I have a conjugate gradient implemented in C without Petsc and when I solve the linear system of my problem (an elliptic pde) it converges with something about 270 iterations. But with Petsc (ksp_type cg pc_type none) the same linear system converges with 510 iterations.
Do someone have any idea why is that happenning? I already checked the matrix and they are equal. The results are quite close and I set the same tolerance to both methods (setting rtol on petsc to a very small number, to use only atol). And besides the number of iterations, the time spent by petsc is not much more than the other method. Thanks, Ronan I'm sorry, I dont know english very well, I did the best I could.
