Register CP-SAT decision strategy to stabilize solve times; Switch to ortools 9.15#452
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OR-Tools >= 9.8 randomized the tie-breaking of CP-SAT's internal
scheduling search heuristic, which the "fixed" subsolver falls back to
when a model has no decision strategy. SLOTHY's models relied on that
heuristic being deterministic: since the change, solve times of large
examples (e.g. ntt_dilithium_123_45678_a55) vary by more than an order
of magnitude between runs, frequently exceeding timeouts and aborting
the external binary search with "No solution found."
Register an explicit decision strategy instead: branch on the
instruction that can be scheduled earliest, trying the earliest
position first, mimicking a greedy scheduling pass. This makes solver
behavior deterministic and fast independently of the OR-Tools version;
the previously failing benchmark solves in flat ~5-7s per attempt on
OR-Tools 9.7, 9.8 and 9.15 alike. The strategy is configurable via
config.solver_search_strategy ("lowest_min" (default),
"program_order", or "auto" for the previous behavior).
Also fix config.solver_random_seed never taking effect: it was
assigned to a non-existent CpSolver attribute instead of the solver
parameter.
Signed-off-by: Matthias J. Kannwischer <matthias@zerorisc.com>
Signed-off-by: Matthias J. Kannwischer <matthias@zerorisc.com>
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