[petsc-users] Question about residue norm in PETSc
Matthew Knepley
knepley at gmail.com
Sun Jul 2 07:32:24 CDT 2023
On Sun, Jul 2, 2023 at 8:19 AM 王赫萌 <wanghemeng at 163.com> wrote:
> Here is the mat and rhs used in code! (May need to change the data path)
>
> mat:
>
> https://studentcupeducn-my.sharepoint.com/:u:/g/personal/wanghemeng_student_cup_edu_cn/Ed76oGtC1ttDriZsObbPR74BCnDPUP8aicVXQEL4sO1AyQ?e=zeszik
> rhs:
>
> https://studentcupeducn-my.sharepoint.com/:u:/g/personal/wanghemeng_student_cup_edu_cn/EdHRqWbzVmtIkAppOLL1UMIBM7tK7ws0gEASESGHuGC3yw?e=SMQSmY
>
> I tried using
> PetscCall(VecAXPY(u, -1.0, b));
> but is just as same as
> PetscCall(VecAXPY(b, -1.0, u));
>
> Thank you so much for that!!!
>
90 KSP unpreconditioned resid norm 9.749157899195e+05 true resid norm
9.749157899195e+05 ||r(i)||/||b|| 1.340218569960e+01
91 KSP unpreconditioned resid norm 1.073123446417e+06 true resid norm
1.073123446417e+06 ||r(i)||/||b|| 1.475224820050e+01
92 KSP unpreconditioned resid norm 1.170251286554e+06 true resid norm
1.170251286554e+06 ||r(i)||/||b|| 1.608746644557e+01
93 KSP unpreconditioned resid norm 1.264719067990e+06 true resid norm
1.264719067990e+06 ||r(i)||/||b|| 1.738611681365e+01
94 KSP unpreconditioned resid norm 1.329446257320e+06 true resid norm
1.329446257320e+06 ||r(i)||/||b|| 1.827592270272e+01
95 KSP unpreconditioned resid norm 1.365944956372e+06 true resid norm
1.365944956372e+06 ||r(i)||/||b|| 1.877767100504e+01
96 KSP unpreconditioned resid norm 1.369513563400e+06 true resid norm
1.369513563400e+06 ||r(i)||/||b|| 1.882672871297e+01
97 KSP unpreconditioned resid norm 1.364905651654e+06 true resid norm
1.364905651654e+06 ||r(i)||/||b|| 1.876338366353e+01
98 KSP unpreconditioned resid norm 1.352584030803e+06 true resid norm
1.352584030803e+06 ||r(i)||/||b|| 1.859399810996e+01
99 KSP unpreconditioned resid norm 1.330589478009e+06 true resid norm
1.330589478009e+06 ||r(i)||/||b|| 1.829163857903e+01
100 KSP unpreconditioned resid norm 1.312782529439e+06 true resid norm
1.312782529439e+06 ||r(i)||/||b|| 1.804684612214e+01
Linear solve did not converge due to DIVERGED_ITS iterations 100
KSPSolve Time: 7579.364000 ms
norm_delta 1312782.529439, norm_b 72743.044439
Norm of error 18.0468 iterations 100
Residue of error 1.31278e+06 iterations 100
I ran with
-pc_type jacobi -ksp_max_it 100
because GAMG takes a long time to setup on my laptop. Those numbers match
exactly.
THanks,
Matt
I'm such a beginner T_T
> ---- Replied Message ----
> From Matthew Knepley<knepley at gmail.com> <knepley at gmail.com>
> Date 7/2/2023 20:10
> To 王赫萌<wanghemeng at 163.com> <wanghemeng at 163.com>
> Cc PETSc<petsc-users at mcs.anl.gov> <petsc-users at mcs.anl.gov>
> Subject Re: [petsc-users] Question about residue norm in PETSc
> On Sun, Jul 2, 2023 at 8:05 AM Matthew Knepley <knepley at gmail.com> wrote:
>
>> On Sun, Jul 2, 2023 at 7:53 AM 王赫萌 <wanghemeng at 163.com> wrote:
>>
>>> Thanks for your reply!
>>> So sorry that I made a mistake in the description.
