The problem is that the fine grid solver is iterating past the prescribed tolerance. It iterates until the maximum number of iterations has been achieved.
Elena ________________________________ From: Mark Adams <[email protected]> Sent: 01 October 2025 13:25:14 To: Barry Smith Cc: Moral Sanchez, Elena; petsc-users Subject: Re: [petsc-users] setting correct tolerances for MG smoother CG at the finest level Sorry to jump in, but what is the problem here? This looks fine to me, other than the coarse grid solver that I mentioned. On Tue, Sep 30, 2025 at 9:27 AM Barry Smith <[email protected]<mailto:[email protected]>> wrote: Would you be able to share your code? I'm at a loss as to why we are seeing this behavior and can much more quickly figure it out by running the code in a debugger. Barry You can send the code [email protected]<mailto:[email protected]> if you don't want to share the code with everyone, On Sep 30, 2025, at 5:05 AM, Moral Sanchez, Elena <[email protected]<mailto:[email protected]>> wrote: This is what I get: Residual norms for mg_levels_1_ solve. 0 KSP Residual norm 2.249726733143e+00 Residual norms for mg_levels_1_ solve. 0 KSP unpreconditioned resid norm 2.249726733143e+00 true resid norm 2.249726733143e+00 ||r(i)||/||b|| 1.000000000000e+00 1 KSP Residual norm 1.433120400946e+00 1 KSP unpreconditioned resid norm 1.433120400946e+00 true resid norm 1.433120400946e+00 ||r(i)||/||b|| 6.370197677051e-01 2 KSP Residual norm 1.169262560123e+00 2 KSP unpreconditioned resid norm 1.169262560123e+00 true resid norm 1.169262560123e+00 ||r(i)||/||b|| 5.197353718108e-01 3 KSP Residual norm 1.323528716607e+00 3 KSP unpreconditioned resid norm 1.323528716607e+00 true resid norm 1.323528716607e+00 ||r(i)||/||b|| 5.883064361148e-01 4 KSP Residual norm 5.006323254234e-01 4 KSP unpreconditioned resid norm 5.006323254234e-01 true resid norm 5.006323254234e-01 ||r(i)||/||b|| 2.225302824775e-01 5 KSP Residual norm 3.569836784785e-01 5 KSP unpreconditioned resid norm 3.569836784785e-01 true resid norm 3.569836784785e-01 ||r(i)||/||b|| 1.586786844906e-01 6 KSP Residual norm 2.493182937513e-01 6 KSP unpreconditioned resid norm 2.493182937513e-01 true resid norm 2.493182937513e-01 ||r(i)||/||b|| 1.108215900529e-01 7 KSP Residual norm 3.038202502298e-01 7 KSP unpreconditioned resid norm 3.038202502298e-01 true resid norm 3.038202502298e-01 ||r(i)||/||b|| 1.350476241198e-01 8 KSP Residual norm 2.780214194402e-01 8 KSP unpreconditioned resid norm 2.780214194402e-01 true resid norm 2.780214194402e-01 ||r(i)||/||b|| 1.235800843473e-01 9 KSP Residual norm 1.676826341491e-01 9 KSP unpreconditioned resid norm 1.676826341491e-01 true resid norm 1.676826341491e-01 ||r(i)||/||b|| 7.453466755710e-02 10 KSP Residual norm 1.209985378713e-01 10 KSP unpreconditioned resid norm 1.209985378713e-01 true resid norm 1.209985378713e-01 ||r(i)||/||b|| 5.378366007245e-02 11 KSP Residual norm 9.445076689969e-02 11 KSP unpreconditioned resid norm 9.445076689969e-02 true resid norm 9.445076689969e-02 ||r(i)||/||b|| 4.198321756516e-02 12 KSP Residual norm 8.308555284580e-02 12 KSP unpreconditioned resid norm 8.308555284580e-02 true resid norm 8.308555284580e-02 ||r(i)||/||b|| 3.693139776569e-02 13 KSP Residual norm 5.472865592585e-02 13 KSP unpreconditioned resid norm 5.472865592585e-02 true resid norm 5.472865592585e-02 ||r(i)||/||b|| 2.432680161532e-02 14 KSP Residual norm 4.357870564398e-02 14 KSP unpreconditioned resid norm 4.357870564398e-02 true resid norm 4.357870564398e-02 ||r(i)||/||b|| 1.937066622447e-02 15 KSP Residual norm 5.079681292439e-02 15 KSP unpreconditioned resid norm 5.079681292439e-02 true resid norm 5.079681292439e-02 ||r(i)||/||b|| 2.257910357558e-02 Linear mg_levels_1_ solve converged due to CONVERGED_ITS iterations 15 Residual norms for mg_levels_1_ solve. 