On Aug 17, 2012, at 8:54 AM, Alexander Grayver <agrayver at gfz-potsdam.de>
wrote:
> On 17.08.2012 15:45, Jed Brown wrote:
>>
>> What I would like to do is very simple: every N iterations I want to take
>> current solution vector x_i from KSP and calculate:
>> x_i = x_i + c,
>> where c is a correction vector.
>>
>> How do you want to compute c?
>
> I was planning to do it inside ShellPCApply.
> The correction itself is calculated as a solution of Poisson equation with
> rhs being divergence of current density and current density is calculated
> based on x_i
A preconditioner step can only be written in the form c = B(b - A
x_i) where B is any linear operator and b is the right hand side and A is
the original operator. So if your calculation of c of this form (note that you
need not form B explicitly as a sparse matrix but it must be some linear
operator. From you description above "with rhs being divergence of current
density and current density is calculated based on x_i" I suspect it cannot be
written in this form.
The way to write this is
KSPSetInitialGuessNonzero(kspmain);
KSPSetTolerance(kspmain, set number of inner iterations you wish to use
before the "filtering step"
for j=1,?.. number outer iterations
KSPSolve(kspmain,b,x)
compute rhs via "rhs being divergence of current density and
current density is calculated based on x_i"
KSPSolve(ksppoisson,rhs,c)
x = x + c
endfor
I don't think you can, or want to "munge" it into some kind of single
complicated KSPSolve
> This trick is often applied to suppress spurious solution arising during
> solution of Maxwell equations and it works very well.
>
>> For a general KSP, this is not allowed because the preconditioner must be
>> constant. (You can do it anyway by checking the iteration number in your
>> PCShell, but it won't converge.) You can play these games with a flexible
>> method like FGMRES, for which you can also do an inner-outer, e.g.
>>
>> -ksp_type fgmres -pc_type composite -pc_composite_type multiplicative
>> -pc_composite_pcs ksp,shell -sub_0_ksp_ksp_type tfqmr -sub_0_ksp_ksp_max_it
>> 10 -sub_0_ksp_pc_type ilu
>
> Looks a bit tricky. The number of fgmres iterations then defines number of
> the cycles correction will be applied, doesn't it?
>
>
> --
> Regards,
> Alexander
>
