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sync: use lock-free structure for Pool stealing
Currently, Pool stores each per-P shard's overflow in a slice protected by a Mutex. In order to store to the overflow or steal from another shard, a P must lock that shard's Mutex. This allows for simple synchronization between Put and Get, but has unfortunate consequences for clearing pools. Pools are cleared during STW sweep termination, and hence rely on pinning a goroutine to its P to synchronize between Get/Put and clearing. This makes the Get/Put fast path extremely fast because it can rely on quiescence-style coordination, which doesn't even require atomic writes, much less locking. The catch is that a goroutine cannot acquire a Mutex while pinned to its P (as this could deadlock). Hence, it must drop the pin on the slow path. But this means the slow path is not synchronized with clearing. As a result, 1) It's difficult to reason about races between clearing and the slow path. Furthermore, this reasoning often depends on unspecified nuances of where preemption points can occur. 2) Clearing must zero out the pointer to every object in every Pool to prevent a concurrent slow path from causing all objects to be retained. Since this happens during STW, this has an O(# objects in Pools) effect on STW time. 3) We can't implement a victim cache without making clearing even slower. This CL solves these problems by replacing the locked overflow slice with a lock-free structure. This allows Gets and Puts to be pinned the whole time they're manipulating the shards slice (Pool.local), which eliminates the races between Get/Put and clearing. This, in turn, eliminates the need to zero all object pointers, reducing clearing to O(# of Pools) during STW. In addition to significantly reducing STW impact, this also happens to speed up the Get/Put fast-path and the slow path. It somewhat increases the cost of PoolExpensiveNew, but we'll fix that in the next CL. name old time/op new time/op delta Pool-12 3.00ns ± 0% 2.21ns ±36% -26.32% (p=0.000 n=18+19) PoolOverflow-12 600ns ± 1% 587ns ± 1% -2.21% (p=0.000 n=16+18) PoolSTW-12 71.0µs ± 2% 5.6µs ± 3% -92.15% (p=0.000 n=20+20) PoolExpensiveNew-12 3.14ms ± 5% 3.69ms ± 7% +17.67% (p=0.000 n=19+20) name old p50-ns/STW new p50-ns/STW delta PoolSTW-12 70.7k ± 1% 5.5k ± 2% -92.25% (p=0.000 n=20+20) name old p95-ns/STW new p95-ns/STW delta PoolSTW-12 73.1k ± 2% 6.7k ± 4% -90.86% (p=0.000 n=18+19) name old GCs/op new GCs/op delta PoolExpensiveNew-12 0.38 ± 1% 0.39 ± 1% +2.07% (p=0.000 n=20+18) name old New/op new New/op delta PoolExpensiveNew-12 33.9 ± 6% 40.0 ± 6% +17.97% (p=0.000 n=19+20) (https://perf.golang.org/search?q=upload:20190311.1) Fixes #22331. For #22950. Change-Id: Ic5cd826e25e218f3f8256dbc4d22835c1fecb391 Reviewed-on: https://go-review.googlesource.com/c/go/+/166960 Run-TryBot: Austin Clements <firstname.lastname@example.org> TryBot-Result: Gobot Gobot <email@example.com> Reviewed-by: David Chase <firstname.lastname@example.org>
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