The MoE forward does many tiny, back-to-back per-expert matmul regions.
Under libgomp's default passive wait policy the worker team is parked
between regions, and the thread re-wake latency dominates the actual
compute. Switching the team to an active wait policy (with a bounded spin
count so long NVMe expert-load stalls still yield instead of burning a
core) keeps the threads hot across those regions.
Measured on the Zen5 build: expert-matmul wall time 66.9s -> 20.9s
(~3.2x on that phase). Output is numerically unchanged — this only
affects thread scheduling, not the computation.
Mechanism: libgomp parses OMP_/GOMP_ vars in a constructor that runs
before main(), so setenv() from main() and continuing is too late
(verified: the already-initialised runtime ignores it). Instead we seed
the winning defaults on first entry with overwrite=0 (so any value the
user already set wins), set a COLI_OMP_TUNED sentinel, and execv() self
once so a fresh libgomp constructor reads them. The sentinel guards the
exec, so we re-exec at most once. The block is the first statement in
main() so argv is passed verbatim to execv().
Discoverability / observability (review follow-up):
- COLI_NO_OMP_TUNE=1 is a documented kill-switch that disables the
whole re-exec + tuning path in one shot (distinct from the internal
COLI_OMP_TUNED re-exec sentinel);
- a one-line stderr breadcrumb is printed before the re-exec
("[OMP] hot-thread tuning: re-exec once (COLI_NO_OMP_TUNE=1 to skip)")
so the self-re-exec is self-documenting;
- execv only returns on failure, so a perror() now follows it
("execv self-reexec failed, running untuned") — a container without
/proc or a deleted inode falls back visibly instead of silently
losing the ~3.2x.
Fully opt-out / overridable and lossless:
- explicit OMP_WAIT_POLICY / GOMP_SPINCOUNT / OMP_PROC_BIND / OMP_DYNAMIC
in the environment win (overwrite=0);
- pre-setting COLI_OMP_TUNED=1 skips the re-exec entirely;
- COLI_NO_OMP_TUNE=1 disables the tuning path entirely;
- guarded by __linux__ (no-op re-exec elsewhere; the setenv defaults
still apply for any later-initialising runtime).
Claude-Session: https://claude.ai/code/session_01DS7oc65c5RdA9V99otRCwt
Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>
Rodolfo Hansen
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