The hot-thread tuning sets OMP_WAIT_POLICY=active and caps libgomp's spin
with GOMP_SPINCOUNT=200000. LLVM libomp reads NEITHER: under active policy it
sets KMP_BLOCKTIME=infinite, so once the answer ends and the engine parks on
stdin, the whole OMP team spins forever — ~100% per thread, the 3000% figure
reported on FreeBSD 15.1 in #341 (clang/libomp is the default toolchain
neighborhood there; the same applies to macOS builds).
KMP_BLOCKTIME=200 keeps the team hot for 200 ms after each parallel region —
plenty to bridge back-to-back per-expert matmuls — and then sleeps it.
setenv with overwrite=0, so a user-provided KMP_BLOCKTIME still wins, and
libgomp builds ignore the variable entirely: on GCC toolchains this commit
is a no-op by construction.
Not reproduced locally (this box is gcc/libgomp and idles clean); shipped as
the mechanism-matching candidate fix with a request on the issue for the
reporter to verify on dev and to name their OpenMP runtime (ldd | grep omp).
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
JustVugg
d5327e2252