0 KSP Residual norm 5.079681292439e-02 Residual norms for mg_levels_1_ solve. 0 KSP unpreconditioned resid norm 5.079681292439e-02 true resid norm 5.079681292439e-02 ||r(i)||/||b|| 2.257910357559e-02 1 KSP Residual norm 2.934938644003e-02 1 KSP unpreconditioned resid norm 2.934938644003e-02 true resid norm 2.934938644003e-02 ||r(i)||/||b|| 1.304575618348e-02 2 KSP Residual norm 3.257065831294e-02 2 KSP unpreconditioned resid norm 3.257065831294e-02 true resid norm 3.257065831294e-02 ||r(i)||/||b|| 1.447760647243e-02 3 KSP Residual norm 4.143063876867e-02 3 KSP unpreconditioned resid norm 4.143063876867e-02 true resid norm 4.143063876867e-02 ||r(i)||/||b|| 1.841585387164e-02 4 KSP Residual norm 4.822471409489e-02 4 KSP unpreconditioned resid norm 4.822471409489e-02 true resid norm 4.822471409489e-02 ||r(i)||/||b|| 2.143580968499e-02 5 KSP Residual norm 3.197538246153e-02 5 KSP unpreconditioned resid norm 3.197538246153e-02 true resid norm 3.197538246153e-02 ||r(i)||/||b|| 1.421300729127e-02 6 KSP Residual norm 3.461217019835e-02 6 KSP unpreconditioned resid norm 3.461217019835e-02 true resid norm 3.461217019835e-02 ||r(i)||/||b|| 1.538505529958e-02 7 KSP Residual norm 3.410193775327e-02 7 KSP unpreconditioned resid norm 3.410193775327e-02 true resid norm 3.410193775327e-02 ||r(i)||/||b|| 1.515825777899e-02 8 KSP Residual norm 4.690424294464e-02 8 KSP unpreconditioned resid norm 4.690424294464e-02 true resid norm 4.690424294464e-02 ||r(i)||/||b|| 2.084886233233e-02 9 KSP Residual norm 3.366148892800e-02 9 KSP unpreconditioned resid norm 3.366148892800e-02 true resid norm 3.366148892800e-02 ||r(i)||/||b|| 1.496247896783e-02 10 KSP Residual norm 4.068015727689e-02 10 KSP unpreconditioned resid norm 4.068015727689e-02 true resid norm 4.068015727689e-02 ||r(i)||/||b|| 1.808226602707e-02 11 KSP Residual norm 2.658836123104e-02 11 KSP unpreconditioned resid norm 2.658836123104e-02 true resid norm 2.658836123104e-02 ||r(i)||/||b|| 1.181848481389e-02 12 KSP Residual norm 2.826244186003e-02 12 KSP unpreconditioned resid norm 2.826244186003e-02 true resid norm 2.826244186003e-02 ||r(i)||/||b|| 1.256261102456e-02 13 KSP Residual norm 2.981793619508e-02 13 KSP unpreconditioned resid norm 2.981793619508e-02 true resid norm 2.981793619508e-02 ||r(i)||/||b|| 1.325402581380e-02 14 KSP Residual norm 3.525455091450e-02 14 KSP unpreconditioned resid norm 3.525455091450e-02 true resid norm 3.525455091450e-02 ||r(i)||/||b|| 1.567059251914e-02 15 KSP Residual norm 2.331539121838e-02 15 KSP unpreconditioned resid norm 2.331539121838e-02 true resid norm 2.331539121838e-02 ||r(i)||/||b|| 1.036365478300e-02 Linear mg_levels_1_ solve converged due to CONVERGED_ITS iterations 15 Residual norms for mg_levels_1_ solve. 0 KSP Residual norm 2.421498365806e-02 Residual norms for mg_levels_1_ solve. 0 KSP unpreconditioned resid norm 2.421498365806e-02 true resid norm 2.421498365806e-02 ||r(i)||/||b|| 1.000000000000e+00 1 KSP Residual norm 1.761072112362e-02 1 KSP unpreconditioned resid norm 1.761072112362e-02 true resid norm 1.761072112362e-02 ||r(i)||/||b|| 7.272654556492e-01 2 KSP Residual norm 1.400842489042e-02 2 KSP unpreconditioned resid norm 1.400842489042e-02 true resid norm 1.400842489042e-02 ||r(i)||/||b|| 5.785023474818e-01 3 KSP Residual norm 1.419665483348e-02 3 KSP unpreconditioned resid norm 1.419665483348e-02 true resid norm 1.419665483348e-02 ||r(i)||/||b|| 5.862756314004e-01 4 