hybrid decode halves (23 → 11 tok/s) whenever a foreign thread lands on a pinned spinning CPU-MoE pool CPU; please detect/warn, consider SCHED_FIFO or a yielding spin #744

Summary

With --moe-strategy hybrid on DeepSeek-V4.1-Flash, decode on one server process alternated between ~23 tok/s and ~11 tok/s for a day and a half. Same request, same flags, no log line in either mode. We traced it to any runnable foreign thread sharing a logical CPU with one of the pinned, spinning CPU-MoE pool threads: every layer's barrier waits for that one worker, so a small intruder costs a lot, and once a thread is parked there the kernel rarely moves it, so it looks like a sticky "mode". Fencing every other process off the pool's CPUs with systemd CPUAffinity removed it (0 slow requests since, under the loads that used to trigger it).

Filing because (a) nothing in the server surfaces this, so it cost us ~20 hours to find, and (b) a pool that spins on pinned cores could defend itself.

Environment

• FreeToken nightly 0.1.3+g8f49e0af9
• DeepSeek-V4.1-Flash-1017, ds_fp4(act_block=32), 40 MoE layers, top-6
• 2× EPYC 7742 (2 NUMA nodes × 64 cores, SMT on, 256 logical CPUs), 1 TB DDR4-2666, RTX 3090 24 GB on node 1 (PCIe Gen4 x16), driver 595.x, Linux 7.0.0
• numactl --physcpubind=64-127,192-255 --membind=1 ft serve --moe-strategy hybrid --disable-moe-prefill-overlap --kv-reserve-tokens 65536 --max-prefill-length 2048 --memory-ratio 0.85
• Startup: CPU MoE executor ready: threads=63 (pinned to cores 64..126) … spin_wait=True handshake=kernel, --moe-hybrid-max-fetch auto: 18.1%, CUDA graphs [1,2,4]

Symptom

Single stream, 150-token completions, temperature 0, thinking off:

Fresh starts came up in either mode (6 of ~12 slow one afternoon). A sched_setaffinity on the main thread flipped fast→slow 4 of 6 times. Nsight (fast mode) shows the GPU idling a median 0.43 ms before each layer's combine kernel, i.e. waiting on the pool; we never got a slow-mode capture because the trigger stopped working once the box went quiet after midnight — which was the clue.

Cause (direct test on a live server, no restart)

A pinned busy thread placed by hand:

Every slow period in our logs coincided with other services on the box (a cron-like scheduler, an API server at ~80 % CPU with a thread observed on CPU 65, a remote-desktop daemon at 100 % on a node-1 CPU). The kernel normally places foreign threads on idle logical CPUs, so unpinned load is harmless most of the time; once one is parked on a pool CPU nothing pulls it off a busy core while the pool spins, so the slow state persists until some rebalance — which is why affinity calls re-rolled it and why idle periods sometimes recovered it.

Things that did NOT matter: FREETOKEN_CPU_MOE_SPIN=0 (slower in both modes, still bimodal), FREETOKEN_CPU_MOE_FLAG_SYNC=0 (always the slow speed), NUMA layout (--interleave=all, 127 threads per #18: still flips), CPU clocks, temperature, DRAM bandwidth, PCIe link state, page faults, GPU power cap.

Fix that worked (host side)

systemd CPUAffinity=0-63,128-191 on every other service (user and system units, including sshd so login shells inherit it), leaving 64-127 to the pool. Since then: 0 slow requests in ~40 probes across the loads that previously triggered it.

Asks

1. Detect and warn. The pool knows its pinned CPUs; a periodic check of /proc/*/task/*/stat (or /proc/schedstat run-queue length per CPU) for another runnable task on a pool CPU, logged as a WARN with the pid/comm, would have pointed straight at this. Even a one-time startup check would help.
2. Per-layer barrier timing under --moe-collect-stats (feat(moe): --moe-collect-stats, so expert-cache behaviour is measurable #231): median/p99 wait at the done barrier per worker, so a straggler core is visible from the stats rather than from Nsight.
3. Consider making the pool robust to it: e.g. SCHED_FIFO/SCHED_RR for pool threads when permitted (CAP_SYS_NICE, or document ulimit -r), or a spin with sched_yield/short backoff so an intruder on a pool CPU gets its 20 % without stalling the barrier; or a documented --moe-cpu-threads sizing note that leaving a few cores free does not help if other processes are not fenced.
4. Docs: a line in the hybrid-mode guide saying "fence everything else off the pool CPUs (systemd CPUAffinity / cpuset); an unrelated process landing on one of them halves decode" would have saved us a day.

Happy to run anything; the probe scripts (pinned intruder, per-thread sched deltas, Nsight gap analysis) are small and reusable.