Schedule the work as early as possible, while still respecting the
utilization percentage on average. The old code tried never to
go above the utilization percentage. The new code is willing
to go above the utilization percentage by one time slice
(but of course after doing that it must wait until the percentage
drops back down to the target before it gets another time slice).

The effect is that for concurrent GCs that can run in a small number
of time slices, the time during which write barriers are enabled is
reduced by one mutator + GC time slice round (possibly 30 ms per GC).

This only affects the fractional GC processor (the remainder of GOMAXPROCS/4),
so it matters most in GOMAXPROCS=1, a bit in GOMAXPROCS=2, and not at
all in GOMAXPROCS=4.

GOMAXPROCS=1
name                                      old mean                new mean        delta
BenchmarkBinaryTree17                12.4s × (0.98,1.03)     13.5s × (0.97,1.04)  +8.84% (p=0.000)
BenchmarkFannkuch11                  4.38s × (1.00,1.01)     4.38s × (1.00,1.01)  ~ (p=0.343)
BenchmarkFmtFprintfEmpty            88.9ns × (0.97,1.10)    90.1ns × (0.93,1.14)  ~ (p=0.224)
BenchmarkFmtFprintfString            356ns × (0.94,1.05)     321ns × (0.94,1.12)  -9.77% (p=0.000)
BenchmarkFmtFprintfInt               344ns × (0.98,1.03)     325ns × (0.96,1.03)  -5.46% (p=0.000)
BenchmarkFmtFprintfIntInt            622ns × (0.97,1.03)     571ns × (0.95,1.05)  -8.09% (p=0.000)
BenchmarkFmtFprintfPrefixedInt       462ns × (0.96,1.04)     431ns × (0.95,1.05)  -6.81% (p=0.000)
BenchmarkFmtFprintfFloat             653ns × (0.98,1.03)     621ns × (0.99,1.03)  -4.90% (p=0.000)
BenchmarkFmtManyArgs                2.32µs × (0.97,1.03)    2.19µs × (0.98,1.02)  -5.43% (p=0.000)
BenchmarkGobDecode                  27.0ms × (0.96,1.04)    20.0ms × (0.97,1.04)  -26.06% (p=0.000)
BenchmarkGobEncode                  26.6ms × (0.99,1.01)    17.8ms × (0.95,1.05)  -33.19% (p=0.000)
BenchmarkGzip                        659ms × (0.98,1.03)     650ms × (0.99,1.01)  -1.34% (p=0.000)
BenchmarkGunzip                      145ms × (0.98,1.04)     143ms × (1.00,1.01)  -1.47% (p=0.000)
BenchmarkHTTPClientServer            111µs × (0.97,1.04)     110µs × (0.96,1.03)  -1.30% (p=0.000)
BenchmarkJSONEncode                 52.0ms × (0.97,1.03)    40.8ms × (0.97,1.03)  -21.47% (p=0.000)
BenchmarkJSONDecode                  127ms × (0.98,1.04)     120ms × (0.98,1.02)  -5.55% (p=0.000)
BenchmarkMandelbrot200              6.04ms × (0.99,1.04)    6.02ms × (1.00,1.01)  ~ (p=0.176)
BenchmarkGoParse                    8.62ms × (0.96,1.08)    8.55ms × (0.93,1.09)  ~ (p=0.302)
BenchmarkRegexpMatchEasy0_32         164ns × (0.98,1.05)     165ns × (0.98,1.07)  ~ (p=0.293)
BenchmarkRegexpMatchEasy0_1K         546ns × (0.98,1.06)     547ns × (0.97,1.07)  ~ (p=0.741)
BenchmarkRegexpMatchEasy1_32         142ns × (0.97,1.09)     141ns × (0.97,1.05)  ~ (p=0.231)
BenchmarkRegexpMatchEasy1_1K         904ns × (0.97,1.07)     900ns × (0.98,1.04)  ~ (p=0.294)
BenchmarkRegexpMatchMedium_32        256ns × (0.98,1.06)     256ns × (0.97,1.04)  ~ (p=0.530)
BenchmarkRegexpMatchMedium_1K       74.2µs × (0.98,1.05)    73.8µs × (0.98,1.04)  ~ (p=0.334)
BenchmarkRegexpMatchHard_32         3.94µs × (0.98,1.07)    3.92µs × (0.98,1.05)  ~ (p=0.356)
BenchmarkRegexpMatchHard_1K          119µs × (0.98,1.07)     119µs × (0.98,1.06)  ~ (p=0.467)
BenchmarkRevcomp                     978ms × (0.96,1.09)     984ms × (0.95,1.07)  ~ (p=0.448)
BenchmarkTemplate                    151ms × (0.96,1.03)     142ms × (0.95,1.04)  -5.55% (p=0.000)
BenchmarkTimeParse                   628ns × (0.99,1.01)     628ns × (0.99,1.01)  ~ (p=0.855)
BenchmarkTimeFormat                  729ns × (0.98,1.06)     734ns × (0.97,1.05)  ~ (p=0.149)

