I see you are using icc. Perhaps our icc changed a bit between versions. These results look like both solves are working and the old does a little better (after two iterations).
Try using jacobi instead of icc. On Apr 17, 2013, at 6:21 PM, Jozsef Bakosi <jbakosi at lanl.gov> wrote: >> On 04.17.2013 15:38, Matthew Knepley wrote: >> >>> On 04.17.2013 14:26, Jozsef Bakosi wrote: >>> >>>> Mark F. Adams mark.adams at columbia.edu >>>> Wed Apr 17 14:25:04 CDT 2013 >>>> >>>> 2) If you get "Indefinite PC" (I am guessing from using CG) it is because >>>> the >>>> preconditioner >>>> really is indefinite (or possible non-symmetric). We improved the >>>> checking >>>> for this in one >>>> of those releases. >>>> >>>> AMG does not guarantee an SPD preconditioner so why persist in trying to >>>> use >>>> CG? >>>> >>>> >>>> AMG is positive if everything is working correctly. >>>> >>>> Are these problems only semidefinite? Singular systems can give erratic >>>> behavior. >>> >>> It is a Laplace operator from Galerkin finite elements. And the PC is fine >>> on >>> ranks 1, 2, 3, and 5 -- indefinite only on 4. I think we can safely say >>> that the >>> same PC should be positive on 4 as well. >> >> Why is it safe? Because it sounds plausible? Mathematics is replete with >> things >> that sound plausible and are false. Are there proofs that suggest this? Is >> there >> computational evidence? Why would I believe you? > > Okay, so here is some additional information: > > I tried both old and new PETSc versions again, but now only taking 2 > iterations > (both with 4 CPUs) and checked the residuals. I get the same exact PC from ML > in > both cases, however, the residuals are different after both iterations: > > Please do a diff on the attached files and you can verify that the ML > diagnostics are exactly the same: same max eigenvalues, nodes aggregated, etc, > while the norm coming out of the solver at the end at both iterations are > different. > > We reproduced the same exact behavior on two different linux platforms. > > Once again: same application source code, same ML source code, different > PETSc: > 3.1-p8 vs. 3.3-p6. > <old.out><new.out>