>>> I set the tolerances by:
>>> PetscCall(KSPSetTolerances(ksp, 1e-12, DBL_MIN, PETSC_DEFAULT,
>>> PETSC_DEFAULT));
>>> and got (by passing `-ksp_norm_type unpreconditioned
>>> -ksp_monitor_true_residual`)
>>> 74 KSP unpreconditioned resid norm 7.256655641876e-08 true resid norm
>>> 7.256655641876e-08 ||r(i)||/||b|| 9.975738158726e-13
>>> I'm wondering why the ` ||r(i)||/||b||` is different with mine which
>>> calculated by:
>>> ```
>>> PetscCall(VecNorm(b, NORM_2, &norm_b)); // (main.c, line 74) (which is
>>> 72743.044439)
>>> PetscCall(VecDuplicate(b, &u)); // (main.c, line 105)
>>> PetscCall(MatMult(A, x, u));
>>> PetscCall(VecAXPY(b, -1.0, u));
>>> PetscCall(VecNorm(b, NORM_2, &norm_delta)); // (which is 0.039608)
>>> ```
>>> and (norm_delta) / (norm_b) = 5.44496e-07 which is higher and different
>>> with the rtol I set (1e-12).
>>> Sorry again for the waste of your time. I would really appreciated if
>>> you could help me again!
>>>
>>
>> 1) 7.256655641876e-08 / 72743.044439 = 9.975738158726e-13 so
>> ||r_i||/||b|| is correct in the output
>>
>> 2) You are asking why you calculate a different residual? I will have to
>> run your code.
>>
>
> I built your code, but you did not send the matrix and rhs.
>
> I suggest using
>
> PetscCall(VecAXPY(u, -1.0, b));
>
> instead so that you do not change b, and keep the residual in u.
>
> Thanks,
>
> Matt
>
> Thanks,
>
> Matt
>
>
>> Thanks,
>>
>> Matt
>>
>>
>>> Best regards!
>>> Hemeng Wang
>>>
>>>
>>> ---- Replied Message ----
>>> From Matthew Knepley<knepley at gmail.com> <knepley at gmail.com>
>>> Date 7/2/2023 18:51
>>> To 王赫萌<wanghemeng at 163.com> <wanghemeng at 163.com>
>>> Cc petsc-users at mcs.anl.gov<petsc-users at mcs.anl.gov>
>>> <petsc-users at mcs.anl.gov>
>>> Subject Re: [petsc-users] Question about residue norm in PETSc
>>> On Sun, Jul 2, 2023 at 2:24 AM 王赫萌 <wanghemeng at 163.com> wrote:
>>>
>>>> Dear PETSc Team,
>>>>
>>>> Sorry to bother! My name is Hemeng Wang, and I am currently learning
>>>> the use of PETSc software package. I am confused while calculating the norm
>>>> of residue.
>>>>
>>>> I calculated residue norm by myself with:
>>>> ```
>>>> PetscCall(VecNorm(b, NORM_2, &norm_b)); // (main.c, line 74)
>>>>
>>>> PetscCall(VecDuplicate(b, &u)); // (main.c, line 105)
>>>> PetscCall(MatMult(A, x, u));
>>>> PetscCall(VecAXPY(b, -1.0, u));
>>>> PetscCall(VecNorm(b, NORM_2, &norm_delta));
>>>> ```
>>>> and check the (norm_delta) / (norm_b). It seems not the `atol` which
>>>> set by `KSPSetTolerances()`.
>>>> (I set atol as 1e-12, but got 5e-7 finally)
>>>> (options: -ksp_type cg -pc_type gamg -ksp_converged_reason
>>>> -ksp_norm_type unpreconditioned -ksp_monitor_true_residual)
>>>>
>>>
>>> If you are using the default convergence test, there is an absolute
>>> tolerance (atol) _and_ a relative tolerance (rtol). It seems likely you hit
>>> the relative tolerance. You can check this using
>>>
>>> -ksp_converged_reason
>>>
>>> You could make rtol really small if you want to just see the atol
>>>
>>> -ksp_rtol 1e-20
>>>
>>> Thanks,
>>>
>>> Matt
>>>
>>>
>>>> I also check the soure code of `KSPSolve_CG` in
>>>> `petsc/src/ksp/ksp/impls/cg/cg.c`. And could not figure out where is the
>>>> difference.
>>>>
>>>> I will really really appreciated if someone can explain the calculation
>>>> of `resid norm` in petsc. And where is my mistake.
>>>>
>>>> To provide you with more context, here are the source code about my
>>>> implementation. And the output of my test.
>>>>
>>>> main.c
>>>> Main code of my program
>>>>
>>>> mmio.h mmloader.h
>>>> Headers for matrix read
>>>>
>>>> Makefile
>>>> For compiling, same as sample provided
>>>>
>>>> task.sh
>>>> A script for running program in `slurm`
>>>>
>>>> slurm-4803840.out
>>>> Output of my test
>>>>
>>>> Thank you very much for your time and attention. I greatly appreciate
>>>> your support and look forward to hearing from you soon.
>>>> Best regards,
>>>> Hemeng Wang
>>>>
>>>>
>>>
>>> --
>>> 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/>
>>>
>>
>>
>> --
>> 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/>
>>
>
>
> --
> 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/>
>
--
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/>
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