[petsc-users] Parallel matrix-vector product with Matrix Shell

[Mario Rossi]

Thanks a lot, Jose!
I looked at the eps folder (where I found the test8.c that has been my
starting point) but I did not look at the nep folder (my fault!)
Thanks again,
Mario

Il giorno ven 17 giu 2022 alle ore 17:34 Jose E. Roman <jroman at dsic.upv.es>
ha scritto:

> You can use VecGetOwnershipRange() to determine the range of global
> indices corresponding to the local portion of a vector, and VecGetArray()
> to access the values. In SLEPc, you can assume that X and Y will have the
> same parallel distribution.
>
> For an example of a shell matrix that implements the matrix-vector product
> in parallel, have a look at this:
> https://slepc.upv.es/documentation/current/src/nep/tutorials/ex21.c.html
> It is a simple tridiagonal example, where neighborwise communication is
> done with two calls to MPI_Sendrecv().
>
> Jose
>
>
> > El 17 jun 2022, a las 17:21, Mario Rossi <dfatiac at gmail.com> escribió:
> >
> > I need to find the largest eigenvalues (say the first three) of a very
> large matrix and I am using
> > a combination of PetSc and SLEPc. In particular, I am using a shell
> matrix. I wrote a "custom"
> > matrix-vector product and everything works fine in serial (one task)
> mode for a "small" case.
> > For the real case, I need multiple (at least 128) tasks for memory
> reasons so I need a parallel variant of the custom matrix-vector product. I
> know exactly how to write the parallel variant
> > (in plain MPI) but I am, somehow, blocked because it is not clear to me
> what each task receives
> > and what is expected to provide in the parallel matrix-vector product.
> > More in detail, with a single task, the function receives the full X
> vector and is expected to provide the full Y vector resulting from Y=A*X.
> > What does it happen with multiple tasks? If I understand correctly
> > in the matrix shell definition, I can choose to split the matrix into
> blocks of rows so that the matrix-vector function should compute a block of
> elements of the vector Y but does it receive only the corresponding subset
> of the X (input vector)? (this is what I guess happens) and in output, does
> > each task return its subset of elements of Y as if it were the whole
> array and then PetSc manages all the subsets? Is there anyone who has a
> working example of a parallel matrix-vector product for matrix shell?
> > Thanks in advance for any help you can provide!
> > Mario
> > i
> >
>
>
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