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Josh Bleecher Snyder 986768de7f context: lazily initialize cancelCtx done channel 
This CL reduces allocations when a context
created with WithCancel either

(1) never has its Done channel used or
(2) gets cancelled before its Done channel is used

This is not uncommon. Many contexts are created
for tasks that end up not using them.

name                                                old time/op    new time/op    delta
ContextCancelTree/depth=1/Root=Background-8            112ns ± 2%      74ns ± 1%  -34.03%  (p=0.000 n=17+18)
ContextCancelTree/depth=1/Root=OpenCanceler-8          601ns ± 3%     544ns ± 1%   -9.56%  (p=0.000 n=20+20)
ContextCancelTree/depth=1/Root=ClosedCanceler-8        367ns ± 4%     257ns ± 1%  -30.01%  (p=0.000 n=20+20)
ContextCancelTree/depth=10/Root=Background-8          2.91µs ± 2%    2.87µs ± 0%   -1.38%  (p=0.000 n=20+18)
ContextCancelTree/depth=10/Root=OpenCanceler-8        4.36µs ± 2%    4.26µs ± 1%   -2.34%  (p=0.000 n=20+18)
ContextCancelTree/depth=10/Root=ClosedCanceler-8      2.02µs ± 2%    1.51µs ± 1%  -25.18%  (p=0.000 n=19+19)
ContextCancelTree/depth=100/Root=Background-8         30.5µs ± 6%    30.5µs ± 1%     ~     (p=0.941 n=20+20)
ContextCancelTree/depth=100/Root=OpenCanceler-8       39.8µs ± 1%    41.1µs ± 1%   +3.15%  (p=0.000 n=18+19)
ContextCancelTree/depth=100/Root=ClosedCanceler-8     17.8µs ± 1%    13.9µs ± 1%  -21.61%  (p=0.000 n=18+20)
ContextCancelTree/depth=1000/Root=Background-8         302µs ± 1%     313µs ± 0%   +3.62%  (p=0.000 n=20+18)
ContextCancelTree/depth=1000/Root=OpenCanceler-8       412µs ± 2%     427µs ± 1%   +3.55%  (p=0.000 n=18+19)
ContextCancelTree/depth=1000/Root=ClosedCanceler-8     178µs ± 1%     139µs ± 1%  -21.80%  (p=0.000 n=19+17)

name                                                old alloc/op   new alloc/op   delta
ContextCancelTree/depth=1/Root=Background-8             176B ± 0%       80B ± 0%  -54.55%  (p=0.000 n=20+20)
ContextCancelTree/depth=1/Root=OpenCanceler-8           544B ± 0%      448B ± 0%  -17.65%  (p=0.000 n=20+20)
ContextCancelTree/depth=1/Root=ClosedCanceler-8         352B ± 0%      160B ± 0%  -54.55%  (p=0.000 n=20+20)
ContextCancelTree/depth=10/Root=Background-8          3.49kB ± 0%    3.39kB ± 0%   -2.75%  (p=0.000 n=20+20)
ContextCancelTree/depth=10/Root=OpenCanceler-8        3.86kB ± 0%    3.76kB ± 0%   -2.49%  (p=0.000 n=20+20)
ContextCancelTree/depth=10/Root=ClosedCanceler-8      1.94kB ± 0%    0.88kB ± 0%  -54.55%  (p=0.000 n=20+20)
ContextCancelTree/depth=100/Root=Background-8         36.6kB ± 0%    36.5kB ± 0%   -0.26%  (p=0.000 n=20+20)
ContextCancelTree/depth=100/Root=OpenCanceler-8       37.0kB ± 0%    36.9kB ± 0%   -0.26%  (p=0.000 n=20+20)
ContextCancelTree/depth=100/Root=ClosedCanceler-8     17.8kB ± 0%     8.1kB ± 0%  -54.55%  (p=0.000 n=20+20)
ContextCancelTree/depth=1000/Root=Background-8         368kB ± 0%     368kB ± 0%   -0.03%  (p=0.000 n=20+20)
ContextCancelTree/depth=1000/Root=OpenCanceler-8       368kB ± 0%     368kB ± 0%   -0.03%  (p=0.000 n=20+20)
ContextCancelTree/depth=1000/Root=ClosedCanceler-8     176kB ± 0%      80kB ± 0%  -54.55%  (p=0.000 n=20+20)