GOMAXPROCS=2
name                                      old mean                new mean        delta
BenchmarkBinaryTree17-2              9.80s × (0.97,1.03)     9.85s × (0.99,1.02)  ~ (p=0.444)
BenchmarkFannkuch11-2                4.35s × (0.99,1.01)     4.40s × (0.98,1.05)  ~ (p=0.099)
BenchmarkFmtFprintfEmpty-2          86.7ns × (0.97,1.05)    85.9ns × (0.98,1.04)  ~ (p=0.409)
BenchmarkFmtFprintfString-2          297ns × (0.98,1.01)     297ns × (0.99,1.01)  ~ (p=0.743)
BenchmarkFmtFprintfInt-2             309ns × (0.98,1.02)     310ns × (0.99,1.01)  ~ (p=0.464)
BenchmarkFmtFprintfIntInt-2          525ns × (0.97,1.05)     518ns × (0.99,1.01)  ~ (p=0.151)
BenchmarkFmtFprintfPrefixedInt-2     408ns × (0.98,1.02)     408ns × (0.98,1.03)  ~ (p=0.797)
BenchmarkFmtFprintfFloat-2           603ns × (0.99,1.01)     604ns × (0.98,1.02)  ~ (p=0.588)
BenchmarkFmtManyArgs-2              2.07µs × (0.98,1.02)    2.05µs × (0.99,1.01)  ~ (p=0.091)
BenchmarkGobDecode-2                19.1ms × (0.97,1.01)    19.3ms × (0.97,1.04)  ~ (p=0.195)
BenchmarkGobEncode-2                16.2ms × (0.97,1.03)    16.4ms × (0.99,1.01)  ~ (p=0.069)
BenchmarkGzip-2                      652ms × (0.99,1.01)     651ms × (0.99,1.01)  ~ (p=0.705)
BenchmarkGunzip-2                    143ms × (1.00,1.01)     143ms × (1.00,1.00)  ~ (p=0.665)
BenchmarkHTTPClientServer-2          149µs × (0.92,1.11)     149µs × (0.91,1.08)  ~ (p=0.862)
BenchmarkJSONEncode-2               34.6ms × (0.98,1.02)    37.2ms × (0.99,1.01)  +7.56% (p=0.000)
BenchmarkJSONDecode-2                117ms × (0.99,1.01)     117ms × (0.99,1.01)  ~ (p=0.858)
BenchmarkMandelbrot200-2            6.10ms × (0.99,1.03)    6.03ms × (1.00,1.00)  ~ (p=0.083)
BenchmarkGoParse-2                  8.25ms × (0.98,1.01)    8.21ms × (0.99,1.02)  ~ (p=0.307)
BenchmarkRegexpMatchEasy0_32-2       162ns × (0.99,1.02)     162ns × (0.99,1.01)  ~ (p=0.857)
BenchmarkRegexpMatchEasy0_1K-2       541ns × (0.99,1.01)     540ns × (1.00,1.00)  ~ (p=0.530)
BenchmarkRegexpMatchEasy1_32-2       138ns × (1.00,1.00)     141ns × (0.98,1.04)  +1.88% (p=0.038)
BenchmarkRegexpMatchEasy1_1K-2       887ns × (0.99,1.01)     894ns × (0.99,1.01)  ~ (p=0.087)
BenchmarkRegexpMatchMedium_32-2      252ns × (0.99,1.01)     252ns × (0.99,1.01)  ~ (p=0.954)
BenchmarkRegexpMatchMedium_1K-2     73.4µs × (0.99,1.02)    72.8µs × (1.00,1.01)  -0.87% (p=0.029)
BenchmarkRegexpMatchHard_32-2       3.95µs × (0.97,1.05)    3.87µs × (1.00,1.01)  -2.11% (p=0.035)
BenchmarkRegexpMatchHard_1K-2        117µs × (0.99,1.01)     117µs × (0.99,1.01)  ~ (p=0.669)
BenchmarkRevcomp-2                   980ms × (0.95,1.03)     993ms × (0.94,1.09)  ~ (p=0.527)
BenchmarkTemplate-2                  136ms × (0.98,1.01)     135ms × (0.99,1.01)  ~ (p=0.200)
BenchmarkTimeParse-2                 630ns × (1.00,1.01)     630ns × (1.00,1.00)  ~ (p=0.634)
BenchmarkTimeFormat-2                705ns × (0.99,1.01)     710ns × (0.98,1.02)  ~ (p=0.174)

