Hi all, I'm writing an algorithm using PETSc that requires part of an (N x k) matrix, where k << N. Specifically, my algorithm loops from j = 1 to k, and uses j columns of the matrix of a time. I've been creating a new matrix for each iteration, and use MatGetColumnVector to get the values for each column vector, scatter them into a new vector, and then use MatSetValue to populate my matrix. MatGetColumnVector is very slow, and I was thinking of using MatCreateSubMatrix. However, I'm not entirely sure how to get the correct IS that would allow me to create a submatrix using the first j columns of the matrix at a time. I'm running in parallel, and the matrix is an MPIAIJ matrix. I've looked at examples of MatCreateSubMatrix as well as examples including the use of IS, but I'm still not very sure how to use them.
Thanks, Rakesh
