5a2d4947ea
Adds XENIA_PARALLEL_PROFILE=1 to the free-run executor: reports, summed across
workers, kernel-lock WAIT (contention), HELD, unlocked REGION, and idle-park
counts. This measured the actual bottleneck before committing to fine-grained
locking — and refuted it:
n=2B free-run profile (worker-thread-time basis):
kernel-lock WAIT = 4.8% <- contention is NOT the wall
kernel-lock HELD = 0.5%
unlocked REGION = 27.1% <- productive
idle_parks = 2.45M ; ~40% = workers blocked at the quiesce barrier
So fine-grained kernel locking would NOT help — the wall is COORDINATION
(the per-tick quiesce barrier + idle-park wake latency) and guest spin-waits.
Also raises the coordinator tick default 200µs -> 2000µs: the barrier is on the
critical path, so the small tick was the dominant cost. This roughly halves the
good-case time (n=2B ~17-18s vs lockstep-JIT ~24s = ~1.35×). BUT free-run is
still BIMODAL — a coordination pathology intermittently latches (~60s). (A/B
tests that looked like "XENIA_GPU_THREAD makes it fast" were run-ordering noise:
use_threaded=true either way — the flag is a no-op here; the split was the
pathology.) Next lever = attack the coordination bimodality (precise cross-slot
wake to kill the ≤50µs idle-park latency; or skip in-flight slots in the ISR
housekeeping to drop the barrier), NOT lock-splitting.
Opt-in (XENIA_PARALLEL_FREERUN=1); lockstep golden byte-identical (interp+JIT).
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>