If you are using MG what is the coarse grid solver? -snes_view might give you that.
On Thu, May 4, 2023 at 8:25 AM Matthew Knepley <knep...@gmail.com> wrote: > On Thu, May 4, 2023 at 8:21 AM Mark Lohry <mlo...@gmail.com> wrote: > >> Do they start very similarly and then slowly drift further apart? >> >> >> Yes, this. I take it this sounds familiar? >> >> See these two examples with 20 fixed iterations pasted at the end. The >> difference for one solve is slight (final SNES norm is identical to 5 >> digits), but in the context I'm using it in (repeated applications to solve >> a steady state multigrid problem, though here just one level) the >> differences add up such that I might reach global convergence in 35 >> iterations or 38. It's not the end of the world, but I was expecting that >> with -np 1 these would be identical and I'm not sure where the root cause >> would be. >> > > The initial KSP residual is different, so its the PC. Please send the > output of -snes_view. If your ASM is using direct factorization, then it > could be randomness in whatever LU you are using. > > Thanks, > > Matt > > >> 0 SNES Function norm 2.801842107848e+04 >> 0 KSP Residual norm 4.045639499595e+01 >> 1 KSP Residual norm 1.917999809040e+01 >> 2 KSP Residual norm 1.616048521958e+01 >> [...] >> 19 KSP Residual norm 8.788043518111e-01 >> 20 KSP Residual norm 6.570851270214e-01 >> Linear solve converged due to CONVERGED_ITS iterations 20 >> 1 SNES Function norm 1.801309983345e+03 >> Nonlinear solve converged due to CONVERGED_ITS iterations 1 >> >> >> Same system, identical initial 0 SNES norm, 0 KSP is slightly different >> >> 0 SNES Function norm 2.801842107848e+04 >> 0 KSP Residual norm 4.045639473002e+01 >> 1 KSP Residual norm 1.917999883034e+01 >> 2 KSP Residual norm 1.616048572016e+01 >> [...] >> 19 KSP Residual norm 8.788046348957e-01 >> 20 KSP Residual norm 6.570859588610e-01 >> Linear solve converged due to CONVERGED_ITS iterations 20 >> 1 SNES Function norm 1.801311320322e+03 >> Nonlinear solve converged due to CONVERGED_ITS iterations 1 >> >> On Wed, May 3, 2023 at 11:05 PM Barry Smith <bsm...@petsc.dev> wrote: >> >>> >>> Do they start very similarly and then slowly drift further apart? That >>> is the first couple of KSP iterations they are almost identical but then >>> for each iteration get a bit further. Similar for the SNES iterations, >>> starting close and then for more iterations and more solves they start >>> moving apart. Or do they suddenly jump to be very different? You can run >>> with -snes_monitor -ksp_monitor >>> >>> On May 3, 2023, at 9:07 PM, Mark Lohry <mlo...@gmail.com> wrote: >>> >>> This is on a single MPI rank. I haven't checked the coloring, was just >>> guessing there. But the solutions/residuals are slightly different from run >>> to run. >>> >>> Fair to say that for serial JFNK/asm ilu0/gmres we should expect bitwise >>> identical results? >>> >>> >>> On Wed, May 3, 2023, 8:50 PM Barry Smith <bsm...@petsc.dev> wrote: >>> >>>> >>>> No, the coloring should be identical every time. Do you see >>>> differences with 1 MPI rank? (Or much smaller ones?). >>>> >>>> >>>> >>>> > On May 3, 2023, at 8:42 PM, Mark Lohry <mlo...@gmail.com> wrote: >>>> > >>>> > I'm running multiple iterations of newtonls with an MFFD/JFNK >>>> nonlinear solver where I give it the sparsity. PC asm, KSP gmres, with >>>> SNESSetLagJacobian -2 (compute once and then frozen jacobian). >>>> > >>>> > I'm seeing slight (<1%) but nonzero differences in residuals from run >>>> to run. I'm wondering where randomness might enter here -- does the >>>> jacobian coloring use a random seed? >>>> >>>> >>> > > -- > 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 > > https://www.cse.buffalo.edu/~knepley/ > <http://www.cse.buffalo.edu/~knepley/> >