[petsc-users] questions about pc_comosite

[Xiangdong]

Hi Barry,

Can you point me the documentation for understanding these iteration
patterns? Thanks a lot.

Best,
Xiangdong

On Fri, Sep 29, 2017 at 11:24 AM, Barry Smith <bsmith at mcs.anl.gov> wrote:

>
>   They are all different iteration patterns. They definitely should
> present different residual results
>
>
> > On Sep 29, 2017, at 8:19 AM, Xiangdong <epscodes at gmail.com> wrote:
> >
> > Hello everyone,
> >
> > I have a questions about residuals reported in pc_composite. For the
> examples in petsc-3.7.6/src/ksp/ksp/examples/tutorials/ex1.c, I found
> that when I use these three options:
> >
> > 1) -pc_type none
> > 2)  -pc_type composite -pc_composite_pcs none,none -pc_composite_type
> multiplicative
> > 3) -pc_type composite -pc_composite_pcs none,none -pc_composite_type
> multiplicative -ksp_norm_type unpreconditioned
> >
> > as shown below, it reported different KSP residuals. Given that I use
> the identity preconditioner (with multiplicative), why does the residual
> vary for different options?
> >
> > Thanks.
> >
> > Best,
> > Xiangdong
> >
> > mpirun -np 1 ./extest -ksp_monitor -pc_type none
> >
> > 0 KSP Residual norm 1.414213562373e+00
> >   1 KSP Residual norm 6.324555320337e-01
> >   2 KSP Residual norm 3.779644730092e-01
> >   3 KSP Residual norm 2.581988897472e-01
> >   4 KSP Residual norm 1.906925178491e-01
> >   5 KSP Residual norm 1.616509124176e-15
> >
> > mpirun -np 1 ./extest -ksp_monitor -pc_type composite -pc_composite_pcs
> none,none -pc_composite_type multiplicative
> >
> >  0 KSP Residual norm 1.414213562373e+00
> >   1 KSP Residual norm 1.176696810829e+00
> >   2 KSP Residual norm 1.096908636191e+00
> >   3 KSP Residual norm 4.389821446437e-01
> >   4 KSP Residual norm 2.088364906576e-01
> >   5 KSP Residual norm 2.725851091482e-13
> >
> > mpirun -np 1 ./extest -ksp_monitor -pc_type composite -pc_composite_pcs
> none,none -pc_composite_type multiplicative -ksp_norm_type unpreconditioned
> > 0 KSP Residual norm 1.414213562373e+00
> >   1 KSP Residual norm 1.290994448736e+00
> >   2 KSP Residual norm 1.249516035344e+00
> >   3 KSP Residual norm 4.836894336642e-01
> >   4 KSP Residual norm 1.467306491390e-01
> >   5 KSP Residual norm 9.881776494390e-14
>
>
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