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. >