On Thu, Dec 22, 2011 at 20:50, Mohamad M. Nasr-Azadani <mmnasr at gmail.com>wrote:
> 1) As mentioned on the web when we use > *-ksp_diagonal_scale_fix: *scale the matrix back AFTER the solve > > with -ksp_diagonal_scale does it also scale back the RHS vector > associated with the KSP solver? > yes > > 2) Also, as mentioned on the web for -ksp_diagonal_scale > "Tells > KSP<http://www.mcs.anl.gov/petsc/petsc-3.1/docs/manualpages/KSP/KSP.html#KSP> > to > symmetrically diagonally scale the system before solving. " > > When I used this option to solve for the Pressure (Poisson equation with > Neumann B.C., descritized via structured fisytenite-difference method), the > number of iterations for convergence was trippled, e.g. without this > option, it converges in 18 iteration, but with this option it goes up to > 50-60 iterations. > (GMRES + BoomerAMG as the preconditioner). > Is this something you would expect? > It's perhaps slightly surprising that it's that much worse, but there is nothing saying that the diagonal scaled matrix will be better. Most preconditioners correct the diagonal scaling. > > 3) How could the linear system resulting from the Poisson equation for > pressure (incompressible flow) can be more diagonally scaled? To elaborate > more, for the uniform grid case and let's say a second order central scheme > finite difference discretization, the sum of the off-diagonal coefficients > are equal to the diagonal coefficient. > You probably don't have a reason to use diagonal scaling. -------------- next part -------------- An HTML attachment was scrubbed... URL: <http://lists.mcs.anl.gov/pipermail/petsc-users/attachments/20111223/fbe3dc46/attachment.htm>
