[petsc-users] Question about residue norm in PETSc

[Barry Smith]

  Also look at https://petsc.org/release/manualpages/KSP/KSPSetPCSide/#kspsetpcside and https://petsc.org/release/manualpages/KSP/KSPSetNormType/#kspsetnormtype  in PETSc different Krylov solvers have different default values for this. 



> On Jul 2, 2023, at 1:47 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)
> 
> 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
> 
> <main.c><Makefile><mmio.h><mmloader.h><slurm-4803840.out><task.sh>

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