[petsc-dev] features of GAMG

Mark F. Adams mark.adams at columbia.edu
Mon Dec 26 12:43:54 CST 2011

I use:

        -@${MPIEXEC} -n 1 ./ex56 -ne 49 -pc_gamg_verbose -alpha 1.e-3 -ksp_monitor -ksp_type cg -pc_gamg_type sa -ksp_rtol 1.e-10

plus -mg_levels_ksp_max_it  to adjust smoothing. 

src/ksp/ksp/examples/tutorials has ex54 ex55 and ex56 that do a 2D scalar Lapacian, 2D elasticity and 3D elasticity respectively.

 -ne is the number of elements on a side so for the 2D problems '-ne 100' is decent and for 3D '-ne 49' is pretty big unless you are running in parallel.

I've just started using -ksp_rtol 1.e-10 and retested all of these test and am seeing a very slight improvement going from 1 to 4.  I would expect to see a significant drop, say 25%, in the number of iterations going from 1 to 2.  The < 5% that I am seeing seems too low but I'm not 100% sure -- maybe there is nothing wrong ...


On Dec 26, 2011, at 10:41 AM, Jed Brown wrote:

> On Mon, Dec 26, 2011 at 00:07, Mark F. Adams <mark.adams at columbia.edu> wrote:
>> Can you show me an example? I have certainly seen it make a different for other problems.
> OK, well we need to think about this then.  Maybe we can take a look at it at Brown in January.  There must be some bad interaction in the way that I'm using it.  The code is around 662 in gamg.c.
> Can you tell me how to run an example that shows this behavior?

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