lru: fastpath before taking resizeMu in maybeGrow - #23546
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…fundable grow before taking resizeMu A full LRU never drops back below curCap, so once the envelope cannot fund the next step the fast-path gate stays armed and every later write re-enters maybeGrow and serialises on the single resizeMu. tailLRU also pays an all-shard Len() ahead of it. Gate on CanReserve instead, which needs no per-cache state and picks up bytes a later Close returns on its own.
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Stacked on #23545.
A full LRU never drops back below
curCap. So once the envelope cannot fund the next grow step, the fast-path gate stays armed forever and every later write re-entersmaybeGrow, takes the one globalresizeMu, fails the sameReserveand returns — a process-wide serialisation point on the cache write path.tailLRU.Addis worse: it pays an all-shardLen()ahead of that call.Gate on
Budget.CanReserveinstead: one atomic load, no per-cache state, and bytes a laterClose/Clearreturns are picked up on the next write with nothing to reset.state_cache.goalready documents "that cache stops growing" — now that is what the code does.Same gate in
GenericCache.putStriped,growLRU.Add,tailLRU.Add.Not what the current
stage_execmutex profile shows (those events are funded grows); this is the cliff behind them.