Measured sustained worker-fact throughput against the pre-feature baseline.
Commit throughput is unregressed (~2.5% delta, within noise) and no per-fact
synchronous commit barrier exists, confirming Req 11.4's core claim. But
apply/observable throughput -- the new hop this spec introduces -- regressed
56% with clearly superlinear scaling (2x volume -> 5.2x elapsed).
Root cause: apply_pump_read_entry has no memory across calls -- every LSN
lookup re-lists WAL segments and rescans its segment from byte 0 -- while its
only caller applies committed entries one at a time in strict ascending
order, turning a sequential access pattern into O(n^2) total work.
Fixed with a minimal segment-read cache on persist_t (fd + offset + expected
next LSN, guarded by the existing apply_pump_lock): a fast path reads the
next expected LSN directly at a remembered offset, falling back unchanged to
full rediscovery on any miss. Correctness never depends on the cache, only
performance does. Invalidated on init, fini, recovery, and WAL truncation --
the four points a stale cursor could otherwise misread from.
Two review rounds: round 1 caught a missing lock guard in the recovery
invalidation path and an inaccurate risk claim about the cache/trim
interaction (a real, bounded behavior change deferred to the existing
APPLY_DIVERGENT fence, not "no worse than before") -- both corrected and
re-verified in round 2. Post-fix apply-throughput scales sublinearly (1.46x
elapsed for 2x volume) and exceeds the pre-feature baseline. Zero new test
failures across the pump, correctness-verification, recovery, and truncation
groups.
An intermittent, pre-existing client hang in the synchronous jobsub path and
a trim-floor/apply-lag architectural gap were surfaced but are explicitly
out of scope for this fix -- flagged for separate follow-up.
Rongsong Shen
40196be2e5