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/>