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

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

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fix(rotated-bc): warn on non-converged linear KSP
lmoresi Jul 4, 2026
89cf405
review(Copilot): treat KSP_CONVERGED_ITERATING (reason=0) as non-conv…
lmoresi Jul 5, 2026
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26 changes: 26 additions & 0 deletions src/underworld3/utilities/rotated_bc.py
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Original file line number Diff line number Diff line change
Expand Up @@ -31,6 +31,30 @@
_ROT_SOLVE_COUNT = 0


def _warn_if_ksp_diverged(ksp, kind):
"""Emit a rank-0 warning if a KSP finished on a KSP_DIVERGED_* reason
(negative converged-reason). ``ksp.solve`` never raises on divergence, so
without this the linear rotated-freeslip solvers here would return a
partial answer to the caller silently — which is exactly what let the 3D
rotation-nullspace bug (#306) go unnoticed until it produced clearly-wrong
physics in a downstream test."""
reason = int(ksp.getConvergedReason())
# Convention: > 0 == converged, < 0 == diverged, 0 == KSP_CONVERGED_ITERATING
# (should not happen after ksp.solve() returns; warn if it does). Matches
# petsc_generic_snes_solvers.pyx:2979.
if reason > 0:
return
its = int(ksp.getIterationNumber())
try:
rnorm = float(ksp.getResidualNorm())
except Exception:
rnorm = float("nan")
from underworld3 import mpi
mpi.pprint(f"[rotated_bc] WARNING: {kind} KSP did NOT converge "
f"(reason={reason}, iterations={its}, |r|={rnorm:.3e}); "
f"proceeding with the last iterate.")


# --------------------------------------------------------------------------- #
# Rotation construction
# --------------------------------------------------------------------------- #
Expand Down Expand Up @@ -291,6 +315,7 @@ def solve_rotated_freeslip(solver, boundaries, remove_rotation_gauge=True, verbo
pc = ksp.getPC(); pc.setType("lu"); pc.setFactorSolverType("mumps")
Uhat = dm.createGlobalVec(); ksp.solve(bhat, Uhat) # returned in info → own it
ksp_reason = ksp.getConvergedReason(); ksp_its = ksp.getIterationNumber()
_warn_if_ksp_diverged(ksp, kind="rotated direct-LU")
else:
Mp = _pressure_mass_schur_pmat(solver)
Uhat, ksp_reason, ctx = _solve_rotated_iterative(
Expand Down Expand Up @@ -797,6 +822,7 @@ def _solve_rotated_iterative(solver, Ahat, bhat, Q, Qt, normal_rows, verbose=Fal

Uhat = Ahat.createVecRight(); Uhat.set(0.0)
ksp.solve(bhat, Uhat)
_warn_if_ksp_diverged(ksp, kind="rotated fieldsplit-Schur")
# An identity constraint row in an ITERATIVE solve only drives its residual
# (= û_i) below tolerance, so û_i ~ tol, not exactly 0. Because zeroRowsColumns
# made these DOFs fully decoupled (row AND column zeroed), û_i affects no other
Expand Down
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