SNES predictor refactoring by bendudson · Pull Request #3506 · boutproject/BOUT-dev

bendudson · 2026-09-29T23:10:38Z

Separate the predictor into a separate Predictor class.

Set predictor type using solver:predictor:type. Options are constant, linear and be_rational that uses a damped relaxation to steady state.

An option solver:predictor:blend calculates the optimum scaling of the previous predictor step, and uses that to update the scaling of the next step.

Adds automatic detection of cases where predictors can cause numerical issues e.g. when the step size is below tolerance.

Includes documentation and unit tests.

Hermes-3 1D-threshold example

linear (default) predictor:

4.789e+03       3853       3.45e+00    89.0    0.0    0.1    1.1    9.7
9.579e+03        323       3.14e-01    81.9    0.0    0.1    8.8    9.1
1.437e+04        325       3.14e-01    82.1    0.0    0.1    8.6    9.2
1.916e+04        549       5.15e-01    84.8    0.0    0.1    5.3    9.7
2.395e+04       1335       1.20e+00    87.7    0.0    0.1    2.3    9.9
...
Run time : 19 s

be_rational predictor:

4.789e+03       1590       1.45e+00    87.5    0.0    0.1    2.5    9.8
9.579e+03        212       2.23e-01    77.1    0.0    0.1   13.1    9.7
1.437e+04        236       2.37e-01    79.0    0.0    0.1   11.7    9.1
1.916e+04        473       4.50e-01    84.2    0.0    0.1    6.2    9.4
2.395e+04        852       8.00e-01    86.2    0.0    0.1    3.5   10.1
...
Run time : 17 s

New enum `BoutSnesPredictor` can be `none` or `linear`. An error message is printed if the previous boolean `predictor` setting is used. This is to enable different types of predictor to be implemented.

Separates the logic of extrapolation from previous steps from the rest of the solver logic. Should make experimenting with other predictor methods easier.

Small updates to documentation and error message.

github-actions

clang-tidy made some suggestions

ZedThree

LGTM, some minor suggested polish though

Use `BOUT_ENUM_CLASS_NS` to put the enums into the `bout` namespace.

@ZedThree

@ZedThree review suggestion, using a struct rather than std::pair.

github-actions

clang-tidy made some suggestions

github-actions · 2026-09-30T16:21:28Z

+  /// `x` must already be allocated and will be overwritten.
+  void predict(SnesPredictor predictor_type, BoutReal time, Vec& x) {
+    // [t0, x0] is the most recent state
+    const auto& [t0, x0] = this->history[0];


warning: 'x0' declared with a const-qualified typedef; results in the type being '_p_Vec *const' instead of 'const _p_Vec *' [misc-misplaced-const]

const auto& [t0, x0] = this->history[0]; ^

Additional context

/usr/lib/petscdir/petsc3.22/x86_64-linux-gnu-real/include/petscvec.h:25: typedef declared here

typedef struct _p_Vec *Vec; ^

github-actions · 2026-09-30T16:21:28Z

+      BOUT_DO_PETSC(VecCopy(x0, x));
+      return;
+    case SnesPredictor::linear: {
+      const auto& [t1, x1] = this->history[1];


warning: 'x1' declared with a const-qualified typedef; results in the type being '_p_Vec *const' instead of 'const _p_Vec *' [misc-misplaced-const]

const auto& [t1, x1] = this->history[1]; ^

Additional context

/usr/lib/petscdir/petsc3.22/x86_64-linux-gnu-real/include/petscvec.h:25: typedef declared here

typedef struct _p_Vec *Vec; ^

Symbols moved into `bout` namespace

- be_rational uses three states to work out a backward Euler damping factor and extrapolate. - Quality measure that evaluates how good predictors are.

Scales the predicted step size by a factor that is adapted. After each step the optimal factor is calculated and used to update the factor for the next step.

Moves predictor options into subsection of solver options. e.g. Rather than `solver:predictor_blend` it's `solver:predictor:blend`

Fixed call to VecAXPY with incorrect argument order. Additional unit testing. Documentation and code docstrings consistent with implementation.

github-actions

clang-tidy made some suggestions

github-actions · 2026-10-13T19:43:54Z

+
+      // Predicted state
+      // `x` always lies between `x0` (constant prediction) and the linear prediction.
+      x_data[i] = x0_data[i] + dt * s0 * damp;


warning: '*' has higher precedence than '+'; add parentheses to explicitly specify the order of operations [readability-math-missing-parentheses]

Suggested change

x_data[i] = x0_data[i] + dt * s0 * damp;

rediction.(

src/solver/impls/snes/snes.cxx:293:

