On Mon, Sep 21, 2009 at 2:29 PM, Barry Smith <bsmith at mcs.anl.gov> wrote:
> > Likely it is too expensive to use the 1-norm for stopping criteria for > fgmres unless you can reformulate fgmres to give you back the residual > efficiently. > I have been told that QMR has a natural 1-norm formulation, but I have never seen it. Matt > Barry > > On Sep 21, 2009, at 2:04 PM, Ryan Yan wrote: > > Hi Barry, >> Thanks for the suggestion. >> >> I am using FGMRES as the krylov solver. Is there a direct access to the >> residual vector for FGMRES. I guess the answer is yes, since that's the >> Krylov method which supports right preconditioning and provides the true >> residual 2_Norm monitor, right? >> >> If so, which header should I include to fetch the residual directly from >> KSP_FGMRES? >> >> Yan >> >> >> >> On Mon, Sep 21, 2009 at 2:43 PM, Barry Smith <bsmith at mcs.anl.gov> wrote: >> >> Yan, >> >> This depends on the Krylov method being used. For example with GMRES the >> residual vector is NOT available at each iteration (the 2-norm of the >> residual is approximated via a recurrence relationship). >> >> You can call KSPBuildResidual() to have the true residual computed for >> you. Note: it is expensive because it actually builds the current solution >> and computes r = b - A*x >> >> What we intended is that if you want the residual efficiently for example >> for the CG method, you determine what Krylov method you want to use the >> include the appropriate private include file and access the residual >> directly from the data structure. This would be efficient. (but like I said >> does not work for all methods). For cg include src/ksp/ksp/impls/cg/cgctx.h >> >> >> Barry >> >> >> On Sep 21, 2009, at 12:44 PM, Ryan Yan wrote: >> >> Hi All, >> The following question is only for test and comparison reason. >> >> Is there a command line option to set up using 1-Norm(sum of absolute >> value of residuals) as convergence monitor and test. >> >> Alternatively, if I can get the residual "r" out at the end of each >> iteration, then it is going to be much simpler, since I can just call >> VecNorm(r, NORM_1, &r_ell1) and pass the r_ell1 into the convergence monitor >> and test. Can anyone inform me if PETSc has an interface for the resuidual >> r? >> >> Or one has to call the convergence test: >> MyKSPConverged(ksp,n,rnorm,flag,dummy); >> and following the steps: >> 1. fetch the exact solution: >> >> KSPBuildSolution(ksp,PETSC_NULL_OBJECT,x,ierr) >> >> 2. calculate the residual: >> >> pass the Right Hand side and the Matrix in to calculate the 1-Norm. But >> this step is *not* obvious, since the matrix and rhs is already distributed >> over each processes. >> >> 3. set up the convergence test based on the 1_Norm residual. >> >> >> Thank you very much, >> >> >> Yan >> >> >> > -- What most experimenters take for granted before they begin their experiments is infinitely more interesting than any results to which their experiments lead. -- Norbert Wiener -------------- next part -------------- An HTML attachment was scrubbed... URL: <http://lists.mcs.anl.gov/pipermail/petsc-users/attachments/20090921/532cda79/attachment.htm>
