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 <[email protected]> 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 >
