[petsc-users] Diagnosing Convergence Issue in Fieldsplit Problem

Barry Smith bsmith at petsc.dev
Wed May 22 16:39:26 CDT 2024 

  Are you using any other command line options or did you hardwire any solver parameters in the code with, like, KSPSetXXX() or PCSetXXX() Please send all of them.

  Something funky definitely happened when the true residual norms jumped up.

  Could you run the same thing with -ksp_view and don't use any thing like -ksp_error_if_not_converged so we can see exactly what is being run.

  Barry


> On May 22, 2024, at 3:21 PM, Colton Bryant <coltonbryant2021 at u.northwestern.edu> wrote:
> 
> This Message Is From an External Sender
> This message came from outside your organization.
> Hello,
> 
> I am solving the Stokes equations on a MAC grid discretized by finite differences using a DMSTAG object. I have tested the solver quite extensively on manufactured problems and it seems to work well. As I am still just trying to get things working and not yet worried about speed I am using the following solver options: 
> -pc_type fieldsplit
> -pc_fieldsplit_detect_saddle_point
> -fieldsplit_0_pc_type lu
> -fieldsplit_1_ksp_rtol 1.e-8
> 
> However I am now using this solver as an inner step of a larger code and have run into issues. The code repeatedly solves the Stokes equations with varying right hand sides coming from changing problem geometry (the solver is a part of an overset grid scheme coupled to a level set method evolving in time). After a couple timesteps I observe the following output when running with -fieldsplit_1_ksp_converged_reason -fieldsplit_1_ksp_monitor_true_residual: 
> 
> Residual norms for back_fieldsplit_1_ solve.
>     0 KSP preconditioned resid norm 2.826514299465e-02 true resid norm 2.826514299465e-02 ||r(i)||/||b|| 1.000000000000e+00
>     1 KSP preconditioned resid norm 7.286621865915e-03 true resid norm 7.286621865915e-03 ||r(i)||/||b|| 2.577953300039e-01
>     2 KSP preconditioned resid norm 1.500598474492e-03 true resid norm 1.500598474492e-03 ||r(i)||/||b|| 5.309007192273e-02
>     3 KSP preconditioned resid norm 3.796396924978e-04 true resid norm 3.796396924978e-04 ||r(i)||/||b|| 1.343137349666e-02
>     4 KSP preconditioned resid norm 8.091057439816e-05 true resid norm 8.091057439816e-05 ||r(i)||/||b|| 2.862556697960e-03
>     5 KSP preconditioned resid norm 3.689113122359e-05 true resid norm 3.689113122359e-05 ||r(i)||/||b|| 1.305181128239e-03
>     6 KSP preconditioned resid norm 2.116450533352e-05 true resid norm 2.116450533352e-05 ||r(i)||/||b|| 7.487846545662e-04
>     7 KSP preconditioned resid norm 3.968234031201e-06 true resid norm 3.968234031200e-06 ||r(i)||/||b|| 1.403932055801e-04
>     8 KSP preconditioned resid norm 6.666949419511e-07 true resid norm 6.666949419506e-07 ||r(i)||/||b|| 2.358717739644e-05
>     9 KSP preconditioned resid norm 1.941522884928e-07 true resid norm 1.941522884931e-07 ||r(i)||/||b|| 6.868965372998e-06
>    10 KSP preconditioned resid norm 6.729545258682e-08 true resid norm 6.729545258626e-08 ||r(i)||/||b|| 2.380863687793e-06
>    11 KSP preconditioned resid norm 3.009070131709e-08 true resid norm 3.009070131735e-08 ||r(i)||/||b|| 1.064586912687e-06
