[petsc-users] Possibilities to VecScatter to a sparse Vector-Format

[Matthew Knepley]

On Thu, Sep 30, 2021 at 9:39 AM Hannes Phil Niklas Brandt <
s6hsbran at uni-bonn.de> wrote:

> Hello,
>
>
>
> I intend to compute a parallel Matrix-Vector-Product (via MPI) and
> therefore would like to scatter the entries of the input MPI-Vec v to a
> local vector containing all entries relevant to the current process.
>
>
>
> To achieve this I tried defining a VecScatter, which scatters from v to a
> sequential Vec v_seq (each process has it's own version of v_seq). However,
> storing v_seq (which has one entry for each global row, thus containing a
> large amount of zero-entries) may demand too much storage space (in
> comparison to my data-sparse Matrix-Storage-Format).
>
>
>
> I am interested in possibilities to scatter v to a sparse Vec-type to
> avoid storing unnecessary large amounts of zero-entries. Is there a sparse
> Vector format in Petsc compatible to the VecScatter procedure or is there
> another efficient way to compute Matrix-Vector-Products without usinglarge
> amounts of storage space on each process?
>
I think you misunderstand VecScatter. Parallel to sequential is one
possibility, but also parallel-parallel, seq-parallel, etc. Second, you can
give
whatever indices you want into it. Thus you can index only a few places in
a large array, or compact a sparse array into a contiguous one. I am
not sure what other possibilities may exist.

  Thanks,

      Matt

> Best Regards
>
> Hannes Brandt
>
>

-- 
What most experimenters take for granted before they begin their
experiments is infinitely more interesting than any results to which their
experiments lead.
-- Norbert Wiener

https://www.cse.buffalo.edu/~knepley/ <http://www.cse.buffalo.edu/~knepley/>
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