2013/11/18 Jed Brown <[email protected]> > Stefano Zampini <[email protected]> writes: > > > Currently, the man page for > > chebychev< > http://www.mcs.anl.gov/petsc/petsc-dev/docs/manualpages/KSP/KSPCHEBYSHEV.html > >states > > that it will only work for symmetric positive (semi-)definite > > systems, but the chebychev code has support for nonsymmetric systems > using > > the hybrid approach. Is the hybrid branch still a work in progress? > > The "hybrid" in cheby.c is nothing special for nonsymmetric problems. > In the paper, it just means that Arnoldi is used instead of Lanczos, but > the spectrum still has to be near the real line for it to be any good. > The tests in the paper are not so strongly nonsymmetric, and the most > nonsymmetric (Problem 4) performs better with Orthomin. > > Cheby is fine more mildly nonsymmetric problems and inadequate for > strongly nonsymmetric. > > In case of BDDC coarse problem, I will not expect strongly unymmetricity. The coarse problem is also structurally symmetric.
> > Who can give me some quick hints on how to work with it? > > > > I wish to make Chebychev the default KSP for the coarse problem of the > > BDDC, and I'd wish to know in which cases it will work or not. > > Actually, I would rather you not change the defaults unless it is > clearly better to the extent we should change other coarse level solvers > in PETSc. It is confusing for users when every component chooses its > own defaults for similar concepts seemingly-arbitrarily. > Sorry, my fault. I didn't explain completely my point. My intent is not of changing the default of the coarse solver (which is ksppreonly+pcredundant), but to find a good ksp in case the user requests a multilevel bddc, i.e. to solve the coarse problem with another level of BDDC. In that case, I would choose very few iterations of a cheap ksp (like richardson or cheby). It seems to me that the rational can be: use cheby if the problem is positive (semi-) definite, and gmres if not. Suggestions? -- Ph. D. Stefano Zampini CINECA SuperComputing Applications and Innovations Department - SCAI Via dei Tizii, 6 00185 Roma - ITALY ------------------------------------------------------------------------------------------------------------------------ Email: [email protected] SkypeID: stefano.zampini GoogleTalk: [email protected] Tel: +39 06.44486.707 ------------------------------------------------------------------------------------------------------------------------
