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The Go programming language
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Currently, the GC uses a moving average of recent scan work ratios to estimate the total scan work required by this cycle. This is in turn used to compute how much scan work should be done by mutators when they allocate in order to perform all expected scan work by the time the allocated heap reaches the heap goal. However, our current scan work estimate can be arbitrarily wrong if the heap topography changes significantly from one cycle to the next. For example, in the go1 benchmarks, at the beginning of each benchmark, the heap is dominated by a 256MB no-scan object, so the GC learns that the scan density of the heap is very low. In benchmarks that then rapidly allocate pointer-dense objects, by the time of the next GC cycle, our estimate of the scan work can be too low by a large factor. This in turn lets the mutator allocate faster than the GC can collect, allowing it to get arbitrarily far ahead of the scan work estimate, which leads to very long GC cycles with very little mutator assist that can overshoot the heap goal by large margins. This is particularly easy to demonstrate with BinaryTree17: $ GODEBUG=gctrace=1 ./go1.test -test.bench BinaryTree17 gc #1 @0.017s 2%: 0+0+0+0+0 ms clock, 0+0+0+0/0/0+0 ms cpu, 4->262->262 MB, 4 MB goal, 1 P gc #2 @0.026s 3%: 0+0+0+0+0 ms clock, 0+0+0+0/0/0+0 ms cpu, 262->262->262 MB, 524 MB goal, 1 P testing: warning: no tests to run PASS BenchmarkBinaryTree17 gc #3 @1.906s 0%: 0+0+0+0+7 ms clock, 0+0+0+0/0/0+7 ms cpu, 325->325->287 MB, 325 MB goal, 1 P (forced) gc #4 @12.203s 20%: 0+0+0+10067+10 ms clock, 0+0+0+0/2523/852+10 ms cpu, 430->2092->1950 MB, 574 MB goal, 1 P 1 9150447353 ns/op Change this estimate to instead use the *current* scannable heap size. This has the advantage of being based solely on the current state of the heap, not on past densities or reachable heap sizes, so it isn't susceptible to falling behind during these sorts of phase changes. This is strictly an over-estimate, but it's better to over-estimate and get more assist than necessary than it is to under-estimate and potentially spiral out of control. Experiments with scaling this estimate back showed no obvious benefit for mutator utilization, heap size, or assist time. This new estimate has little effect for most benchmarks, including most go1 benchmarks, x/benchmarks, and the 6g benchmark. It has a huge effect for benchmarks that triggered the bad pacer behavior: name old mean new mean delta BinaryTree17 10.0s × (1.00,1.00) 3.5s × (0.98,1.01) -64.93% (p=0.000) Fannkuch11 2.74s × (1.00,1.01) 2.65s × (1.00,1.00) -3.52% (p=0.000) FmtFprintfEmpty 56.4ns × (0.99,1.00) 57.8ns × (1.00,1.01) +2.43% (p=0.000) FmtFprintfString 187ns × (0.99,1.00) 185ns × (0.99,1.01) -1.19% (p=0.010) FmtFprintfInt 184ns × (1.00,1.00) 183ns × (1.00,1.00) (no variance) FmtFprintfIntInt 321ns × (1.00,1.00) 315ns × (1.00,1.00) -1.80% (p=0.000) FmtFprintfPrefixedInt 266ns × (1.00,1.00) 263ns × (1.00,1.00) -1.22% (p=0.000) FmtFprintfFloat 353ns × (1.00,1.00) 353ns × (1.00,1.00) -0.13% (p=0.035) FmtManyArgs 1.21µs × (1.00,1.00) 1.19µs × (1.00,1.00) -1.33% (p=0.000) GobDecode 9.69ms × (1.00,1.00) 9.59ms × (1.00,1.00) -1.07% (p=0.000) GobEncode 7.89ms × (0.99,1.01) 7.74ms × (1.00,1.00) -1.92% (p=0.000) Gzip 391ms × (1.00,1.00) 392ms × (1.00,1.00) ~ (p=0.522) Gunzip 97.1ms × (1.00,1.00) 97.0ms × (1.00,1.00) -0.10% (p=0.000) HTTPClientServer 55.7µs × (0.99,1.01) 56.7µs × (0.99,1.01) +1.81% (p=0.001) JSONEncode 19.1ms × (1.00,1.00) 19.0ms × (1.00,1.00) -0.85% (p=0.000) JSONDecode 66.8ms × (1.00,1.00) 66.9ms × (1.00,1.00) ~ (p=0.288) Mandelbrot200 4.13ms × (1.00,1.00) 4.12ms × (1.00,1.00) -0.08% (p=0.000) GoParse 3.97ms × (1.00,1.01) 4.01ms × (1.00,1.00) +0.99% (p=0.000) RegexpMatchEasy0_32 114ns × (1.00,1.00) 115ns × (0.99,1.00) ~ (p=0.070) RegexpMatchEasy0_1K 376ns × (1.00,1.00) 376ns × (1.00,1.00) ~ (p=0.900) RegexpMatchEasy1_32 94.9ns × (1.00,1.00) 96.3ns × (1.00,1.01) +1.53% (p=0.001) RegexpMatchEasy1_1K 568ns × (1.00,1.00) 567ns × (1.00,1.00) -0.22% (p=0.001) RegexpMatchMedium_32 159ns × (1.00,1.00) 159ns × (1.00,1.00) ~ (p=0.178) RegexpMatchMedium_1K 46.4µs × (1.00,1.00) 46.6µs × (1.00,1.00) +0.29% (p=0.000) RegexpMatchHard_32 2.37µs × (1.00,1.00) 2.37µs × (1.00,1.00) ~ (p=0.722) RegexpMatchHard_1K 71.1µs × (1.00,1.00) 71.2µs × (1.00,1.00) ~ (p=0.229) Revcomp 565ms × (1.00,1.00) 562ms × (1.00,1.00) -0.52% (p=0.000) Template 81.0ms × (1.00,1.00) 80.2ms × (1.00,1.00) -0.97% (p=0.000) TimeParse 380ns × (1.00,1.00) 380ns × (1.00,1.00) ~ (p=0.148) TimeFormat 405ns × (0.99,1.00) 385ns × (0.99,1.00) -5.00% (p=0.000) Change-Id: I11274158bf3affaf62662e02de7af12d5fb789e4 Reviewed-on: https://go-review.googlesource.com/9696 Reviewed-by: Russ Cox <rsc@golang.org> Run-TryBot: Austin Clements <austin@google.com> |
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The Go Programming Language
Go is an open source programming language that makes it easy to build simple, reliable, and efficient software.
For documentation about how to install and use Go, visit https://golang.org/ or load doc/install-source.html in your web browser.
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To contribute, please read the contribution guidelines: https://golang.org/doc/contribute.html
Please note that we do not use pull requests.
Unless otherwise noted, the Go source files are distributed under the BSD-style license found in the LICENSE file.
--
Binary Distribution Notes
If you have just untarred a binary Go distribution, you need to set the environment variable $GOROOT to the full path of the go directory (the one containing this file). You can omit the variable if you unpack it into /usr/local/go, or if you rebuild from sources by running all.bash (see doc/install-source.html). You should also add the Go binary directory $GOROOT/bin to your shell's path.
For example, if you extracted the tar file into $HOME/go, you might put the following in your .profile:
export GOROOT=$HOME/go
export PATH=$PATH:$GOROOT/bin
See https://golang.org/doc/install or doc/install.html for more details.