[petsc-users] Creating a larger matrix from submatrices

[Anush Krishnan]

Thanks, Jed. That was helpful.

On 9 November 2013 21:15, Jed Brown <jedbrown at mcs.anl.gov> wrote:

> Anush Krishnan <anush at bu.edu> writes:
> > Thank you very much! That worked.
>
> Great!
>
> > I also have a couple of general questions regarding this:
> >
> > 1. In this case, I followed your advice and created a single matrix G of
> > type MPIAIJ and assembled it. How do I decide if I should rather use
> Block
> > Matrices (MPIBAIJ) or MatGetLocalSubMatrix to set it up?
>
> BAIJ only does square blocks and you don't have block structurex to
> begin with (because you don't interlace the x- and y-components of the
> fields).
>
> MatGetLocalSubMatrix is a convenience when it makes sense to assembled
> blocks on their own.  That probably doesn't apply here because you don't
> have a reason to assemble, say, the gradient of pressure in the
> y-direction only.
>
> > 2. Various functions exist to help us access the correct indices when
> > required (e.g. DMDAGetAO, DMCompositeGetGlobalISs, DMDAGetCorners,
> > VecGetOwnershipRange, etc.) And sometimes, there are multiple ways by
> which
> > we can obtain the same indices. In such cases, are some of these
> functions
> > preferable to others (either because they are faster or need to allocate
> > less memory)?
>
> Many uses of AO are fairly heavy-weight because any global-to-local
> operation either needs non-scalable memory or a lookup structure that is
> slower than an array.
>
> DMDAGetCorners() is extremely cheap and doesn't use much memory, so if
> that is all you need, use that.  DMCompositeGetGlobalISs() is cheap on
> its own, but accessing the resulting ISs often boils down to an array of
> indices, which would mildly increase your memory bandwidth demands if
> you do it in a performance-sensitive traversal.  Usually these things
> are only needed during setup, at which point they are cheap as long as
> they are scalable.
>
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