[petsc-users] -ksp_diagonal_scale

[Mohamad M. Nasr-Azadani]

Hi guys,

Three questions:
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?

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?

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.

Thanks,
Best,
Mohamad
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