>    12 KSP preconditioned resid norm 7.849353009588e-09 true resid norm 7.849353009903e-09 ||r(i)||/||b|| 2.777043445840e-07
>    13 KSP preconditioned resid norm 2.306283345754e-09 true resid norm 2.306283346677e-09 ||r(i)||/||b|| 8.159461097060e-08
>    14 KSP preconditioned resid norm 9.336302495083e-10 true resid norm 9.336302502503e-10 ||r(i)||/||b|| 3.303115255517e-08
>    15 KSP preconditioned resid norm 6.537456143401e-10 true resid norm 6.537456141617e-10 ||r(i)||/||b|| 2.312903968982e-08
>    16 KSP preconditioned resid norm 6.389159552788e-10 true resid norm 6.389159550304e-10 ||r(i)||/||b|| 2.260437724130e-08
>    17 KSP preconditioned resid norm 6.380905134246e-10 true resid norm 6.380905136023e-10 ||r(i)||/||b|| 2.257517372981e-08
>    18 KSP preconditioned resid norm 6.380440605992e-10 true resid norm 6.380440604688e-10 ||r(i)||/||b|| 2.257353025207e-08
>    19 KSP preconditioned resid norm 6.380427156582e-10 true resid norm 6.380427157894e-10 ||r(i)||/||b|| 2.257348267830e-08
>    20 KSP preconditioned resid norm 6.380426714897e-10 true resid norm 6.380426714004e-10 ||r(i)||/||b|| 2.257348110785e-08
>    21 KSP preconditioned resid norm 6.380426656970e-10 true resid norm 6.380426658839e-10 ||r(i)||/||b|| 2.257348091268e-08
>    22 KSP preconditioned resid norm 6.380426650538e-10 true resid norm 6.380426650287e-10 ||r(i)||/||b|| 2.257348088242e-08
>    23 KSP preconditioned resid norm 6.380426649918e-10 true resid norm 6.380426645888e-10 ||r(i)||/||b|| 2.257348086686e-08
>    24 KSP preconditioned resid norm 6.380426649803e-10 true resid norm 6.380426644294e-10 ||r(i)||/||b|| 2.257348086122e-08
>    25 KSP preconditioned resid norm 6.380426649796e-10 true resid norm 6.380426649774e-10 ||r(i)||/||b|| 2.257348088061e-08
>    26 KSP preconditioned resid norm 6.380426649795e-10 true resid norm 6.380426653788e-10 ||r(i)||/||b|| 2.257348089481e-08
>    27 KSP preconditioned resid norm 6.380426649795e-10 true resid norm 6.380426646744e-10 ||r(i)||/||b|| 2.257348086989e-08
>    28 KSP preconditioned resid norm 6.380426649795e-10 true resid norm 6.380426650818e-10 ||r(i)||/||b|| 2.257348088430e-08
>    29 KSP preconditioned resid norm 6.380426649795e-10 true resid norm 6.380426649518e-10 ||r(i)||/||b|| 2.257348087970e-08
>    30 KSP preconditioned resid norm 6.380426652142e-10 true resid norm 6.380426652142e-10 ||r(i)||/||b|| 2.257348088898e-08
>    31 KSP preconditioned resid norm 6.380426652141e-10 true resid norm 6.380426646799e-10 ||r(i)||/||b|| 2.257348087008e-08
>    32 KSP preconditioned resid norm 6.380426652141e-10 true resid norm 6.380426648077e-10 ||r(i)||/||b|| 2.257348087460e-08
>    33 KSP preconditioned resid norm 6.380426652141e-10 true resid norm 6.380426649048e-10 ||r(i)||/||b|| 2.257348087804e-08
>    34 KSP preconditioned resid norm 6.380426652141e-10 true resid norm 6.380426648142e-10 ||r(i)||/||b|| 2.257348087483e-08
>    35 KSP preconditioned resid norm 6.380426652141e-10 true resid norm 6.380426651079e-10 ||r(i)||/||b|| 2.257348088522e-08
>    36 KSP preconditioned resid norm 6.380426652141e-10 true resid norm 6.380426650433e-10 ||r(i)||/||b|| 2.257348088294e-08
