All of these answers are equally correct, they are due to different round-offs due to different order of operations due to different communication patterns between the processes in each run.
Barry On Mar 21, 2014, at 9:49 PM, Fande Kong <[email protected]> wrote: > Hi, > > I run the code src/snes/examples/tutorials/ex5.c with options: > > mpirun -n 8 ./ex5 -pc_type mg \ > -ksp_monitor\ > -pc_mg_levels 3 \ > -pc_mg_galerkin \ > -da_grid_x 17 \ > -da_grid_y 17 \ > -mg_levels_ksp_norm_type unpreconditioned \ > -snes_monitor \ > -mg_levels_ksp_chebyshev_estimate_eigenvalues 0.5,1.1 \ > -mg_levels_pc_type sor \ > -pc_mg_type \ > > > I run this script several times, but at each time, the residual norms have > some tiny differences ( these should not happen). For example: > > Case 1: > > 0 SNES Function norm 1.188788066192e+00 > 0 KSP Residual norm 1.573384253521e+00 > 1 KSP Residual norm 3.616321708396e-02 > 2 KSP Residual norm 2.780221563755e-04 > 3 KSP Residual norm 2.194662354037e-06 > 1 SNES Function norm 5.125240595190e-03 > 0 KSP Residual norm 1.217284338266e-01 > 1 KSP Residual norm 1.774247017346e-04 > 2 KSP Residual norm 2.557118591292e-06 > 3 KSP Residual norm 1.622173269367e-08 > 2 SNES Function norm 3.922335995111e-05 > 0 KSP Residual norm 9.960745924876e-04 > 1 KSP Residual norm 1.273916336665e-06 > 2 KSP Residual norm 1.571259383270e-08 > 3 KSP Residual norm 1.250266145356e-10 > 3 SNES Function norm 2.662898279023e-09 > > > Case 2: > > > 0 SNES Function norm 1.188788066192e+00 > 0 KSP Residual norm 1.573384253521e+00 > 1 KSP Residual norm 3.616321708396e-02 > 2 KSP Residual norm 2.780221563755e-04 > 3 KSP Residual norm 2.194662354037e-06 > 1 SNES Function norm 5.125240595190e-03 > 0 KSP Residual norm 1.217284338266e-01 > 1 KSP Residual norm 1.774247017347e-04 > 2 KSP Residual norm 2.557118591292e-06 > 3 KSP Residual norm 1.622173269367e-08 > 2 SNES Function norm 3.922335995108e-05 > 0 KSP Residual norm 9.960745924862e-04 > 1 KSP Residual norm 1.273916336654e-06 > 2 KSP Residual norm 1.571259383257e-08 > 3 KSP Residual norm 1.250266145346e-10 > 3 SNES Function norm 2.662898285038e-09 > > These differences are marked by red color. So here, I was wondering there are > any explanations why these happen? > > Thanks, > > Fande, > >
