[petsc-users] Poor multigrid convergence in parallel

[Lawrence Mitchell]

Hello all,

I'm implementing a multigrid solver using PCMG, starting with a simple Poisson equation (with strong boundary conditions) to ensure I'm doing things right.  Everything works fine in serial, but when running on two processes, with the default chebyshev smoother, convergence goes to pot, in particular, the preconditioner becomes indefinite.

I've determined that the operators I'm building on each level are indeed symmetric, furthermore, if I switch the smoother to jacobi (or sor) preconditioned richardson iterations then I get good convergence in both serial and parallel runs.  The eigenvalue estimates for the chebyshev smoother look plausible in both cases.  Any suggestions as to where to look next to figure out what's wrong in my code?  I can try and untangle it such that it demonstrates the problem standalone, but that may take a little time.

Cheers,

Lawrence
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