A long cold-request workload flooded the prefill log (~177MB / 32K lines in
8min, all 8 CP ranks) with cp_host_reservation_plan_insufficient +
prepare_write_backup_reservation_failed even though the host pool was <42%
used and the writes physically fit.
Root cause: _free_room_deficit folds the proactive 20% free-room target
(hicache_host_free_room_ratio) into the admission deficit once a lane dips
below the 10% trigger. _evict_cp_host_for_write_admission then failed -- and
evicted nothing -- whenever it could not reclaim that full 20% target, which
is impossible while the residual is locked/pending. The lane never drained,
so every subsequent backup re-issued the same impossible demand and re-logged
on every tick. The reservation failure is itself a graceful skip, so there is
no crash; the symptom is the log storm.
Gate write-backup admission on the HARD deficit -- max(0, required-available)
over the target and draft lanes (draft_available is 2**62 with no draft pool,
a no-op) -- and evict toward the watermark best-effort, admitting whenever the
write itself fits. This drains the stuck lane and backs the request up instead
of skipping+flooding. The proactive watermark stays as the eviction target
(deficit_by_owner unchanged); it is no longer a hard admission gate. The
decision is computed from rank-replicated state on the collective-free reserve
path, so every CP rank makes the same admit/skip choice (opus-reviewed
rank-safe).
Also rate-limit the four host-reservation fallback warnings (once per 10s per
fallback_name, with a suppressed count) so any genuine exhaustion degrades
quietly instead of producing a log storm.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
leavelet
8d73919d02