Hi guys, I am trying to narrow down an issue with my Poisson solver. I have the following problem setup
Laplace(f) = rhs(x,z,y) 0 <= x,y,z <= (Lx,Ly,Lz) I solve the Poisson equation in three dimensions with the analytical function f(x,y,z) defined by f(x,z,y) = cos(2*pi*x/Lx)*cos(2*pi*y/Ly)*cos(2*pi*z/Lz) + K where Lx = Ly =Lz = 1.0 and K is a constant I use to set f(Lx,Ly,Lz) = 0.0. Second order descritization is used for the Poisson equation. Also, Neumann boundary condition is used everywhere, but I set the top-right-front node's value to zero to get rid of the Nullspaced matrix manually. I use 20 grid points in each direction. The problem is: I use GMRES(20) without any preconditioners (rtol = 1e-12) to solve the linear system. It takes 77,000 iterations to converge!!!! For the size of only 8,000 unknowns, even though the lsys is not preconditioned, I guess that is a LOT of iterations. Next, I setup the exact same problem in MATLAB and use their GMRES solver function. I set the same parameters and MATLAB tells me that it converges using only 3870 iterations. I know that there might be some internal differences between MATLAB and PETSc's implementations of this method, but given the fact that these two solvers are not preconditioned, I am wondering about this big difference? Any ideas? Best, Mohamad -------------- next part -------------- An HTML attachment was scrubbed... URL: <http://lists.mcs.anl.gov/pipermail/petsc-users/attachments/20120107/b5f939c1/attachment.htm>
