* perf(mount): add graduated write backpressure before buffer cap
Introduce soft (80%) and hard (95%) throttling thresholds in
WriteBufferAccountant. When write buffer usage approaches the cap,
Reserve() inserts brief sleeps to slow writers gradually rather than
blocking them completely at the cap. This smooths out write latency
under sustained load.
* fix(mount): address review feedback for graduated backpressure
- Use projected usage (used + n) for threshold checks so a single
reservation crossing a threshold is throttled immediately.
- Widen no-throttle test timing tolerance to softThrottleDelay to
avoid CI flakes from scheduling jitter.
- Replace fragile timing upper-bound in recovery test with
counter-based invariant (hardThrottleCount stays at 1).
* docs(mount): clarify single-shot throttle design in WriteBufferAccountant
Add a comment explaining why graduated throttling runs once per Reserve
call rather than inside the blocking loop: once at the cap, the evictor
+ cond.Wait mechanism frees actual capacity, which time-based sleeps
cannot do.
Chris LuandGitHub
216b52c13a