14 recorded "harbor seed 2 scored 71 then 73 on byte-identical re-runs --
parallel/BLAS non-determinism", and b8g carried that forward as noise
widening the error bars on every A/B run at n_workers>1. The premise does
not survive measurement. Nothing is non-deterministic:
score a frozen .dom, 20 repeats in one process bit-identical
same .dom, 8 processes, varied PYTHONHASHSEED bit-identical
full search, harbor seeds 0-3, n_workers 1..4,
repeated across processes bit-identical PER count
the same with OMP/OPENBLAS/MKL_NUM_THREADS=1 IDENTICAL to unpinned
The last line matters most: b8g proposed "likely a one-line env pin in the
worker pool initializer". Pinning BLAS threads changes nothing bit-for-bit,
so shipping that would have looked like a fix, done nothing, and retired
the issue.
What is real is not noise: the trajectory is a deterministic function of
n_workers. harbor seed 3, budget 1500 -- w=1/2/3 all give 64 fails with
identical bits, w=4 gives 65. Each stable across processes. The mechanism
is batch_n = min(n_workers, ...) children bred from ONE population snapshot
before any is admitted, with the shared rng consumed in a different
pattern; at w=1 each child sees the population its predecessor updated. A
4-worker run is partly generational, a 1-worker run steady-state -- same
seed, different search. Divergence is occasional (seeds 0/1/2 agreed, seed
3 did not), which is how it reads as noise when sampled.
14's observation was most likely homemaker-py-xcy, the as_completed
admission-ordering bug, which WAS non-deterministic and is fixed.
Shipped instead of a no-op env pin: driver.search's docstring states the
contract; test_search_is_reproducible_at_a_fixed_worker_count parametrises
over 2/3/4 workers, asserting each is internally stable and deliberately
NOT that they agree; test_scoring_a_frozen_design_is_deterministic guards
the floor.
The run_*_ab.sh harnesses already pin WORKERS=4, so arms inside one harness
are sound. The exposure is comparing across harnesses, or against a
historical figure whose worker count was never recorded.
Closes homemaker-py-b8g.
Lint at parity (46); tests 384 passed (3 new), 0 failed.
Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01MJ84Feep79Hhm3E4zZJmnB
f6eeee7191