[petsc-dev] VecNest

[Pierre Jolivet]

> On 12 Jul 2022, at 2:04 PM, Mark Adams <mfadams at lbl.gov> wrote:
> 
> It looks like the RHS is zero in the 2nd case (0 KSP unpreconditioned resid norm 0.000000000000e+00), but the true residual is the same.
> 
> It looks like you added "nest_subvec" to our example.

It’s a new (dumb) example (ex1111).

> You can start by looking at the vectors with -vec_view (there is code that you can view vectors explicitly in your code to get more control if this is too noisy)

The Vec are exactly the same (as reported by -ksp_view_rhs) in both cases.

> You could send us the diffs in ex111. We may be able to see something wrong.

I believe there is something weird inside KSPGMRES + PCFIELDSPLIT + right preconditioning.
Because if I switch to KSPHPDDM implementation of GMRES, then all is good.
In the below traces, I switched VecSet(part of RHS,0.0) to VecSet(part of RHS,1.0) in ex1111.c to make the error more visible.

Thanks,
Pierre

$ ./ex1111 -pc_type fieldsplit -ksp_converged_reason -fieldsplit_pc_type lu -ksp_pc_side right -ksp_view_final_residual -nest_subvec false -ksp_view_rhs -ksp_view_solution          
Linear solve did not converge due to DIVERGED_BREAKDOWN iterations 30 < garbage
KSP final norm of residual 1.75544 < garbage
Vec Object: 1 MPI process
  type: nest
  VecNest, rows=2,  structure: 
  (0) : type=seq, rows=5 
    Vec Object: 1 MPI process
      type: seq
    0.720032
    0.061794
    0.0100223
    0.144639
    0.397778
  (1) : type=seq, rows=5 
    Vec Object: 1 MPI process
      type: seq
    1.
    1.
    1.
    1.
    1.
Vec Object: 1 MPI process
  type: nest
  VecNest, rows=2,  structure: 
  (0) : type=seq, rows=5 
    Vec Object: 1 MPI process
      type: seq
    1.73518 < garbage
    2.07583e-16
    -8.13135e-16
    -4.24749e-17
    8.18337e-16
  (1) : type=seq, rows=5 
    Vec Object: 1 MPI process
      type: seq
    -1.73518 < garbage
    -1.73518 < garbage
    -1.73518 < garbage
    -1.73518 < garbage
    -1.73518 < garbage

$ ./ex1111 -pc_type fieldsplit -ksp_converged_reason -fieldsplit_pc_type lu -ksp_pc_side right -ksp_view_final_residual -nest_subvec false -ksp_view_rhs -ksp_view_solution -ksp_type hpddm
Linear solve converged due to CONVERGED_RTOL iterations 1 < correct
KSP final norm of residual 7.89955e-16
Vec Object: 1 MPI process
  type: nest
  VecNest, rows=2,  structure: 
  (0) : type=seq, rows=5 
    Vec Object: 1 MPI process
      type: seq
    0.720032
    0.061794
    0.0100223
    0.144639
    0.397778
  (1) : type=seq, rows=5 
    Vec Object: 1 MPI process
      type: seq
    1.
    1.
    1.
    1.
    1.
Vec Object: 1 MPI process
  type: nest
  VecNest, rows=2,  structure: 
  (0) : type=seq, rows=5 
    Vec Object: 1 MPI process
      type: seq
    1. < correct
    1.79841e-18
    -5.45696e-17
    8.83987e-18
    2.82354e-17
  (1) : type=seq, rows=5 
    Vec Object: 1 MPI process
      type: seq
    -1. < correct
    -1. < correct
    -1. < correct
    -1. < correct
    -1. < correct

$ ./ex1111 -pc_type fieldsplit -ksp_converged_reason -fieldsplit_pc_type lu -ksp_pc_side right -ksp_view_final_residual -nest_subvec true -ksp_view_rhs -ksp_view_solution
Linear solve converged due to CONVERGED_RTOL iterations 1 < correct
KSP final norm of residual 2.74152e-16
Vec Object: 1 MPI process
  type: nest
  VecNest, rows=2,  structure: 
  (0) : type=seq, rows=5 
    Vec Object: 1 MPI process
      type: seq
    0.720032
    0.061794
    0.0100223
    0.144639
    0.397778
  (1) : type=seq, rows=5 
    Vec Object: 1 MPI process
      type: seq
    1.
    1.
    1.
    1.
    1.
Vec Object: 1 MPI process
  type: nest
  VecNest, rows=2,  structure: 
  (0) : type=seq, rows=5 
    Vec Object: 1 MPI process
      type: seq
    1. < correct
    -3.75571e-17
    2.51808e-16
    7.69081e-18
    -2.65477e-16
  (1) : type=seq, rows=5 
    Vec Object: 1 MPI process
      type: seq
    -1. < correct
    -1. < correct
    -1. < correct
    -1. < correct
    -1. < correct

> Mark
> 
> On Mon, Jul 11, 2022 at 2:17 PM Pierre Jolivet <pierre at joliv.et <mailto:pierre at joliv.et>> wrote:
> Hello,
> Could anyone help me understand what is going on in the following example, please?
> I have a VecNest.
> I either: a) initialize all values to 0.0, then set a specific part of the vector to nonzero or b) initialize a part of the vector to 0.0 and set the other part to nonzero.
> I don’t see why a) and b) produce different results.
> 
> $ ./ex1111 -pc_type fieldsplit -ksp_monitor_true_residual -ksp_converged_reason -fieldsplit_pc_type jacobi -ksp_pc_side right -ksp_view_final_residual -nest_subvec true 
>   0 KSP unpreconditioned resid norm 8.375635517980e-01 true resid norm 8.375635517980e-01 ||r(i)||/||b|| 1.000000000000e+00
>   1 KSP unpreconditioned resid norm 4.748816884247e-01 true resid norm 4.748816884247e-01 ||r(i)||/||b|| 5.669798875623e-01
>   2 KSP unpreconditioned resid norm 4.713006778679e-01 true resid norm 4.713006778679e-01 ||r(i)||/||b|| 5.627043784990e-01
>   3 KSP unpreconditioned resid norm 7.092979927129e-02 true resid norm 7.092979927129e-02 ||r(i)||/||b|| 8.468587144106e-02
>   4 KSP unpreconditioned resid norm 1.457836310255e-02 true resid norm 1.457836310255e-02 ||r(i)||/||b|| 1.740567992870e-02
>   5 KSP unpreconditioned resid norm 1.625040500524e-14 true resid norm 1.633468028779e-14 ||r(i)||/||b|| 1.950261595401e-14
> Linear solve converged due to CONVERGED_RTOL iterations 5
> KSP final norm of residual 1.63347e-14
> $ ./ex1111 -pc_type fieldsplit -ksp_monitor_true_residual -ksp_converged_reason -fieldsplit_pc_type jacobi -ksp_pc_side right -ksp_view_final_residual -nest_subvec false
>   0 KSP unpreconditioned resid norm 0.000000000000e+00 true resid norm 8.375635517980e-01 ||r(i)||/||b||            inf
> Linear solve converged due to CONVERGED_ATOL iterations 0
> KSP final norm of residual 0.837564
> 
> Thanks,
> Pierre
> 

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