name                                                old allocs/op  new allocs/op  delta
ContextCancelTree/depth=1/Root=Background-8             3.00 ± 0%      2.00 ± 0%  -33.33%  (p=0.000 n=20+20)
ContextCancelTree/depth=1/Root=OpenCanceler-8           8.00 ± 0%      7.00 ± 0%  -12.50%  (p=0.000 n=20+20)
ContextCancelTree/depth=1/Root=ClosedCanceler-8         6.00 ± 0%      4.00 ± 0%  -33.33%  (p=0.000 n=20+20)
ContextCancelTree/depth=10/Root=Background-8            48.0 ± 0%      47.0 ± 0%   -2.08%  (p=0.000 n=20+20)
ContextCancelTree/depth=10/Root=OpenCanceler-8          53.0 ± 0%      52.0 ± 0%   -1.89%  (p=0.000 n=20+20)
ContextCancelTree/depth=10/Root=ClosedCanceler-8        33.0 ± 0%      22.0 ± 0%  -33.33%  (p=0.000 n=20+20)
ContextCancelTree/depth=100/Root=Background-8            498 ± 0%       497 ± 0%   -0.20%  (p=0.000 n=20+20)
ContextCancelTree/depth=100/Root=OpenCanceler-8          503 ± 0%       502 ± 0%   -0.20%  (p=0.000 n=20+20)
ContextCancelTree/depth=100/Root=ClosedCanceler-8        303 ± 0%       202 ± 0%  -33.33%  (p=0.000 n=20+20)
ContextCancelTree/depth=1000/Root=Background-8         5.00k ± 0%     5.00k ± 0%   -0.02%  (p=0.000 n=20+20)
ContextCancelTree/depth=1000/Root=OpenCanceler-8       5.00k ± 0%     5.00k ± 0%   -0.02%  (p=0.000 n=20+20)
ContextCancelTree/depth=1000/Root=ClosedCanceler-8     3.00k ± 0%     2.00k ± 0%  -33.33%  (p=0.000 n=20+20)

Change-Id: Ibd7a0c3d5c847861cf1497f8fead34329413d26d
Reviewed-on: https://go-review.googlesource.com/34979
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Sameer Ajmani <sameer@golang.org>
 		2017-02-01 20:08:41 +00:00
.github doc: improve issue template 2016-08-29 03:33:28 +00:00
api Revert "cmd/go: note when some Go files were ignored on no-Go-files errors" 2016-12-21 05:25:57 +00:00
doc all: merge dev.typealias into master 2017-01-31 13:01:31 -05:00
lib/time lib/time: update tzdata to 2016j 2017-01-05 01:00:53 +00:00
misc runtime: reorder modules so main.main comes first 2017-01-25 22:33:57 +00:00
src context: lazily initialize cancelCtx done channel 2017-02-01 20:08:41 +00:00
test all: merge dev.inline into master 2017-02-01 09:47:23 -05:00
.gitattributes .gitattributes: prevent all magic line ending changes 2014-12-12 23:14:54 +00:00
.gitignore .gitignore: fix attempt at rooted paths 2017-01-05 01:29:25 +00:00
AUTHORS A+C: automated update 2016-12-21 03:22:22 +00:00
CONTRIBUTING.md doc: update CONTRIBUTING.md a bit, mention proposal process 2017-01-06 23:19:01 +00:00
CONTRIBUTORS A+C: automated update 2016-12-21 03:22:22 +00:00
LICENSE doc: revert copyright date to 2009 2016-06-01 22:40:04 +00:00
PATENTS
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README.md README.md: update and simplify 2017-01-17 21:26:43 +00:00
favicon.ico website: recreate 16px and 32px favicon 2016-08-25 15:43:32 +00:00
robots.txt
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README.md

The Go Programming Language

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