[petsc-dev] [petsc-users] Problem with AMG packages

[Pierre Jolivet]

> "Mark F. Adams" <mfadams at lbl.gov> writes:
>> There are many parameters, including stupid things like solver
>> parameters and stupider things like bugs, that can kill an iterative
>> solver and we are trying to figure out what is wrong here.  You should
>> be running with about 1/10 or 1/100, or even 1/1000, the number of
>> processors that you are using here.  If you eventually want to run in
>> this regime then that is fine but for debugging it helps to be in a
>> more normal regime, because then you (or us) can start to see
>> secondary things like super slow flop rates that can provide a hint of
>> what is going wrong.  It is also better to start in serial so there
>> are less moving parts
>
> Pierre, can you scale the problem down to 10k or 100k degrees of freedom
> and make sure that you can reproduce the qualitative convergence
> behavior.
>
> Write the matrix to a file using
>
>   -ksp_view_mat binary:matrix-100k-jolivet
>
> then put the file matrix-100k-jolivet and matrix-100k-jolivet.info
> somewhere we can find it or anonymous ftp to ftp.mcs.anl.gov, cd
> incoming, and upload the file.
>
> This will let us experiment quickly.
>

I really don't want to bother you more than I already have, but since you
asked for it, I uploaded a P_1 3D Poisson matrix on ANL's ftp. On my
desktop, using 2 processes (~70k unkowns/proc), AMG is 8x slower (setup ~8
seconds) than BoomerAMG or GASM (setup ~1 second).

Please feel free to drop the matter, I'll make some more runs at larger
scale and just pick the best results.

Thanks again for your help.