KSPSetTolerances and processors size

[Barry Smith]

  Ben,


On Sat, 16 Jun 2007, Ben Tay wrote:

> Hi,
> 
> I am trying to use PETSc to solve my poisson eqn. In the past, when I'm not
> using MPI, I set KSPSetTolerances to 1.0e-5. This seems to be good enuff.
> 
> Now, I'm trying to use PETSc with MPI to solve the eqn. I am using a very
> small no. of pts, only 16x4. On 1 processor, the ans is the same as the one
> without MPI. However, on 2 processors, I found that I've to increase the
> KSPSetTolerances to 1.0e-6, else those grid pts on the edge will be
> different. On 4 processors, KSPSetTolerances has to be decreased to 1.0e-7.
> On 8 processors, when KSPSetTolerances is 1.0e-8, the difference of the last
> pt between 1 and 8 processors is about 3%.
>

   Are you using the same number of grid points when changing the number of
processors?

   Run the cases with -ksp_monitor_true_residual and send us ALL the output.

   Barry


 
> May I know why this is so? Also, does this happens only on very small grids?
> Does it happen on large grids as well? I usually solve grids with approx
> 20000 to 55000 pts.
> 
> Thank you.
>