[petsc-users] How to use multigrid?

w_ang_temp w_ang_temp at 163.com
Sat Nov 3 10:52:22 CDT 2012

Is there something that need attention when setting up PETSc? The -log_summary
is no use in my system.

>At 2012-11-03 23:31:52,"Jed Brown" <jedbrown at mcs.anl.gov> wrote:
>1. Always send -log_summary when asking about performance.
>2. AMG setup costs more, the solve should be faster, especially for large problems.
>3. 30k degrees of freedom is not large.

>>On Sat, Nov 3, 2012 at 10:27 AM, w_ang_temp <w_ang_temp at 163.com> wrote:

>>    I have tried AMG, but there are some problems. I use the command:
>>    mpiexec -n 4 ./ex4f -ksp_type gmres -pc_type gamg -pc_gamg_agg_nsmooths 1 -ksp_gmres_restart 170 -ksp_rtol 1.0e-15 ->>ksp_converged_reason.
>>    The matrix has a size of 30000. However, compared with -pc_type asm,
>>the amg need more time:asm needs 4.9s, amg needs 13.7s. I did several tests
>>and got the same conclusion. When it begins, the screen shows the information:
>>[0]PCSetData_AGG bs=1 MM=7601. I do not know the meaning. And if there is some
>>parameters that affect the performance of AMG?
>>    Besides, I want to confirm a conception. In my view, AMG itself can be a solver
>>like gmres. It can also be used as a preconditioner like jacobi and is used by combining
>>with other solver. Is it right? If it is right, how use AMG solver?
>>    My codes are attached.
>>    Thanks.
>>                                             Jim

>At 2012-11-01 22:00:28,"Jed Brown" <jedbrown at mcs.anl.gov> wrote:

>Yes, it's faster to understand this error message than to have "mysteriously slow performance".

>* Preallocation routines now automatically set MAT_NEW_NONZERO_ALLOCATION_ERR, if you intentionally preallocate less than necessary then >use MatSetOption(mat,MAT_NEW_NONZERO_ALLOCATION_ERR,PETSC_FALSE) to disable the error generation.


>On Thu, Nov 1, 2012 at 8:57 AM, w_ang_temp <w_ang_temp at 163.com> wrote:

>Do you mean that the two versions have a different in this point? If I use the new version, I have to
>make some modifications on my codes?

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