- mp; + mp);

github-actions · 2026-10-13T19:43:54Z

+}
+
+BoutReal Predictor::quality(Vec x0, Vec x_new, Vec x_pred) {
+  PetscReal norm_num, norm_den;


warning: variable 'norm_num' is not initialized [cppcoreguidelines-init-variables]

, Vec x_pred) { ^

this fix will not be applied because it overlaps with another fix

github-actions · 2026-10-13T19:43:54Z

+}
+
+BoutReal Predictor::optimal_weight(Vec x0, Vec x_new, Vec x_pred) {
+  PetscReal norm_delta, norm_align;


warning: variable 'norm_delta' is not initialized [cppcoreguidelines-init-variables]

, Vec x_pred) { ^

this fix will not be applied because it overlaps with another fix

Makes printing of the `quality` metric optional. Renamed `quality` to `prediction_error_ratio` because it's a metric where smaller is better; minimising quality doesn't seem right.

Only prints blending diagnostics when `diagnose=true`

ZedThree

LGTM! Just wondering if the use of this-> is a deliberate choice, or just an LLMism?

bendudson · 2026-10-14T14:52:43Z

Thsnks @ZedThree ! The this-> is an @oparry-ukaea 'ism. I quite like it so used it here.

bendudson added 3 commits September 28, 2026 10:36

SNESSolver: Replace predictor with predictor_type

7e6d9e8

New enum `BoutSnesPredictor` can be `none` or `linear`. An error message is printed if the previous boolean `predictor` setting is used. This is to enable different types of predictor to be implemented.

SNESSolver: Refactor predictor into Predictor class

d176e53

Separates the logic of extrapolation from previous steps from the rest of the solver logic. Should make experimenting with other predictor methods easier.

SNESSolver predictor unit tests

f3ca1d7

Small updates to documentation and error message.

bendudson added work in progress Not ready for merging performance labels Sep 29, 2026

github-actions Bot reviewed Sep 29, 2026

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Comment thread src/solver/impls/snes/snes.hxx Outdated

Comment thread src/solver/impls/snes/snes.hxx Outdated

ZedThree previously approved these changes Sep 30, 2026

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Comment thread src/solver/impls/snes/snes.hxx Outdated

Comment thread src/solver/impls/snes/snes.hxx Outdated

Comment thread src/solver/impls/snes/snes.hxx

bendudson added 2 commits September 30, 2026 08:51

SNESSolver: Move enums into bout namespace

64584b5

Use `BOUT_ENUM_CLASS_NS` to put the enums into the `bout` namespace.

SNES Predictor: Use struct State

b955c24

@ZedThree review suggestion, using a struct rather than std::pair.

bendudson dismissed ZedThree’s stale review via b955c24 September 30, 2026 16:18

github-actions Bot reviewed Sep 30, 2026

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bendudson added 4 commits September 30, 2026 09:26

SNES Predictor: Fix unit tests

9dea7b9

Symbols moved into `bout` namespace

SNES predictor: be_rational method and quality measure

1192eaa

- be_rational uses three states to work out a backward Euler damping factor and extrapolate. - Quality measure that evaluates how good predictors are.

SNES predictor: Add adaptive blending

59a5e89

Scales the predicted step size by a factor that is adapted. After each step the optimal factor is calculated and used to update the factor for the next step.

SNES predictor: Move predictor_ prefix to section

c53cc16

Moves predictor options into subsection of solver options. e.g. Rather than `solver:predictor_blend` it's `solver:predictor:blend`

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SNES Predictor: Codex tiding, fixes and docs

8874e63

Fixed call to VecAXPY with incorrect argument order. Additional unit testing. Documentation and code docstrings consistent with implementation.

github-actions Bot reviewed Oct 13, 2026

View reviewed changes

SNES Predictor: Clang tidying

a65346f

bendudson removed the work in progress Not ready for merging label Oct 13, 2026

bendudson added 2 commits October 13, 2026 21:58

SNES Predictor: Add diagnose flag

1ec4008

Makes printing of the `quality` metric optional. Renamed `quality` to `prediction_error_ratio` because it's a metric where smaller is better; minimising quality doesn't seem right.

SNES Predictor: Make blend diagnostics optional

6f5f6f9

Only prints blending diagnostics when `diagnose=true`

ZedThree approved these changes Oct 14, 2026

View reviewed changes

bendudson merged commit 603debf into next Oct 14, 2026
23 checks passed

bendudson deleted the snes-predictor branch October 14, 2026 16:08

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