KSP Residual norm 1.617590701667e-02 4 KSP unpreconditioned resid norm 1.617590701667e-02 true resid norm 1.617590701667e-02 ||r(i)||/||b|| 6.680123036665e-01 5 KSP Residual norm 1.354824081005e-02 5 KSP unpreconditioned resid norm 1.354824081005e-02 true resid norm 1.354824081005e-02 ||r(i)||/||b|| 5.594982429624e-01 6 KSP Residual norm 1.387252917475e-02 6 KSP unpreconditioned resid norm 1.387252917475e-02 true resid norm 1.387252917475e-02 ||r(i)||/||b|| 5.728902967950e-01 7 KSP Residual norm 1.514043102087e-02 7 KSP unpreconditioned resid norm 1.514043102087e-02 true resid norm 1.514043102087e-02 ||r(i)||/||b|| 6.252505157414e-01 8 KSP Residual norm 1.275811124745e-02 8 KSP unpreconditioned resid norm 1.275811124745e-02 true resid norm 1.275811124745e-02 ||r(i)||/||b|| 5.268684640721e-01 9 KSP Residual norm 1.241039155981e-02 9 KSP unpreconditioned resid norm 1.241039155981e-02 true resid norm 1.241039155981e-02 ||r(i)||/||b|| 5.125087728764e-01 10 KSP Residual norm 9.585207801652e-03 10 KSP unpreconditioned resid norm 9.585207801652e-03 true resid norm 9.585207801652e-03 ||r(i)||/||b|| 3.958378802565e-01 11 KSP Residual norm 9.022641230732e-03 11 KSP unpreconditioned resid norm 9.022641230732e-03 true resid norm 9.022641230732e-03 ||r(i)||/||b|| 3.726057121550e-01 12 KSP Residual norm 1.187709152046e-02 12 KSP unpreconditioned resid norm 1.187709152046e-02 true resid norm 1.187709152046e-02 ||r(i)||/||b|| 4.904852172597e-01 13 KSP Residual norm 1.084880112494e-02 13 KSP unpreconditioned resid norm 1.084880112494e-02 true resid norm 1.084880112494e-02 ||r(i)||/||b|| 4.480201712351e-01 14 KSP Residual norm 8.194750346781e-03 14 KSP unpreconditioned resid norm 8.194750346781e-03 true resid norm 8.194750346781e-03 ||r(i)||/||b|| 3.384165136140e-01 15 KSP Residual norm 7.614246199165e-03 15 KSP unpreconditioned resid norm 7.614246199165e-03 true resid norm 7.614246199165e-03 ||r(i)||/||b|| 3.144435819857e-01 Linear mg_levels_1_ solve converged due to CONVERGED_ITS iterations 15 Residual norms for mg_levels_1_ solve. 0 KSP Residual norm 7.614246199165e-03 Residual norms for mg_levels_1_ solve. 0 KSP unpreconditioned resid norm 7.614246199165e-03 true resid norm 7.614246199165e-03 ||r(i)||/||b|| 3.144435819857e-01 1 KSP Residual norm 5.620014684145e-03 1 KSP unpreconditioned resid norm 5.620014684145e-03 true resid norm 5.620014684145e-03 ||r(i)||/||b|| 2.320883120759e-01 2 KSP Residual norm 6.643368363907e-03 2 KSP unpreconditioned resid norm 6.643368363907e-03 true resid norm 6.643368363907e-03 ||r(i)||/||b|| 2.743494878096e-01 3 KSP Residual norm 8.708642393659e-03 3 KSP unpreconditioned resid norm 8.708642393659e-03 true resid norm 8.708642393659e-03 ||r(i)||/||b|| 3.596385823189e-01 4 KSP Residual norm 6.401852907459e-03 4 KSP unpreconditioned resid norm 6.401852907459e-03 true resid norm 6.401852907459e-03 ||r(i)||/||b|| 2.643756856440e-01 5 KSP Residual norm 7.230576215262e-03 5 KSP unpreconditioned resid norm 7.230576215262e-03 true resid norm 7.230576215262e-03 ||r(i)||/||b|| 2.985992605803e-01 6 KSP Residual norm 6.204081601285e-03 6 KSP unpreconditioned resid norm 6.204081601285e-03 true resid norm 6.204081601285e-03 ||r(i)||/||b|| 2.562083744880e-01 7 KSP Residual norm 7.038656665944e-03 7 KSP unpreconditioned resid norm 7.038656665944e-03 true resid norm 7.038656665944e-03 ||r(i)||/||b|| 2.906736079337e-01 8 KSP Residual norm 7.194079694050e-03 8 KSP unpreconditioned resid norm 7.194079694050e-03 true resid norm 7.194079694050e-03 ||r(i)||/||b|| 2.970920730585e-01 9 KSP Residual norm 6.353576889135e-03 9 KSP unpreconditioned resid norm 