[petsc-users] questions about parallel vectors

[Shri]

----- Original Message -----
> Hello everyone,
> 
> I am trying to create a sparse matrix A, which depends on a parallel
> vector v. For example, my function looks like this: Mat
> myfun(MPI_Comm comm, Vec v, other parameters). When I set the value
> A(i,j) = v[k], v[k] may not be obtained by VecGetValues since that
> operation can only get values on the same processor. I am thinking
> 
> 1) create v as an array and pass this array into myfun.
> 2) create another vector v2, which is a full copy of parallel v
> through VecScatter.

Do you need all the vector elements on each processor to set the matrix values or just a subset of them? '
If you need all the vector elements then you can use VecScattertoAll
http://www.mcs.anl.gov/petsc/petsc-as/snapshots/petsc-current/docs/manualpages/Vec/VecScatterCreateToAll.html
If you only need a subset then you could create v as a ghosted vector. See the example
http://www.mcs.anl.gov/petsc/petsc-as/snapshots/petsc-current/src/vec/vec/examples/tutorials/ex9.c.html

> 3) when I first create the initial vec v, using
> VecCreate(PETSC_COMM_SELF,v) or VecCreateSeq. Does this guarantee that
> all the processors creating matrix A have all the components of vector
> v?
> 
> I think 1) and 2) are going to work, but not sure about option 3). I
> have no idea which would have better performance. Can you give me some
> suggestions on how to handle this problem? Thanks.
> 
> Another quick question, what is the difference between
> PetscViewerSetFormat and PetscViewerPushFormat?
> 
> Best,
> Xiangdong