[petsc-users] Some tricky problem in my multilevel feti dp code

[Thomas Witkowski]

I have some problem in the implementation of my multilevel FETI DP code, 
where two block structured matrices must be multiplied. I'll give my 
best to explain the problem, may be one of you have an idea how to 
implement it. I think, the best is to make a small example: lets assume 
we have 16 subdomains, uniformly subdividing a unit square. Each of the 
subdomain matrices is purely local, thus they have the communicator 
PETSC_COMM_SELF. Each of them is of size n x n. There is a coarse grid 
matrix, with communicator PETSC_COMM_WORLD and of size m x m. The 
coupling matrices between the global coarse grid and the local matrices 
are also global, so they are of size 16n x m and m x 16n, respectively. 
So far, everything is fine and works perfectly. Now I introduce four 
"local coarse grids", each of them couples four local subdomains, and is 
defined on a subset communicator of PETSC_COMM_WORLD. Say, each "local 
coarse grid" matrix is of size p x p, and there are also coupling 
matrices of size 4n x p and p x 4n. Now I have to perform a MatMatMult 
of the local coarse coupling matrices p x 4n with the global coupling 
matrix 16n x m. So the final matrix is of size 4p x m. But I cannot 
perform the MatMatMult, as the matrix sizes do not fit and the 
communicators are not compatible.

Is it possible to understand, what I want to do? :) Any idea, how to 
implement it?

Thomas