6.353576889135e-03 true resid norm 6.353576889135e-03 ||r(i)||/||b|| 2.623820432363e-01 10 KSP Residual norm 7.313589502731e-03 10 KSP unpreconditioned resid norm 7.313589502731e-03 true resid norm 7.313589502731e-03 ||r(i)||/||b|| 3.020274391264e-01 11 KSP Residual norm 6.643320423193e-03 11 KSP unpreconditioned resid norm 6.643320423193e-03 true resid norm 6.643320423193e-03 ||r(i)||/||b|| 2.743475080142e-01 12 KSP Residual norm 7.235443182108e-03 12 KSP unpreconditioned resid norm 7.235443182108e-03 true resid norm 7.235443182108e-03 ||r(i)||/||b|| 2.988002504681e-01 13 KSP Residual norm 4.971292307201e-03 13 KSP unpreconditioned resid norm 4.971292307201e-03 true resid norm 4.971292307201e-03 ||r(i)||/||b|| 2.052981896416e-01 14 KSP Residual norm 5.357933842147e-03 14 KSP unpreconditioned resid norm 5.357933842147e-03 true resid norm 5.357933842147e-03 ||r(i)||/||b|| 2.212652264320e-01 15 KSP Residual norm 5.841682994497e-03 15 KSP unpreconditioned resid norm 5.841682994497e-03 true resid norm 5.841682994497e-03 ||r(i)||/||b|| 2.412424917146e-01 Linear mg_levels_1_ solve converged due to CONVERGED_ITS iterations 15 Cheers, Elena ________________________________ From: Barry Smith <[email protected]<mailto:[email protected]>> Sent: 29 September 2025 20:31:26 To: Moral Sanchez, Elena Cc: Mark Adams; petsc-users Subject: Re: [petsc-users] setting correct tolerances for MG smoother CG at the finest level Thanks. I missed something earlier in the KSPView using UNPRECONDITIONED norm type for convergence test Please add the options -ksp_monitor_true_residual -mg_levels_ksp_monitor_true_residual It is using the unpreconditioned residual norms for convergence testing but we are printing the preconditioned norms. Barry On Sep 29, 2025, at 11:12 AM, Moral Sanchez, Elena <[email protected]<mailto:[email protected]>> wrote: This is the output: Residual norms for mg_levels_1_ solve. 0 KSP Residual norm 2.249726733143e+00 1 KSP Residual norm 1.433120400946e+00 2 KSP Residual norm 1.169262560123e+00 3 KSP Residual norm 1.323528716607e+00 4 KSP Residual norm 5.006323254234e-01 5 KSP Residual norm 3.569836784785e-01 6 KSP Residual norm 2.493182937513e-01 7 KSP Residual norm 3.038202502298e-01 8 KSP Residual norm 2.780214194402e-01 9 KSP Residual norm 1.676826341491e-01 10 KSP Residual norm 1.209985378713e-01 11 KSP Residual norm 9.445076689969e-02 12 KSP Residual norm 8.308555284580e-02 13 KSP Residual norm 5.472865592585e-02 14 KSP Residual norm 4.357870564398e-02 15 KSP Residual norm 5.079681292439e-02 Linear mg_levels_1_ solve converged due to CONVERGED_ITS iterations 15 Residual norms for mg_levels_1_ solve. 0 KSP Residual norm 5.079681292439e-02 1 KSP Residual norm 2.934938644003e-02 2 KSP Residual norm 3.257065831294e-02 3 KSP Residual norm 4.143063876867e-02 4 KSP Residual norm 4.822471409489e-02 5 KSP Residual norm 3.197538246153e-02 6 KSP Residual norm 3.461217019835e-02 7 KSP Residual norm 3.410193775327e-02 8 KSP Residual norm 4.690424294464e-02 9 KSP Residual norm 3.366148892800e-02 10 KSP Residual norm 4.068015727689e-02 11 KSP Residual norm 2.658836123104e-02 12 KSP Residual norm 2.826244186003e-02 13 KSP Residual norm 2.981793619508e-02 14 KSP Residual norm 3.525455091450e-02 15 KSP Residual norm 2.331539121838e-02 Linear mg_levels_1_ solve converged due to CONVERGED_ITS iterations 15 Residual norms for mg_levels_1_ solve. 