[petsc-users] Parallel vs. single processor

[Jed Brown]

On Tue, Nov 13, 2012 at 8:16 AM, Rob Kunz <rfk102 at arl.psu.edu> wrote:

>  I have successfully implemented the GAMG and I am getting convergence
> now as expected from a multigrid solver!****
>
> Two issues:****
>
> **1)      **Every iteration if the CFD solver the pressure equation takes
> more solution time. Iteration 1 the code executes at 5e-5 wall
> secs/cell*iteration. Iteration 1000 the code executes at 3e-4 wall
> secs/cell*iteration. Steady increase. Why does this happen?
>
The most common cause is a memory leak. Maybe you are creating an object
and not freeing it? Run a small model with -malloc_dump.


> ****
>
> **2)      **I would like to see some documentation on how to control the
> GAMG, the online docs are too limited for my modest experience. Also there
> is only one example and that has very basic implementation. I am
> particularly interested in trying to get the solver to act only in the
> strong coefficient direction, with multiple cell agglomeration – i.e., try
> and make the solver act like a block correction multigrid.
>
I'm afraid there aren't comprehensive docs at this time. If you are
familiar with multigrid (see, e.g., Trottenberg's Multigrid book), the
multigrid options should be easy to understand. You'll have to read about
smoothed aggregation to interpret some of the -pc_gamg_* options.

We are refactoring some of the components in GAMG (should not affect the
high-level interface) and will write better documentation once that is
complete.
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