fix(rotated-bc): warn on non-converged linear KSP by lmoresi · Pull Request #315 · underworldcode/underworld3

lmoresi · 2026-07-04T03:38:06Z

Summary

ksp.solve never raises on KSP_DIVERGED_*, so the two linear paths in
src/underworld3/utilities/rotated_bc.py (the default fieldsplit-Schur iterative
solve in _solve_rotated_iterative and the opt-in direct MUMPS LU in
solve_rotated_freeslip) both returned partial answers silently.

That silence is what let the 3D rotation-nullspace bug fixed by #306 look
like a working solve for as long as it did: on the 3D Zhong
SphericalShellInternalBoundary case (test_1064 setup) the outer Krylov
hit its `ksp_max_it` ceiling (300 in the pre-#306 code) with a residual
around 2.7e-7 (target rtol was 1e-7) while producing rotated-frame
solutions that the caller then rotated back and returned. Diagnosed under
the /remote-control session that surfaced #306.

Change

Add a small _warn_if_ksp_diverged(ksp, kind) helper at the top of the
module and call it immediately after each of the two `ksp.solve` sites.
Rank-0 warning only (via `uw.mpi.pprint`), same style as the existing
nonlinear-driver warning at the tail of `solve_rotated_freeslip_nonlinear`.

Impact

Happy path: no behaviour change — the helper is a no-op unless the
KSP finished on a negative converged-reason.
Failure path: a divergence that previously was silent now emits a clear
rank-0 warning with the reason code, iteration count, and final
residual, so a caller inspecting stdout can see it.

Tests

`tests/test_1018_rotated_freeslip.py`: 14/14 pass (13 linear + 1
nonlinear-guard, unchanged from post-#306 tip). No new tests here — the
guard fires only on divergence, and forcing a divergence in-test would
require patching in a deliberately-broken preconditioner.

`ksp.solve` never raises on `KSP_DIVERGED_*`, so the two linear paths in `rotated_bc` (the default fieldsplit-Schur iterative solve at `_solve_rotated_iterative` and the opt-in direct MUMPS LU at `solve_rotated_freeslip`) both returned partial answers silently. That silence is what let the 3D rotation-nullspace bug fixed by #306 look like a working solve for as long as it did: the outer Krylov hit its `ksp_max_it` ceiling (300 in the pre-#306 code) with a residual well above tolerance, and neither the KSP nor the caller flagged it. The solution then propagated into downstream diagnostics that eventually exposed it as wrong physics rather than as a solver failure. Add a small `_warn_if_ksp_diverged` helper and call it immediately after each of the two `ksp.solve` sites. Rank-0 warning only (via `uw.mpi.pprint`), same style as the existing nonlinear-driver warning at the end of `solve_rotated_freeslip_nonlinear`. No behaviour change on the happy path; non-convergence is now loud instead of silent. test_1018_rotated_freeslip.py: 14/14 pass (2D behaviour unchanged). Underworld development team with AI support from Claude Code

Copilot

Pull request overview

Adds a small safety guard to the rotated free-slip linear solves so that PETSc KSP divergences (which ksp.solve() does not raise) are no longer silent, improving diagnosability of partial / non-converged rotated-frame solutions.

Changes:

Introduces _warn_if_ksp_diverged(ksp, kind) to emit a rank-0 warning when KSP finishes with a non-converged reason.
Calls the helper after both ksp.solve() sites (direct LU path and iterative fieldsplit-Schur path).

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+    reason = int(ksp.getConvergedReason())
+    if reason >= 0:
+        return


…erged Copilot review on #315 noted the convention split: * KSP.getConvergedReason() > 0 → converged * KSP.getConvergedReason() == 0 → KSP_CONVERGED_ITERATING (initial state; unexpected after ksp.solve() returns) * KSP.getConvergedReason() < 0 → diverged The previous `reason >= 0` early-return silently accepted a hypothetical `reason == 0` outcome, undermining the guard's purpose. Change to `reason > 0` so a stray `KSP_CONVERGED_ITERATING` also fires the warning, matching the project convention documented in petsc_generic_snes_solvers.pyx:2979. test_1018_rotated_freeslip.py: 14/14 pass (unchanged). Underworld development team with AI support from Claude Code

lmoresi · 2026-07-05T09:44:53Z

Thanks @copilot-pull-request-reviewer — good catch on the reason==0 case. Applied in 89cf405: reason > 0 matches the snes.getConvergedReason() > 0 convention documented in petsc_generic_snes_solvers.pyx:2979. test_1018 still green (14/14).

