Currently, every Pool is cleared completely at the start of each GC.
This is a problem for heavy users of Pool because it causes an
allocation spike immediately after Pools are clear, which impacts both
throughput and latency.

This CL fixes this by introducing a victim cache mechanism. Instead of
clearing Pools, the victim cache is dropped and the primary cache is
moved to the victim cache. As a result, in steady-state, there are
(roughly) no new allocations, but if Pool usage drops, objects will
still be collected within two GCs (as opposed to one).

This victim cache approach also improves Pool's impact on GC dynamics.
The current approach causes all objects in Pools to be short lived.
However, if an application is in steady state and is just going to
repopulate its Pools, then these objects impact the live heap size *as
if* they were long lived. Since Pooled objects count as short lived
when computing the GC trigger and goal, but act as long lived objects
in the live heap, this causes GC to trigger too frequently. If Pooled
objects are a non-trivial portion of an application's heap, this
increases the CPU overhead of GC. The victim cache lets Pooled objects
affect the GC trigger and goal as long-lived objects.

This has no impact on Get/Put performance, but substantially reduces
the impact to the Pool user when a GC happens. PoolExpensiveNew
demonstrates this in the substantially reduction in the rate at which
the "New" function is called.

name                 old time/op     new time/op     delta
Pool-12                 2.21ns ±36%     2.00ns ± 0%     ~     (p=0.070 n=19+16)
PoolOverflow-12          587ns ± 1%      583ns ± 1%   -0.77%  (p=0.000 n=18+18)
PoolSTW-12              5.57µs ± 3%     4.52µs ± 4%  -18.82%  (p=0.000 n=20+19)
PoolExpensiveNew-12     3.69ms ± 7%     1.25ms ± 5%  -66.25%  (p=0.000 n=20+19)

name                 old p50-ns/STW  new p50-ns/STW  delta
PoolSTW-12               5.48k ± 2%      4.53k ± 2%  -17.32%  (p=0.000 n=20+20)

name                 old p95-ns/STW  new p95-ns/STW  delta
PoolSTW-12               6.69k ± 4%      5.13k ± 3%  -23.31%  (p=0.000 n=19+18)

name                 old GCs/op      new GCs/op      delta
PoolExpensiveNew-12       0.39 ± 1%       0.32 ± 2%  -17.95%  (p=0.000 n=18+20)

name                 old New/op      new New/op      delta
PoolExpensiveNew-12       40.0 ± 6%       12.4 ± 6%  -68.91%  (p=0.000 n=20+19)

(https://perf.golang.org/search?q=upload:20190311.2)

Fixes #22950.

Change-Id: If2e183d948c650417283076aacc20739682cdd70
Reviewed-on: https://go-review.googlesource.com/c/go/+/166961
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: David Chase <drchase@google.com>