[petsc-users] -ksp_diagonal_scale

[Alexander Grayver]

Hi,

Sorry for small offtopic.
I would also be interested more to know how it scales system? I mean 
what is idea behind this scaling.

Regards,
Alexander

On 23.12.2011 03:50, Mohamad M. Nasr-Azadani wrote:
> 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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