[petsc-users] Size of off-diagonal matrix

[John Fettig]

On Thu, Jun 23, 2011 at 11:06 AM, John Fettig <john.fettig at gmail.com> wrote:
> 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.

Is there any way to access the map from global numbering to the
compressed numbering?

John