[petsc-users] a strange bug on using DMDA_STENCIL_BOX

[Barry Smith]

  Your interpretation is correct. If DMDA_STENTIL_STAR works then one absolutely expects that DMDA_STENTIL_BOX would produce the same answer.  If you are running with linux could you run with valgrind to see if there is some strange memory corruption or lack of initialization http://www.mcs.anl.gov/petsc/documentation/faq.html#valgrind

   What happens with -ksp_monitor on both cases? Same convergence history or different?

   Barry



On Jul 10, 2014, at 7:49 PM, Sun, Hui <hus003 at ucsd.edu> wrote:

> I'm using KSP for 2D Navier Stokes, finite difference. I have DM object created with DMDA_STENTIL_STAR. I'm only using one single process to run the code. It works fine, the linear system converges to the correct solution with correct order, as expected. However, if I change it to DMDA_STENTIL_BOX, it does not converge, the residue \|Ax-b\|_2 remains large. 
> 
> I have checked that the rhs for both cases are exactly the same, and the matrices also seem to be the same because I have checked the output of A*y, where y is the analytic solution, and both cases give exactly the same answer. If the linear system is exactly the same, then it seems very strange to me that DMDA_STENTIL_STAR works while DMDA_STENTIL_BOX does not work, for I use the same solver, there shouldn't be any difference. 
> 
> If you happen to encounter this kind of bug before and have some idea what's going on, please let me know. Thank you very much! If you want to look at the code I've written, feel free to ask.