Problem with iterative solver for linear equation (bicgstab, gmres) versus mldivide

Hi

I have to solve a large sparse linear equation system for a system of PDEs using FEM.

So far, I just used backslash \ (mldivide) to solve the system which works fine. I now also tried to use iterative algorithms such as bicgstab or gmres. These algorithms do not work at all (bicgstab) or take way more time (gmres) than mldivide. Even though I use the preconditioner proposed in the respective Matlab docs.

I have no clue how to make these solvers work and I'm happy with any hint.

Remarks: - The matrix is square, but not symmetric. - I plan to use iterative solvers because I need to solve even larger systems on large parallel computers where an iterative solution is strongly recommended. Probably, I won't be able to store the matrix then but just provide a matrix vector product.

Thanks a lot, Michael