[petsc-users] Size of off-diagonal matrix

[John Fettig]

On Thu, Jun 23, 2011 at 11:03 AM, Jed Brown <jed at 59a2.org> wrote:
> 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.

Thanks for the explanation, it makes sense.

John