0 KSP Residual norm 2.421498365806e-02 1 KSP Residual norm 1.761072112362e-02 2 KSP Residual norm 1.400842489042e-02 3 KSP Residual norm 1.419665483348e-02 4 KSP Residual norm 1.617590701667e-02 5 KSP Residual norm 1.354824081005e-02 6 KSP Residual norm 1.387252917475e-02 7 KSP Residual norm 1.514043102087e-02 8 KSP Residual norm 1.275811124745e-02 9 KSP Residual norm 1.241039155981e-02 10 KSP Residual norm 9.585207801652e-03 11 KSP Residual norm 9.022641230732e-03 12 KSP Residual norm 1.187709152046e-02 13 KSP Residual norm 1.084880112494e-02 14 KSP Residual norm 8.194750346781e-03 15 KSP Residual norm 7.614246199165e-03 Linear mg_levels_1_ solve converged due to CONVERGED_ITS iterations 15 Residual norms for mg_levels_1_ solve. 0 KSP Residual norm 7.614246199165e-03 1 KSP Residual norm 5.620014684145e-03 2 KSP Residual norm 6.643368363907e-03 3 KSP Residual norm 8.708642393659e-03 4 KSP Residual norm 6.401852907459e-03 5 KSP Residual norm 7.230576215262e-03 6 KSP Residual norm 6.204081601285e-03 7 KSP Residual norm 7.038656665944e-03 8 KSP Residual norm 7.194079694050e-03 9 KSP Residual norm 6.353576889135e-03 10 KSP Residual norm 7.313589502731e-03 11 KSP Residual norm 6.643320423193e-03 12 KSP Residual norm 7.235443182108e-03 13 KSP Residual norm 4.971292307201e-03 14 KSP Residual norm 5.357933842147e-03 15 KSP Residual norm 5.841682994497e-03 Linear mg_levels_1_ solve converged due to CONVERGED_ITS iterations 15 ________________________________ From: Barry Smith <[email protected]<mailto:[email protected]>> Sent: 29 September 2025 15:56:33 To: Moral Sanchez, Elena Cc: Mark Adams; petsc-users Subject: Re: [petsc-users] setting correct tolerances for MG smoother CG at the finest level I asked you to run with -ksp_monitor -mg_levels_ksp_monitor -ksp_converged_reason -mg_levels_ksp_converged_reason you chose not to, delaying the process of understanding what is happening. Please run with those options and send the output. My guess is that you are computing the "residual norms" in your own monitor code, and it is doing so differently than what PETSc does, thus resulting in the appearance of a sufficiently small residual norm, whereas PETSc may not have calculated something that small. Barry On Sep 29, 2025, at 8:39 AM, Moral Sanchez, Elena <[email protected]<mailto:[email protected]>> wrote: Thanks for the hint. I agree that the coarse solve should be much more "accurate". However, for the moment I am just trying to understand what the MG is doing exactly. I am puzzled to see that the fine grid smoother ("lvl 0") does not stop when the residual becomes less than 1e-1. It should converge due to the atol. ________________________________ From: Mark Adams <[email protected]<mailto:[email protected]>> Sent: 29 September 2025 14:20:56 To: Moral Sanchez, Elena Cc: Barry Smith; petsc-users Subject: Re: [petsc-users] setting correct tolerances for MG smoother CG at the finest level Oh I see the coarse grid solver in your full solver output now. You still want an accurate coarse grid solve. Usually (the default in GAMG) you use a direct solver on one process, and cousin until the coarse grid is small enough to make that cheap. On Mon, Sep 29, 2025 at 8:07 AM Moral Sanchez, Elena <[email protected]<mailto:[email protected]>> wrote: Hi, I doubled the system size and changed the tolerances just to show a better example of the problem. This is the output of the callbacks in the first iteration: CG Iter 0/1 | res = 2.25e+00/1.00e-09 | 0.1 s MG lvl 0 (s=884): CG Iter 0/15 | res = 2.25e+00/1.00e-01 | 0.3 s MG lvl 0 (s=884): CG Iter 1/15 | res = 1.43e+00/1.00e-01 | 0.2 s MG lvl 0 (s=884): CG Iter 2/15 | res = 1.17e+00/1.00e-01 | 0.2 s MG lvl 0 (s=884): CG Iter 3/15 | res = 1.32e+00/1.00e-01 | 0.1 s MG lvl 0 (s=884): CG Iter 4/15 | res = 5.01e-01/1.00e-01 | 0.2 s MG lvl 0 (s=884): CG Iter 5/15 | res = 