[petsc-users] Size of off-diagonal matrix

[Jed Brown]

On Thu, Jun 23, 2011 at 16:59, John Fettig <john.fettig at gmail.com> wrote:

> I understand everything you say here except what "compressing" means.
> It is compressed by virtue of being in CSR format, but then you do
> some additional mapping with the column indices that further
> compresses it? What purpose does changing the column indices serve?
>

It operates on the dense vector consisting of only those entries on other
processors that interact with the current one. If this thing operated on the
full global vector, there would either need to be a matrix format that could
operator on "sparse vectors" (i.e. an additional layer of indirection) or
the whole dense global vector would need to be gathered to every process
which would be horribly non-scalable.
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