamg failure with petsc-dev

[Jed Brown]

Stephan Kramer <s.kramer at imperial.ac.uk> writes:
> Yes indeed. I've come to realize this now by looking into how smoothed
> aggregation with a near null space actually works. We currently have
> our dofs numbered the wrong way around (vertices on the inside,
> velocity component on the outside - which made sense for other eqns we
> solve with the model) so will take a bit of work, but might well be
> worth the effort

The memory streaming and cache reuse is much better if you interlace the
degrees of freedom.  This is as true now as it was at the time of the
PETSc-FUN3D papers.  When evaluating the "physics", it can be useful to
pack the interlaced degrees of freedom into a vector-friendly ordering.

The AMG solve is plenty expensive that you can pack/solve/unpack an
interlaced vector at negligible cost without changing the rest of your
code.

Mark, should we provide some more flexible way to label "fields"?  It
will be more complicated than the present code and I think packing into
interlaced format is faster anyway.
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