[petsc-users] A similar question of "Enquiry regarding log summary results"

[Jed Brown]

On Fri, Oct 5, 2012 at 2:33 PM, Zhenglun (Alan) Wei
<zhenglun.wei at gmail.com>wrote:

> Dear Dr. Brown,
>      Thanks for your reply. I checked my grid partition by output the
> corner points of the local domain. There is no problem according to that.
>      I ran the same case with smaller grid size with 8 cores and find that
> the code occupy a lot of memory and slow if I use the original script file
> which is
> *mpiexec -np 8 ./ex45 -pc_type exotic -ksp_type fgmres
> -mg_levels_ksp_type gmres -mg_levels_ksp_max_it 1 -mg_levels_pc_type
> bjacobi -ksp_rtol 1.0e-7 -ksp_monitor_true_residual -ksp_converged_reason
> -log_summary > out*
>
>      If I simply use
> *mpiexec -np 8 ./ex45 -ksp_monitor_true_residual -ksp_converged_reason
> -log_summary > out*
>
>      Everything works fine. Therefore, I did more tests and found out that
> the "-pc_type exotic" has some problem.
>

PCEXOTIC is a special purpose, and mostly experimental, method.


> Now I changed it to "-pc_type gamg"; like,
> *mpiexec -np 8 ./ex45 -pc_type gamg -ksp_type fgmres -mg_levels_ksp_type
> gmres -mg_levels_ksp_max_it 1 -mg_levels_pc_type bjacobi -ksp_rtol 1.0e-7
> -ksp_monitor_true_residual -ksp_converged_reason -log_summary > out*
>

Add -pc_gamg_agg_nsmooths 1 to the options above and it will likely
converge much faster (in exchange for somewhat higher setup, but you are
spending almost no time in setup).

Why are you using GMRES on levels? It will probably go faster if you drop
all the -mg_levels_* options above (which will use Chebyshev and SOR,
should converge fast, but has higher setup), perhaps going with
-mg_levels_ksp_max_it 1 -mg_levels_ksp_type richardson which will probably
also works fine without the setup cost.


> * *
>      it is super fast now, the log file is attached. However, I have
> another question that it always show that "unpreconditioned " although I
> already put pc_type over there.
>       " 0 KSP unpreconditioned resid norm 1.800295456670e+01 true resid
> norm 1.800295456670e+01 ||r(i)||/||b|| 1.000000000000e+00"
>
>       However, if I changed the 'ksp_type' from 'fgmres' to 'cg', it would
> give me:
>       "0 KSP preconditioned resid norm 1.117293929796e+02 true resid norm
> 1.800293604638e+01 ||r(i)||/||b|| 1.000000000000e+00"
>

This "preconditioned" versus "unpreconditioned" is reporting the norm used
by the method, not whether preconditioning is being used. Note that you can
only use CG (or normal GMRES) if the preconditioner is linear.
Changing/removing the option -mg_levels_ksp_type gmres will make your
method linear. Note that you can use normal GMRES in unpreconditioned norm
(-ksp_norm_type unpreconditioned) so you don't have to pay for the extra
memory and work done by FGMRES.


>
>       The finial question: what are the choice for the *'*-mg_levels_ksp_type'.
> Are they as the same as 'ksp_type'?
>

Yes, you can also tell this via -help. Usually you want to use a stationary
method in the smoother, typically chebyshev or richardson. A "Krylov"
method like GMRES or CG will make the multigrid cycle nonlinear.
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