On Thu, Oct 8, 2009 at 10:14 AM, Matt Funk <mafunk at nmsu.edu> wrote:
> Hi Matt, > > > i don't understand how it can be the condition of the system. > After all, the matrix as well as the RHS vector is EXACTLY the same between > the 2 runs. This is what puzzles me so much. > The order of operations is different in the serial and parallel cases. With a very ill-conditioned matrix, which it sounds like you have, the reordering produces noticeably different residuals, even though they both satisfy the error bound. Matt > The only difference is whether i solve it using 1 core vs 4 cores. > > > Of course i could be missing something. > > > > matt > > > > > On Wednesday 07 October 2009, Matthew Knepley wrote: > > This sounds like it has to do with the condition of your system, not any > > parallel problem. Errors > > in the solution can only be reduced to about (condition number) * > (residual > > norm). > > > > Matt > > > > On Wed, Oct 7, 2009 at 5:26 PM, Matt Funk <mafunk at nmsu.edu> wrote: > > > Hi, > > > > > > > > > i have a problem for which i am not exaclty sure about what to do. > > > I set up a simple 2D rectangular domain and decompose it into four > equal > > > boxes. I then build the petsc matrix based on this layout as well as > the > > > corresponsing RHS vector. > > > > > > > > > I print out the matrix and RHS vector right before my KSPSolve call, > and > > > right after that call i print out the solution vector 'x'. > > > > > > > > > I do this for 2 runs. > > > 1) 1 processor > > > 2) 4 processors. > > > > > > > > > For both runs i do a difference (i.e. on the output files using diff) > on > > > all 3 quantities (the matrix, the RHS vector and the solution vector). > > > > > > > > > The 'diff' command reports no difference between the files for the > matrix > > > and RHS vector. > > > > > > > > > However, the soltution vector is different between the 2 runs. How > > > different depends a little on what precond/solver combination i use and > > > the tolerances. > > > > > > > > > However, for example for BJacobi/GMRES with reltol=abstol=1e-12 the > > > vector element with the maximum difference is on the order 1e-05. This > is > > > only after the first timestep. My problem has some nonlinearlity to it > > > such that this will become a problem later on. > > > > > > > > > The worst difference i have seen is if i use hypre's euclid. It was on > > > the order of 1e-02. > > > > > > > > > > > > So my question is whether someone has an idea why this is happening (i > > > suppose it is related to the parallel communication) and if there is > way > > > to fix it. > > > > > > > > > > > > thanks > > > matt > > > > -- 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/20091008/49285133/attachment.htm>
