<div>Hi,</div>
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<div>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. </div>
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<div>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%.
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<div>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.</div>
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<div>Thank you.</div>
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