…tion (#291) (#318) * fix(rotated-bc): warn on non-converged linear KSP `ksp.solve` never raises on `KSP_DIVERGED_*`, so the two linear paths in `rotated_bc` (the default fieldsplit-Schur iterative solve at `_solve_rotated_iterative` and the opt-in direct MUMPS LU at `solve_rotated_freeslip`) both returned partial answers silently. That silence is what let the 3D rotation-nullspace bug fixed by #306 look like a working solve for as long as it did: the outer Krylov hit its `ksp_max_it` ceiling (300 in the pre-#306 code) with a residual well above tolerance, and neither the KSP nor the caller flagged it. The solution then propagated into downstream diagnostics that eventually exposed it as wrong physics rather than as a solver failure. Add a small `_warn_if_ksp_diverged` helper and call it immediately after each of the two `ksp.solve` sites. Rank-0 warning only (via `uw.mpi.pprint`), same style as the existing nonlinear-driver warning at the end of `solve_rotated_freeslip_nonlinear`. No behaviour change on the happy path; non-convergence is now loud instead of silent. test_1018_rotated_freeslip.py: 14/14 pass (2D behaviour unchanged). Underworld development team with AI support from Claude Code * review(Copilot): treat KSP_CONVERGED_ITERATING (reason=0) as non-converged Copilot review on #315 noted the convention split: * KSP.getConvergedReason() > 0 → converged * KSP.getConvergedReason() == 0 → KSP_CONVERGED_ITERATING (initial state; unexpected after ksp.solve() returns) * KSP.getConvergedReason() < 0 → diverged The previous `reason >= 0` early-return silently accepted a hypothetical `reason == 0` outcome, undermining the guard's purpose. Change to `reason > 0` so a stray `KSP_CONVERGED_ITERATING` also fires the warning, matching the project convention documented in petsc_generic_snes_solvers.pyx:2979. test_1018_rotated_freeslip.py: 14/14 pass (unchanged). Underworld development team with AI support from Claude Code * fix(constrained): guard empty stratum IS in interior-multiplier reduction (#291) `Stokes_Constrained` segfaulted at np>1 in `_constrain_interior_multipliers_in_section`: on a rank owning zero points with a given boundary label value, `dm.getLabel("UW_Boundaries").getStratumIS(bvalue)` returns a VALID PETSc IS with `getSize() == 0` — not None, non-zero handle, `bool(IS) == True`. The existing `if bd_is is not None:` guard passed through, and the subsequent `bd_is.getIndices()` crashed with SIGSEGV in the PETSc IS binding. The failure was reliably reproduced by #291's repro (StructuredQuadBox + free-slip on all four walls, np=2) and by a lighter unit-viscosity variant which now solves bit-identically at np=1/2/4 with the fix. Fix: replace `if bd_is is not None:` with `if bd_is and bd_is.getSize() > 0:`. `bool(bd_is)` short-circuits the null-handle case (project convention, see `utilities/boundary_flux.py:70`); `.getSize() > 0` catches the valid-but-empty case that neither the None check nor the truthiness check protects against. ### Verification Lightweight unit-viscosity repro (16x16 quad, sinusoidal body force, free-slip on all four walls; `_reduce_interior_multiplier = True` default): np=1: |u|_inf = 2.533e-02 (6.6 s) np=2: |u|_inf = 2.533e-02 (8.5 s) [previously: SIGSEGV] np=4: |u|_inf = 2.533e-02 (5.0 s) [previously: SIGSEGV] Bit-identical parallel-vs-serial in this regime. `tests/test_1062_constrained_solcx.py` (canonical SolCx eta_B=1e6 test): serial passes (198 s); np=2 with-mpi rerun exceeds the default 300 s timeout under this session's compute — a longer-timeout parallel check is warranted as follow-up (probably belongs in `tests/parallel/`). Underworld development team with AI support from Claude Code * test(#291): add lightweight np≥2 regression + un-skip custom-FMG parallel test Copilot review on #318 rightly pointed out that a parallel-only SIGSEGV regression should not merge without a re-enabled parallel test. Two changes: ## 1. New lightweight parallel regression (tests/parallel/test_1062_constrained_stratum_guard_parallel.py) Unit-viscosity 16×16 quad box, sinusoidal body force, free-slip on all four walls at np≥2. On a 2-way axis-aligned partition (the default) this puts one full boundary on each rank and zero points on the other — the exact empty-stratum configuration that #291's guard now handles. Runs in ~10 s at np=2 (vs the canonical test_1062 SolCx eta_B=1e6 which timed out at 300 s in this session's parallel runs, per #244). Asserts `|u|_inf` matches the serial GOLDEN of 2.53305e-02 to rtol 1e-3 (actual serial-vs-parallel spread on this workload is < 1e-9). ## 2. Un-skip test_parallel_custom_fmg_stokes_constrained (tests/parallel/test_1017_custom_mg_parallel_mpi.py) Its skip reason explicitly cited "#291; auto-enables once #291 is fixed." The guard fix is now in place, so the test can run. Marked `slow` (SolCx eta_B=1e6 is not a fast benchmark) so CI selects it via `pytest -m slow` or `-m "not slow"` per environment. Underworld development team with AI support from Claude Code

lmoresi · 2026-07-23T05:58:21Z

Closing as superseded: the entire change — _warn_if_ksp_diverged and both call sites (direct-LU and fieldsplit-Schur) — is already on development, carried in by the PR #318 squash-merge on 2026-07-05 (commit b25453a; the two fixes came from the same session). Development has since also grown the nonlinear-path variant of the warning. The conflicts on this branch are just its pre-#318 base showing its age.

Underworld development team with AI support from Claude Code

Copilot AI review requested due to automatic review settings July 4, 2026 03:38

Copilot started reviewing on behalf of lmoresi July 4, 2026 03:38 View session

Copilot AI reviewed Jul 4, 2026

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Comment thread src/underworld3/utilities/rotated_bc.py

Comment on lines +41 to +43

reason = int(ksp.getConvergedReason())

if reason >= 0:

return

Copilot started work on behalf of lmoresi July 5, 2026 09:45 View session

Copilot finished work on behalf of lmoresi July 5, 2026 09:45

lmoresi closed this Jul 23, 2026

lmoresi deleted the bugfix/rotated-freeslip-silent-nonconvergence branch July 23, 2026 05:58

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fix(rotated-bc): warn on non-converged linear KSP#315

fix(rotated-bc): warn on non-converged linear KSP#315
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