CG is a better method than GMRES for symmetric positive definite problems.
You want to use a better preconditioner. Try '-pc_type gamg -pc_gamg_agg_nsmooths 1' On Feb 19, 2013, at 11:36 AM, Marcelo Xavier Guterres <m.guterres at gmail.com> wrote: > Esteemed colleagues, > > > > My problem is: > > Div ( Grad ( phi ) ) = 0 > > mesh: > > 0 < x < 1; > 0 < y < 1; > > with boundary conditions: > > phi ( x , 0 ) = x; > phi ( 0 ,y ) = x; > phi ( x , 1 ) = x; > phi ( 1 ,y ) = x; > > with serial processing. > > > solved with ( KSPCG x PCJACOBI ) and (KSPGMRS x PCJACOBI). I have a > question, why the "norm error of the solution" of GMRES is high, when the > mesh is large. It is a problem in methods or truncation error? > > > the results were: > > > m n m x n error ( KSPCG x PCJACOBI ) error ( KSPGMRS x > PCJACOBI ) > 3 3 9 1,92E?16 4,42E?16 > > 4 4 16 2,08E?16 6,46E?16 > > 5 5 25 4,41E?16 9,63E?16 > > 6 6 36 8,77E?16 8,26E?16 > > 7 7 49 2,37E?06 2,52E?06 > > 8 8 64 1,17E?05 1,33E?05 > > 9 9 81 9,32E?06 1,26E?05 > > 10 10 100 7,33E?06 9,93E?06 > > 20 20 400 4,22E?05 3,16E?04 > > 30 30 900 1,06E?04 2,37E?02 > > 40 40 1600 2,14E?04 3,77E?02 > > 50 50 2500 3,37E?04 9,36E?03 > > 100 100 10000 1,27E?03 6,53E?02 > > 200 200 40000 4,32E?03 3,67E?01 > > 300 300 90000 9,53E?02 1,02E+00 > > 400 400 160000 1,68E?02 2,10E+00 > > 500 500 250000 2,61E?02 3,66E+00 > > > how can I improve the performance of GMRES? > > > a good week for all. > > > -- > Ph.d student Marcelo Xavier Guterres > Rio de Janeiro , Brazil. -------------- next part -------------- An HTML attachment was scrubbed... URL: <http://lists.mcs.anl.gov/pipermail/petsc-users/attachments/20130219/d204c2be/attachment-0001.html>