GOMAXPROCS=4
BenchmarkBinaryTree17-4              9.87s × (0.96,1.04)     9.75s × (0.96,1.03)  ~ (p=0.178)
BenchmarkFannkuch11-4                4.35s × (1.00,1.01)     4.40s × (0.99,1.04)  ~ (p=0.071)
BenchmarkFmtFprintfEmpty-4          85.8ns × (0.98,1.06)    85.6ns × (0.98,1.04)  ~ (p=0.858)
BenchmarkFmtFprintfString-4          306ns × (0.99,1.03)     304ns × (0.97,1.02)  ~ (p=0.470)
BenchmarkFmtFprintfInt-4             317ns × (0.98,1.01)     315ns × (0.98,1.02)  -0.92% (p=0.044)
BenchmarkFmtFprintfIntInt-4          527ns × (0.99,1.01)     525ns × (0.98,1.01)  ~ (p=0.164)
BenchmarkFmtFprintfPrefixedInt-4     421ns × (0.98,1.03)     417ns × (0.99,1.02)  ~ (p=0.092)
BenchmarkFmtFprintfFloat-4           623ns × (0.98,1.02)     618ns × (0.98,1.03)  ~ (p=0.172)
BenchmarkFmtManyArgs-4              2.09µs × (0.98,1.02)    2.09µs × (0.98,1.02)  ~ (p=0.679)
BenchmarkGobDecode-4                18.6ms × (0.99,1.01)    18.6ms × (0.98,1.03)  ~ (p=0.595)
BenchmarkGobEncode-4                15.0ms × (0.98,1.02)    15.1ms × (0.99,1.01)  ~ (p=0.301)
BenchmarkGzip-4                      659ms × (0.98,1.04)     660ms × (0.97,1.02)  ~ (p=0.724)
BenchmarkGunzip-4                    145ms × (0.98,1.04)     144ms × (0.99,1.04)  ~ (p=0.671)
BenchmarkHTTPClientServer-4          139µs × (0.97,1.02)     138µs × (0.99,1.02)  ~ (p=0.392)
BenchmarkJSONEncode-4               35.0ms × (0.99,1.02)    35.1ms × (0.98,1.02)  ~ (p=0.777)
BenchmarkJSONDecode-4                119ms × (0.98,1.01)     118ms × (0.98,1.02)  ~ (p=0.710)
BenchmarkMandelbrot200-4            6.02ms × (1.00,1.00)    6.02ms × (1.00,1.00)  ~ (p=0.289)
BenchmarkGoParse-4                  7.96ms × (0.99,1.01)    7.96ms × (0.99,1.01)  ~ (p=0.884)
BenchmarkRegexpMatchEasy0_32-4       164ns × (0.98,1.04)     166ns × (0.97,1.04)  ~ (p=0.221)
BenchmarkRegexpMatchEasy0_1K-4       540ns × (0.99,1.01)     552ns × (0.97,1.04)  +2.10% (p=0.018)
BenchmarkRegexpMatchEasy1_32-4       140ns × (0.99,1.04)     142ns × (0.97,1.04)  ~ (p=0.226)
BenchmarkRegexpMatchEasy1_1K-4       896ns × (0.99,1.03)     907ns × (0.97,1.04)  ~ (p=0.155)
BenchmarkRegexpMatchMedium_32-4      255ns × (0.99,1.04)     255ns × (0.98,1.04)  ~ (p=0.904)
BenchmarkRegexpMatchMedium_1K-4     73.4µs × (0.99,1.04)    73.8µs × (0.98,1.04)  ~ (p=0.560)
BenchmarkRegexpMatchHard_32-4       3.93µs × (0.98,1.04)    3.95µs × (0.98,1.04)  ~ (p=0.571)
BenchmarkRegexpMatchHard_1K-4        117µs × (1.00,1.01)     119µs × (0.98,1.04)  +1.48% (p=0.048)
BenchmarkRevcomp-4                   990ms × (0.94,1.08)     989ms × (0.94,1.10)  ~ (p=0.957)
BenchmarkTemplate-4                  137ms × (0.98,1.02)     137ms × (0.99,1.01)  ~ (p=0.996)
BenchmarkTimeParse-4                 629ns × (1.00,1.00)     629ns × (0.99,1.01)  ~ (p=0.924)
BenchmarkTimeFormat-4                710ns × (0.99,1.01)     716ns × (0.98,1.02)  +0.84% (p=0.033)

Change-Id: I43a04e0f6ad5e3ba9847dddf12e13222561f9cf4
Reviewed-on: https://go-review.googlesource.com/9543Reviewed-by: Austin Clements <austin@google.com>