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

[Zhenglun (Alan) Wei]

Dear Dr. Brown,
      Thank you so much for your help. Your reply is really helpful and 
knowledgeable.

sincerely,
Alan
On 10/5/2012 2:42 PM, Jed Brown wrote:
> On Fri, Oct 5, 2012 at 2:33 PM, Zhenglun (Alan) Wei 
> <zhenglun.wei at gmail.com <mailto: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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