mirror of https://github.com/golang/go.git
82d14d77da
Currently the concurrent root scan is performed in its entirety by the GC coordinator before entering concurrent mark (which enables GC workers). This scan is done sequentially, which can prolong the scan phase, delay the mark phase, and means that the scan phase does not obey the 25% CPU goal. Furthermore, there's no need to complete the root scan before starting marking (in fact, we already allow GC assists to happen during the scan phase), so this acts as an unnecessary barrier between root scanning and marking. This change shifts the root scan work out of the GC coordinator and in to the GC workers. The coordinator simply sets up the scan state and enqueues the right number of root scan jobs. The GC workers then drain the root scan jobs prior to draining heap scan jobs. This parallelizes the root scan process, makes it obey the 25% CPU goal, and effectively eliminates root scanning as an isolated phase, allowing the system to smoothly transition from root scanning to heap marking. This also eliminates a major non-STW responsibility of the GC coordinator, which will make it easier to switch to a decentralized state machine. Finally, it puts us in a good position to perform root scanning in assists as well, which will help satisfy assists at the beginning of the GC cycle. This is mostly straightforward. One tricky aspect is that we have to deal with preemption deadlock: where two non-preemptible gorountines are trying to preempt each other to perform a stack scan. Given the context where this happens, the only instance of this is two background workers trying to scan each other. We avoid this by simply not scanning the stacks of background workers during the concurrent phase; this is safe because we'll scan them during mark termination (and their stacks are *very* small and should not contain any new pointers). This change also switches the root marking during mark termination to use the same gcDrain-based code path as concurrent mark. This shouldn't affect performance because STW root marking was already parallel and tasks switched to heap marking immediately when no more root marking tasks were available. However, it simplifies the code and unifies these code paths. This has negligible effect on the go1 benchmarks. It slightly slows down the garbage benchmark, possibly by making GC run slightly more frequently. name old time/op new time/op delta XBenchGarbage-12 5.10ms ± 1% 5.24ms ± 1% +2.87% (p=0.000 n=18+18) name old time/op new time/op delta BinaryTree17-12 3.25s ± 3% 3.20s ± 5% -1.57% (p=0.013 n=20+20) Fannkuch11-12 2.45s ± 1% 2.46s ± 1% +0.38% (p=0.019 n=20+18) FmtFprintfEmpty-12 49.7ns ± 3% 49.9ns ± 4% ~ (p=0.851 n=19+20) FmtFprintfString-12 170ns ± 2% 170ns ± 1% ~ (p=0.775 n=20+19) FmtFprintfInt-12 161ns ± 1% 160ns ± 1% -0.78% (p=0.000 n=19+18) FmtFprintfIntInt-12 267ns ± 1% 270ns ± 1% +1.04% (p=0.000 n=19+19) FmtFprintfPrefixedInt-12 238ns ± 2% 238ns ± 1% ~ (p=0.133 n=18+19) FmtFprintfFloat-12 311ns ± 1% 310ns ± 2% -0.35% (p=0.023 n=20+19) FmtManyArgs-12 1.08µs ± 1% 1.06µs ± 1% -2.31% (p=0.000 n=20+20) GobDecode-12 8.65ms ± 1% 8.63ms ± 1% ~ (p=0.377 n=18+20) GobEncode-12 6.49ms ± 1% 6.52ms ± 1% +0.37% (p=0.015 n=20+20) Gzip-12 319ms ± 3% 318ms ± 1% ~ (p=0.975 n=19+17) Gunzip-12 41.9ms ± 1% 42.1ms ± 2% +0.65% (p=0.004 n=19+20) HTTPClientServer-12 61.7µs ± 1% 62.6µs ± 1% +1.40% (p=0.000 n=18+20) JSONEncode-12 16.8ms ± 1% 16.9ms ± 1% ~ (p=0.239 n=20+18) JSONDecode-12 58.4ms ± 1% 60.7ms ± 1% +3.85% (p=0.000 n=19+20) Mandelbrot200-12 3.86ms ± 0% 3.86ms ± 1% ~ (p=0.092 n=18+19) GoParse-12 3.75ms ± 2% 3.75ms ± 2% ~ (p=0.708 n=19+20) RegexpMatchEasy0_32-12 100ns ± 1% 100ns ± 2% +0.60% (p=0.010 n=17+20) RegexpMatchEasy0_1K-12 341ns ± 1% 342ns ± 2% ~ (p=0.203 n=20+19) RegexpMatchEasy1_32-12 82.5ns ± 2% 83.2ns ± 2% +0.83% (p=0.007 n=19+19) RegexpMatchEasy1_1K-12 495ns ± 1% 495ns ± 2% ~ (p=0.970 n=19+18) RegexpMatchMedium_32-12 130ns ± 2% 130ns ± 2% +0.59% (p=0.039 n=19+20) RegexpMatchMedium_1K-12 39.2µs ± 1% 39.3µs ± 1% ~ (p=0.214 n=18+18) RegexpMatchHard_32-12 2.03µs ± 2% 2.02µs ± 1% ~ (p=0.166 n=18+19) RegexpMatchHard_1K-12 61.0µs ± 1% 60.9µs ± 1% ~ (p=0.169 n=20+18) Revcomp-12 533ms ± 1% 535ms ± 1% ~ (p=0.071 n=19+17) Template-12 68.1ms ± 2% 73.0ms ± 1% +7.26% (p=0.000 n=19+20) TimeParse-12 355ns ± 2% 356ns ± 2% ~ (p=0.530 n=19+20) TimeFormat-12 357ns ± 2% 347ns ± 1% -2.59% (p=0.000 n=20+19) [Geo mean] 62.1µs 62.3µs +0.31% name old speed new speed delta GobDecode-12 88.7MB/s ± 1% 88.9MB/s ± 1% ~ (p=0.377 n=18+20) GobEncode-12 118MB/s ± 1% 118MB/s ± 1% -0.37% (p=0.015 n=20+20) Gzip-12 60.9MB/s ± 3% 60.9MB/s ± 1% ~ (p=0.944 n=19+17) Gunzip-12 464MB/s ± 1% 461MB/s ± 2% -0.64% (p=0.004 n=19+20) JSONEncode-12 115MB/s ± 1% 115MB/s ± 1% ~ (p=0.236 n=20+18) JSONDecode-12 33.2MB/s ± 1% 32.0MB/s ± 1% -3.71% (p=0.000 n=19+20) GoParse-12 15.5MB/s ± 2% 15.5MB/s ± 2% ~ (p=0.702 n=19+20) RegexpMatchEasy0_32-12 320MB/s ± 1% 318MB/s ± 2% ~ (p=0.094 n=18+20) RegexpMatchEasy0_1K-12 3.00GB/s ± 1% 2.99GB/s ± 1% ~ (p=0.194 n=20+19) RegexpMatchEasy1_32-12 388MB/s ± 2% 385MB/s ± 2% -0.83% (p=0.008 n=19+19) RegexpMatchEasy1_1K-12 2.07GB/s ± 1% 2.07GB/s ± 1% ~ (p=0.964 n=19+18) RegexpMatchMedium_32-12 7.68MB/s ± 1% 7.64MB/s ± 2% -0.57% (p=0.020 n=19+20) RegexpMatchMedium_1K-12 26.1MB/s ± 1% 26.1MB/s ± 1% ~ (p=0.211 n=18+18) RegexpMatchHard_32-12 15.8MB/s ± 1% 15.8MB/s ± 1% ~ (p=0.180 n=18+19) RegexpMatchHard_1K-12 16.8MB/s ± 1% 16.8MB/s ± 2% ~ (p=0.236 n=20+19) Revcomp-12 477MB/s ± 1% 475MB/s ± 1% ~ (p=0.071 n=19+17) Template-12 28.5MB/s ± 2% 26.6MB/s ± 1% -6.77% (p=0.000 n=19+20) [Geo mean] 100MB/s 99.0MB/s -0.82% Change-Id: I875bf6ceb306d1ee2f470cabf88aa6ede27c47a0 Reviewed-on: https://go-review.googlesource.com/16059 Reviewed-by: Rick Hudson <rlh@golang.org> Run-TryBot: Austin Clements <austin@google.com> TryBot-Result: Gobot Gobot <gobot@golang.org> |
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