3.57e-01/1.00e-01 | 0.2 s MG lvl 0 (s=884): CG Iter 6/15 | res = 2.49e-01/1.00e-01 | 0.2 s MG lvl 0 (s=884): CG Iter 7/15 | res = 3.04e-01/1.00e-01 | 0.2 s MG lvl 0 (s=884): CG Iter 8/15 | res = 2.78e-01/1.00e-01 | 0.2 s MG lvl 0 (s=884): CG Iter 9/15 | res = 1.68e-01/1.00e-01 | 0.2 s MG lvl 0 (s=884): CG Iter 10/15 | res = 1.21e-01/1.00e-01 | 0.1 s MG lvl 0 (s=884): CG Iter 11/15 | res = 9.45e-02/1.00e-01 | 0.2 s MG lvl 0 (s=884): CG Iter 12/15 | res = 8.31e-02/1.00e-01 | 0.2 s MG lvl 0 (s=884): CG Iter 13/15 | res = 5.47e-02/1.00e-01 | 0.2 s MG lvl 0 (s=884): CG Iter 14/15 | res = 4.36e-02/1.00e-01 | 0.2 s MG lvl 0 (s=884): CG Iter 15/15 | res = 5.08e-02/1.00e-01 | 0.1 s ConvergedReason MG lvl 0: 4 MG lvl -1 (s=524): CG Iter 0/15 | res = 8.15e-02/1.00e-01 | 3.0 s ConvergedReason MG lvl -1: 3 MG lvl 0 (s=884): CG Iter 0/15 | res = 5.08e-02/1.00e-01 | 0.3 s MG lvl 0 (s=884): CG Iter 1/15 | res = 2.93e-02/1.00e-01 | 0.2 s MG lvl 0 (s=884): CG Iter 2/15 | res = 3.26e-02/1.00e-01 | 0.2 s MG lvl 0 (s=884): CG Iter 3/15 | res = 4.14e-02/1.00e-01 | 0.2 s MG lvl 0 (s=884): CG Iter 4/15 | res = 4.82e-02/1.00e-01 | 0.2 s MG lvl 0 (s=884): CG Iter 5/15 | res = 3.20e-02/1.00e-01 | 0.2 s MG lvl 0 (s=884): CG Iter 6/15 | res = 3.46e-02/1.00e-01 | 0.3 s MG lvl 0 (s=884): CG Iter 7/15 | res = 3.41e-02/1.00e-01 | 0.2 s MG lvl 0 (s=884): CG Iter 8/15 | res = 4.69e-02/1.00e-01 | 0.2 s MG lvl 0 (s=884): CG Iter 9/15 | res = 3.37e-02/1.00e-01 | 0.2 s MG lvl 0 (s=884): CG Iter 10/15 | res = 4.07e-02/1.00e-01 | 0.2 s MG lvl 0 (s=884): CG Iter 11/15 | res = 2.66e-02/1.00e-01 | 0.1 s MG lvl 0 (s=884): CG Iter 12/15 | res = 2.83e-02/1.00e-01 | 0.1 s MG lvl 0 (s=884): CG Iter 13/15 | res = 2.98e-02/1.00e-01 | 0.2 s MG lvl 0 (s=884): CG Iter 14/15 | res = 3.53e-02/1.00e-01 | 0.1 s MG lvl 0 (s=884): CG Iter 15/15 | res = 2.33e-02/1.00e-01 | 0.2 s ConvergedReason MG lvl 0: 4 CG Iter 1/1 | res = 2.42e-02/1.00e-09 | 5.6 s MG lvl 0 (s=884): CG Iter 0/15 | res = 2.42e-02/1.00e-01 | 0.2 s MG lvl 0 (s=884): CG Iter 1/15 | res = 1.76e-02/1.00e-01 | 0.2 s MG lvl 0 (s=884): CG Iter 2/15 | res = 1.40e-02/1.00e-01 | 0.2 s MG lvl 0 (s=884): CG Iter 3/15 | res = 1.42e-02/1.00e-01 | 0.1 s MG lvl 0 (s=884): CG Iter 4/15 | res = 1.62e-02/1.00e-01 | 0.2 s MG lvl 0 (s=884): CG Iter 5/15 | res = 1.35e-02/1.00e-01 | 0.1 s MG lvl 0 (s=884): CG Iter 6/15 | res = 1.39e-02/1.00e-01 | 0.1 s MG lvl 0 (s=884): CG Iter 7/15 | res = 1.51e-02/1.00e-01 | 0.2 s MG lvl 0 (s=884): CG Iter 8/15 | res = 1.28e-02/1.00e-01 | 0.2 s MG lvl 0 (s=884): CG Iter 9/15 | res = 1.24e-02/1.00e-01 | 0.2 s MG lvl 0 (s=884): CG Iter 10/15 | res = 9.59e-03/1.00e-01 | 0.1 s MG lvl 0 (s=884): CG Iter 11/15 | res = 9.02e-03/1.00e-01 | 0.1 s MG lvl 0 (s=884): CG Iter 12/15 | res = 1.19e-02/1.00e-01 | 0.1 s MG lvl 0 (s=884): CG Iter 13/15 | res = 1.08e-02/1.00e-01 | 0.1 s MG lvl 0 (s=884): CG Iter 14/15 | res = 8.19e-03/1.00e-01 | 0.1 s MG lvl 0 (s=884): CG Iter 15/15 | res = 7.61e-03/1.00e-01 | 0.1 s ConvergedReason MG lvl 0: 4 MG lvl -1 (s=524): CG Iter 0/15 | res = 1.38e-02/1.00e-01 | 5.2 s ConvergedReason MG lvl -1: 3 MG lvl 0 (s=884): CG Iter 0/15 | res = 7.61e-03/1.00e-01 | 0.2 s MG lvl 0 (s=884): CG Iter 1/15 | res = 5.62e-03/1.00e-01 | 0.1 s MG lvl 0 (s=884): CG Iter 2/15 | res = 6.64e-03/1.00e-01 | 0.1 s MG lvl 0 (s=884): CG Iter 3/15 | res = 8.71e-03/1.00e-01 | 0.1 s MG lvl 0 (s=884): CG Iter 4/15 | res = 6.40e-03/1.00e-01 | 0.1 s MG lvl 0 (s=884): CG Iter 5/15 | res = 7.23e-03/1.00e-01 | 0.1 s MG lvl 0 (s=884): CG Iter 6/15 | res = 6.20e-03/1.00e-01 | 0.1 s MG lvl 0 (s=884): CG Iter 7/15 | res = 7.04e-03/1.00e-01 | 0.1 s MG lvl 0 (s=884): CG Iter 8/15 | res = 7.19e-03/1.00e-01 | 0.1 s MG lvl 0 (s=884): CG Iter 9/15 | res = 6.35e-03/1.00e-01 | 0.1 s MG lvl 0 (s=884): CG Iter 10/15 | res = 7.31e-03/1.00e-01 | 0.1 s MG lvl 0 (s=884): CG Iter 11/15 | res = 6.64e-03/1.00e-01 | 0.1 s MG lvl 0 (s=884): CG Iter 12/15 | res = 7.24e-03/1.00e-01 | 0.1 s MG lvl 0 (s=884): CG Iter 13/15 | res = 4.97e-03/1.00e-01 | 0.1 s MG lvl 0 (s=884): CG Iter 14/15 | res = 5.36e-03/1.00e-01 | 0.1 s MG lvl 0 (s=884): CG Iter 15/15 | res = 5.84e-03/1.00e-01 | 0.1 s ConvergedReason MG lvl 0: 4 CG ConvergedReason: -3 For completeness, I add here the -ksp_view of the whole solver: KSP Object: 1 MPI process type: cg variant HERMITIAN maximum iterations=1, nonzero initial guess tolerances: relative=1e-08, absolute=1e-09, divergence=10000. left preconditioning using UNPRECONDITIONED norm type for convergence test PC Object: 1 MPI process type: mg type is MULTIPLICATIVE, levels=2 cycles=v Cycles per PCApply=1 Not using Galerkin computed coarse grid matrices Coarse grid solver -- level 0 ------------------------------- KSP Object: (mg_coarse_) 1 MPI process type: cg variant HERMITIAN maximum iterations=15, nonzero initial guess tolerances: relative=0.1, absolute=0.1, divergence=1e+30 left preconditioning using UNPRECONDITIONED norm type for convergence test PC Object: (mg_coarse_) 1 MPI process type: none linear system matrix = precond matrix: Mat Object: 1 MPI process type: python rows=524, cols=524 Python: Solver_petsc.LeastSquaresOperator Down solver (pre-smoother) on level 1 ------------------------------- KSP Object: (mg_levels_1_) 1 MPI process type: cg variant HERMITIAN maximum iterations=15, nonzero initial guess tolerances: relative=0.1, absolute=0.1, divergence=1e+30 left preconditioning using UNPRECONDITIONED norm type for convergence test PC Object: (mg_levels_1_) 1 MPI process type: none linear system matrix = precond matrix: Mat Object: 1 MPI process type: python rows=884, cols=884 Python: Solver_petsc.LeastSquaresOperator Up solver (post-smoother) same as down solver (pre-smoother) linear system matrix = precond matrix: Mat Object: 1 MPI process type: python rows=884, cols=884 Python: Solver_petsc.LeastSquaresOperator Regarding Mark's Email: What do you mean with "the whole solver doesn't have a coarse grid"? I am using my own Restriction and Interpolation operators. Thanks for the help, Elena ________________________________ From: Mark Adams <[email protected]<mailto:[email protected]>> Sent: 28 September 2025 20:13:54 To: Barry Smith Cc: Moral Sanchez, Elena; petsc-users Subject: Re: [petsc-users] setting correct tolerances for MG smoother CG at the finest level Not sure why your "whole"solver does not have a coarse grid but this is wrong: KSP Object: (mg_coarse_) 1 MPI process type: cg variant HERMITIAN maximum iterations=100, initial guess is zero tolerances: relative=0.1, absolute=0.1, divergence=1e+30 The coarse grid has to be accurate. The defaults are a good place to start: max_it=10.000, rtol=1e-5, atol=1e-30 (ish) On Fri, Sep 26, 2025 at 3:21 PM Barry Smith <[email protected]<mailto:[email protected]>> wrote: Looks reasonable. Send the output running with -ksp_monitor -mg_levels_ksp_monitor -ksp_converged_reason -mg_levels_ksp_converged_reason On Sep 26, 2025, at 1:19 PM, Moral Sanchez, Elena <[email protected]<mailto:[email protected]>> wrote: Dear Barry, This is -ksp_view for the smoother at the finest level: KSP Object: (mg_levels_1_) 1 MPI process type: cg variant HERMITIAN maximum iterations=10, nonzero initial guess tolerances: relative=0.1, absolute=0.1, divergence=1e+30 left preconditioning