[petsc-users] SNES always ends at iteration 0

Sun Dec 12 18:47:46 CST 2021

Thank you so much for your reply !

In fact I didn't know how to set tolerances, so I proceeded without
specifying the tolerances, hoping that this could lead to the
implementation of default values...

I just added *SNESSetFromOptions(snes); *However, it doesn't make any
difference... But it's true that the "tol" values are somehow set to
zero...

*atol = 0.000000, rtol = 0.000000, stol = 0.000000, maxit = 50, maxf =
10000    0 SNES Function norm 7.604910424038e+02*

Is it possible that it has something to do with my makefile ?

Since I didn't figure out the PETSc makefile format (which seems to be
different from standard C makefile format), I named my source code as
*ex1.c* to make use of the default settings for PETSc example programs...

And in my makefile I wrote :

*include ${PETSC_DIR}/lib/petsc/conf/variablesinclude
${PETSC_DIR}/lib/petsc/conf/testex1: ex1.o*

Is it possible the "tol" values are set to 0 by the default setting used
for example programs ?

Thank you so much for your help.

PS: I just tried the same code with less degrees of freedom and this time
it worked... But for a large system it didn't...

*atol = 0.000000, rtol = 0.000000, stol = 0.000000, maxit = 50, maxf =
10000    0 SNES Function norm 1.164809703659e+00   1 SNES Function norm
1.311388740736e-01   2 SNES Function norm 7.232579319557e-02   3 SNES
Function norm 4.984271911548e-02   4 SNES Function norm 3.224387373805e-02
  5 SNES Function norm 6.898280568053e-03   6 SNES Function norm
6.297558001575e-03   7 SNES Function norm 5.358028396052e-03   8 SNES
Function norm 4.591005105466e-03   9 SNES Function norm 4.063981130201e-03
 10 SNES Function norm 3.715929394609e-03  11 SNES Function norm
3.428330101253e-03  12 SNES Function norm 3.177113603032e-03  13 SNES
Function norm 2.958574186594e-03  14 SNES Function norm 2.769227811865e-03
 15 SNES Function norm 2.605947870584e-03  16 SNES Function norm
2.465934405221e-03  17 SNES Function norm 2.346761136962e-03  18 SNES
Function norm 2.246362261451e-03  19 SNES Function norm 2.163102452591e-03
 20 SNES Function norm 2.095849101382e-03  21 SNES Function norm
2.043740325461e-03  22 SNES Function norm 2.005106316761e-03  23 SNES
Function norm 1.975748994170e-03  24 SNES Function norm 1.949413335428e-03
 25 SNES Function norm 1.920795414593e-03  26 SNES Function norm
1.886883259141e-03  27 SNES Function norm 1.846374653045e-03  28 SNES
Function norm 1.799050087038e-03  29 SNES Function norm 1.745284156916e-03
 30 SNES Function norm 1.685885151987e-03  31 SNES Function norm
1.621850994665e-03  32 SNES Function norm 1.554258940064e-03  33 SNES
Function norm 1.484213253375e-03  34 SNES Function norm 1.412768267404e-03
 35 SNES Function norm 1.340893218332e-03  36 SNES Function norm
1.269412489589e-03  37 SNES Function norm 1.199029202116e-03  38 SNES
Function norm 1.130300263372e-03  39 SNES Function norm 1.063694395854e-03
 40 SNES Function norm 9.995826338243e-04  41 SNES Function norm
9.383610129089e-04  42 SNES Function norm 8.807543352645e-04  43 SNES
Function norm 8.288695938590e-04  44 SNES Function norm 7.898873173876e-04
 45 SNES Function norm 7.752509690373e-04  46 SNES Function norm
7.625724154377e-04  47 SNES Function norm 7.503152403370e-04  48 SNES
Function norm 7.364744378378e-04  49 SNES Function norm 7.202926541551e-04
 50 SNES Function norm 7.015245603442e-04 *

Mark Adams <mfadams at lbl.gov> 于2021年12月13日周一 01:11写道：

> The three "tol" values should be finite. It sounds like you set them to 0.
> Don't do that and use the defaults to start.
> The behavior with zero tolerances is not defined.
> You can use -snes_monitor to print out the iterations.
>
> On Sun, Dec 12, 2021 at 6:22 PM celestechevali at gmail.com <
> celestechevali at gmail.com> wrote:
>
>> Hello,
>>
>> I encountered a strange problem concerning the convergence of SNES.
>>
>> In my recent test runs I found that SNES always stops at iteration 0...
>>
>> At first I thought there may be an error with the tolerance setting, so I
>> output the tolerances :
>>
>>
>>
>> *atol = 0.000000, rtol = 0.000000, stol = 0.000000, maxit = 50, maxf =
>> 10000  Norm of error 760.491 Iterations 0*
>>
>> Which are exactly the default values that I always used. However, for the
>> same tolerance settings, the SNES solver converges successfully if I
>> decrease the number of degrees of freedom in my system...
>>
>> I wish to know if anyone has experienced the same type of problems or has
>> an idea about what could possibly cause the problem...
>>
>> Thank you so much in advance.
>>
>> I appreciate any advice that you provide.
>>
>>
>>
>>
>>
>>
>>
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