>    37 KSP preconditioned resid norm 6.380426652141e-10 true resid norm 6.380426649765e-10 ||r(i)||/||b|| 2.257348088057e-08
>    38 KSP preconditioned resid norm 6.380426652141e-10 true resid norm 6.380426650364e-10 ||r(i)||/||b|| 2.257348088269e-08
>    39 KSP preconditioned resid norm 6.380426652141e-10 true resid norm 6.380426650051e-10 ||r(i)||/||b|| 2.257348088159e-08
>    40 KSP preconditioned resid norm 6.380426652141e-10 true resid norm 6.380426651154e-10 ||r(i)||/||b|| 2.257348088549e-08
>    41 KSP preconditioned resid norm 6.380426652141e-10 true resid norm 6.380426650246e-10 ||r(i)||/||b|| 2.257348088227e-08
>    42 KSP preconditioned resid norm 6.380426652141e-10 true resid norm 6.380426650702e-10 ||r(i)||/||b|| 2.257348088389e-08
>    43 KSP preconditioned resid norm 6.380426652141e-10 true resid norm 6.380426651686e-10 ||r(i)||/||b|| 2.257348088737e-08
>    44 KSP preconditioned resid norm 6.380426652141e-10 true resid norm 6.380426650870e-10 ||r(i)||/||b|| 2.257348088448e-08
>    45 KSP preconditioned resid norm 6.380426652141e-10 true resid norm 6.380426651208e-10 ||r(i)||/||b|| 2.257348088568e-08
>    46 KSP preconditioned resid norm 6.380426652141e-10 true resid norm 6.380426651441e-10 ||r(i)||/||b|| 2.257348088650e-08
>    47 KSP preconditioned resid norm 6.380426652141e-10 true resid norm 6.380426650955e-10 ||r(i)||/||b|| 2.257348088478e-08
>    48 KSP preconditioned resid norm 6.380426652141e-10 true resid norm 6.380426650877e-10 ||r(i)||/||b|| 2.257348088451e-08
>    49 KSP preconditioned resid norm 6.380426652141e-10 true resid norm 6.380426651240e-10 ||r(i)||/||b|| 2.257348088579e-08
>    50 KSP preconditioned resid norm 6.380426652141e-10 true resid norm 6.380426650534e-10 ||r(i)||/||b|| 2.257348088329e-08
>    51 KSP preconditioned resid norm 6.380426652141e-10 true resid norm 6.380426648615e-10 ||r(i)||/||b|| 2.257348087651e-08
>    52 KSP preconditioned resid norm 6.380426652141e-10 true resid norm 6.380426649523e-10 ||r(i)||/||b|| 2.257348087972e-08
>    53 KSP preconditioned resid norm 6.380426652140e-10 true resid norm 6.380426652601e-10 ||r(i)||/||b|| 2.257348089061e-08
>    54 KSP preconditioned resid norm 6.380426652125e-10 true resid norm 6.380427512852e-10 ||r(i)||/||b|| 2.257348393411e-08
>    55 KSP preconditioned resid norm 6.380426651849e-10 true resid norm 6.380603444402e-10 ||r(i)||/||b|| 2.257410636701e-08
>    56 KSP preconditioned resid norm 6.380426646751e-10 true resid norm 6.439925413105e-10 ||r(i)||/||b|| 2.278398313542e-08
>    57 KSP preconditioned resid norm 6.380426514019e-10 true resid norm 2.674218007058e-09 ||r(i)||/||b|| 9.461186902765e-08
>    58 KSP preconditioned resid norm 6.380425077384e-10 true resid norm 2.406759314486e-08 ||r(i)||/||b|| 8.514937691775e-07
>    59 KSP preconditioned resid norm 6.380406171326e-10 true resid norm 3.100137288622e-07 ||r(i)||/||b|| 1.096805803957e-05
>     Linear back_fieldsplit_1_ solve did not converge due to DIVERGED_BREAKDOWN iterations 60
> 
> Any advice on steps I could take to elucidate the issue would be greatly appreciated. Thanks so much for any help in advance!
> 
> Best,
> Colton Bryant 

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