using UNPRECONDITIONED norm type for convergence test PC Object: (mg_levels_1_) 1 MPI process type: none linear system matrix = precond matrix: Mat Object: 1 MPI process type: python rows=524, cols=524 Python: Solver_petsc.LeastSquaresOperator And at the coarsest level: KSP Object: (mg_coarse_) 1 MPI process type: cg variant HERMITIAN maximum iterations=100, initial guess is zero tolerances: relative=0.1, absolute=0.1, divergence=1e+30 left preconditioning using UNPRECONDITIONED norm type for convergence test PC Object: (mg_coarse_) 1 MPI process type: none linear system matrix = precond matrix: Mat Object: 1 MPI process type: python rows=344, cols=344 Python: Solver_petsc.LeastSquaresOperator And for the whole solver: KSP Object: 1 MPI process type: cg variant HERMITIAN maximum iterations=100, nonzero initial guess tolerances: relative=1e-08, absolute=1e-09, divergence=10000. left preconditioning using UNPRECONDITIONED norm type for convergence test PC Object: 1 MPI process type: mg type is MULTIPLICATIVE, levels=2 cycles=v Cycles per PCApply=1 Not using Galerkin computed coarse grid matrices Coarse grid solver -- level 0 ------------------------------- KSP Object: (mg_coarse_) 1 MPI process type: cg variant HERMITIAN maximum iterations=100, initial guess is zero tolerances: relative=0.1, absolute=0.1, divergence=1e+30 left preconditioning using UNPRECONDITIONED norm type for convergence test PC Object: (mg_coarse_) 1 MPI process type: none linear system matrix = precond matrix: Mat Object: 1 MPI process type: python rows=344, cols=344 Python: Solver_petsc.LeastSquaresOperator Down solver (pre-smoother) on level 1 ------------------------------- KSP Object: (mg_levels_1_) 1 MPI process type: cg variant HERMITIAN maximum iterations=10, nonzero initial guess tolerances: relative=0.1, absolute=0.1, divergence=1e+30 left preconditioning using UNPRECONDITIONED norm type for convergence test PC Object: (mg_levels_1_) 1 MPI process type: none linear system matrix = precond matrix: Mat Object: 1 MPI process type: python rows=524, cols=524 Python: Solver_petsc.LeastSquaresOperator Up solver (post-smoother) same as down solver (pre-smoother) linear system matrix = precond matrix: Mat Object: 1 MPI process type: python rows=524, cols=524 Python: Solver_petsc.LeastSquaresOperator Best, Elena ________________________________ From: Barry Smith <[email protected]<mailto:[email protected]>> Sent: 26 September 2025 19:05:02 To: Moral Sanchez, Elena Cc: [email protected]<mailto:[email protected]> Subject: Re: [petsc-users] setting correct tolerances for MG smoother CG at the finest level Send the output using -ksp_view Normally one uses a fixed number of iterations of smoothing on level with multigrid rather than a tolerance, but yes PETSc should respect such a tolerance. Barry On Sep 26, 2025, at 12:49 PM, Moral Sanchez, Elena <[email protected]<mailto:[email protected]>> wrote: Hi, I am using multigrid (multiplicative) as a preconditioner with a V-cycle of two levels. At each level, I am setting CG as the smoother with certain tolerance. What I observe is that in the finest level the CG continues iterating after the residual norm reaches the tolerance (atol) and it only stops when reaching the maximum number of iterations at that level. At the coarsest level this does not occur and the CG stops when the tolerance is reached. I double-checked that the smoother at the finest level has the right tolerance. And I am using a Monitor function to track the residual. Do you know how to make the smoother at the finest level stop when reaching the tolerance? Cheers, Elena.
