Evidence (history.md session 5, findings 10-11, 20-core jobrunner run):
rq-mutex collapses with thread count on queue-heavy load (yield-storm
6.9x slower than striped at 20T; attributed cause of the baseline's
multi-thread regression on yield_many/chained_spawn), while rq-mpmc is
best-or-close everywhere: best 1-thread, best slot-on ping-pong
(1237us vs mutex 6754us at 20T), -23%/-41 cyc per yield roundtrip on
real hardware (single-core switch_cost, interleaved). rq-striped
remains the churn-heavy many-core option, selectable per build.
Docs + compile_error hints in run_queue.rs updated to name rq-mpmc as
the default. slot_state.rs untouched, no loom-relevant changes; lib
(60) + scheduler/channel/supervisor/park_wake/wake_slot/preempt (41)
pass under the new default.
Re-measured via jobrunner (taskset -c 0-19 of 24, rust:1.97-slim,
sweep.py run --save-baseline, 5 sets). Supersedes the old 24-thread
baseline; multi-thread labels are now 'smarm 20-thread'. Taken under
the rq-mutex default *before* the backend flip — the yield_many /
chained_spawn multi-thread regression reproduces here and is attributed
to rq-mutex contention (history.md session 5, finding 10).
Every resume paid an unconditional AtomicPtr::swap (lock xchg, full
barrier) to check for a first-resume closure that is null on all
resumes after the first. A Relaxed null-load fast path is sound:
store_closure runs only before publish_queued, whose Release pairing
with try_claim's Acquire orders it before this call, so no writer can
race the load within an occupancy.
Measured on switch_cost (1-core sandbox, rq-mutex, cycles): mean
roundtrip 350-355 -> 324-328, ~7.5%. All lib + scheduler/channel/
supervisor tests pass.
switch_to_actor takes the target sp in rdi and returns the actor's
next saved sp in rax (handed over by switch_to_scheduler's shim).
Deletes the ACTOR_SP thread-local and halves the helper calls per
one-way switch (2 -> 1); the scheduler loop also drops its
set_actor_sp/get_actor_sp TLS round-trips. SCHEDULER_SP stays: a
yielding actor at arbitrary call depth has no argument channel back.
asm before/after in outputs/history.md session 1-2. Tests: 354 pass.
claude-asm-audit