[petsc-users] convergence with PCLU

[Ataollah Mesgarnejad]

Dear all,

I was running an small problem (~9000 unknowns) and I decided to try the direct solver with PCLU. What is puzzling me is that when it converges if I check the residual with KSPGetResidualNorm it gives me zero but when I calculate the residual myself ( ||r||:=||Ax-b||) it gives me ||r||~8.8e-6? -I know my matrix is ill-conditioned (I'm solving a very bad 4th order problem) and I get a poor convergence with gmres.-  Is there a remedy for this or I should forget using direct solvers all together.

Thanks for your time,
Ata