[petsc-dev] coarse grid in gamg (for the OPTE paper)

[Barry Smith]

  Forwarding this to people who might have a clue.

On Sep 17, 2013, at 5:09 PM, Jungho Lee <julee at mcs.anl.gov> wrote:

> I'm looking into the 6*6 grid case, with dof=3, so the system matrix size is 108*108. 
> 
> 1) It turns out that using
> 
> -pc_type gamg -pc_gamg_type agg
> 
> results in two levels with the coarse problem being 13*13. Wouldn't it be natural for the coarse problem to be of size that's an integer multiple of 3, though? It turned out that KSP->mat->rmap->bs = 1 for the finest level matrix, so I added 
> 
> MatSetBlockSize(user.M,3)
> 
> in the main code, but it seems like this information somehow gets lost by the time lower-level functions (such as graph, coarsen, createlevel...) in gamg.c, agg.c, tools.c, etc., are called. 
> 
> 2) For the specific set of parameters I'm using for testing purposes, the smallest nonzero entry of the finest level matrix is of order e-7. For the coarse level matrix (size 13*13), whose entries are determined by MatPtAP called in createlevel (in gamg.c), the smallest nonzero entry is of order e-24 - this jumped out at me as a potential sign of  something wrong.
> 
> Thoughts?
> 
> Thanks,
> Jungho