On Nov 11, 2011, at 11:20 AM, Jed Brown wrote: > On Fri, Nov 11, 2011 at 10:08, Mark F. Adams <mark.adams at columbia.edu> > wrote: > First, "my" matrices (actually XGC1 matrices from one of my projects and I > don't even know exactly what they do) put 1.0 on the diagonal for BCs and god > knows how the thing is scaled (1e13 apparently). But this does not matter > because the RHS and initial guess are 0.0 so the BCs have been completely > removed from the algebra, even if they are still in the data structures. > > So what happens when you use the same method with inhomogeneous Dirichlet > conditions?
Jed, I agreed with this point in my email. And I can now see your point that using the preconditioned residual is a good default for PETSc, or at least I'm willing to defer to your judgment. > This is especially bad if the interior is scaled by, say, 1e-13 instead of > 1e+13, because the boundary conditions dominate the initial residual. > > > As I recall the preconditioned residual was confusing because since these are > Laplacian matrices with a scale of 1e13, as you found, the residual dropped > like 10 orders of magnitude in the first iteration, which was pretty > confusing. > > The preconditioned residual fixes the scaling. If you evaluate the initial > unpreconditioned residual, then you see the confusing scaling. But > preconditioning, even with just Jacobi, fixes the scaling. All I recall was that I was confused because the residual dropped 10 orders of magnitude, apparently, in the first iteration and Barry explained the issue, I added 'unpreconditioned' residual and it was fine. So if I am recalling this correctly this default confused me but apparently, or what I infer from this discussion, more users get confused with using the unpreconditioned residual so you've chosen preconditioned residual as the default. Mark > > > So I don't see these XGC1 problems as being arguments for preconditioned > residual, in fact they argue against it, right? > > The preconditioned residual fixes the scaling, assuming it is no worse than > Jacobi. -------------- next part -------------- An HTML attachment was scrubbed... URL: <http://lists.mcs.anl.gov/pipermail/petsc-dev/attachments/20111111/e23023ed/attachment.html>
