KSPSetTolerances and processors size

[Ben Tay]

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%.

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.
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