faa7a7e8ae
This commit implements stack barriers to minimize the amount of stack re-scanning that must be done during mark termination. Currently the GC scans stacks of active goroutines twice during every GC cycle: once at the beginning during root discovery and once at the end during mark termination. The second scan happens while the world is stopped and guarantees that we've seen all of the roots (since there are no write barriers on writes to local stack variables). However, this means pause time is proportional to stack size. In particularly recursive programs, this can drive pause time up past our 10ms goal (e.g., it takes about 150ms to scan a 50MB heap). Re-scanning the entire stack is rarely necessary, especially for large stacks, because usually most of the frames on the stack were not active between the first and second scans and hence any changes to these frames (via non-escaping pointers passed down the stack) were tracked by write barriers. To efficiently track how far a stack has been unwound since the first scan (and, hence, how much needs to be re-scanned), this commit introduces stack barriers. During the first scan, at exponentially spaced points in each stack, the scan overwrites return PCs with the PC of the stack barrier function. When "returned" to, the stack barrier function records how far the stack has unwound and jumps to the original return PC for that point in the stack. Then the second scan only needs to proceed as far as the lowest barrier that hasn't been hit. For deeply recursive programs, this substantially reduces mark termination time (and hence pause time). For the goscheme example linked in issue #10898, prior to this change, mark termination times were typically between 100 and 500ms; with this change, mark termination times are typically between 10 and 20ms. As a result of the reduced stack scanning work, this reduces overall execution time of the goscheme example by 20%. Fixes #10898. The effect of this on programs that are not deeply recursive is minimal: name old time/op new time/op delta BinaryTree17 3.16s ± 2% 3.26s ± 1% +3.31% (p=0.000 n=19+19) Fannkuch11 2.42s ± 1% 2.48s ± 1% +2.24% (p=0.000 n=17+19) FmtFprintfEmpty 50.0ns ± 3% 49.8ns ± 1% ~ (p=0.534 n=20+19) FmtFprintfString 173ns ± 0% 175ns ± 0% +1.49% (p=0.000 n=16+19) FmtFprintfInt 170ns ± 1% 175ns ± 1% +2.97% (p=0.000 n=20+19) FmtFprintfIntInt 288ns ± 0% 295ns ± 0% +2.73% (p=0.000 n=16+19) FmtFprintfPrefixedInt 242ns ± 1% 252ns ± 1% +4.13% (p=0.000 n=18+18) FmtFprintfFloat 324ns ± 0% 323ns ± 0% -0.36% (p=0.000 n=20+19) FmtManyArgs 1.14µs ± 0% 1.12µs ± 1% -1.01% (p=0.000 n=18+19) GobDecode 8.88ms ± 1% 8.87ms ± 0% ~ (p=0.480 n=19+18) GobEncode 6.80ms ± 1% 6.85ms ± 0% +0.82% (p=0.000 n=20+18) Gzip 363ms ± 1% 363ms ± 1% ~ (p=0.077 n=18+20) Gunzip 90.6ms ± 0% 90.0ms ± 1% -0.71% (p=0.000 n=17+18) HTTPClientServer 51.5µs ± 1% 50.8µs ± 1% -1.32% (p=0.000 n=18+18) JSONEncode 17.0ms ± 0% 17.1ms ± 0% +0.40% (p=0.000 n=18+17) JSONDecode 61.8ms ± 0% 63.8ms ± 1% +3.11% (p=0.000 n=18+17) Mandelbrot200 3.84ms ± 0% 3.84ms ± 1% ~ (p=0.583 n=19+19) GoParse 3.71ms ± 1% 3.72ms ± 1% ~ (p=0.159 n=18+19) RegexpMatchEasy0_32 100ns ± 0% 100ns ± 1% -0.19% (p=0.033 n=17+19) RegexpMatchEasy0_1K 342ns ± 1% 331ns ± 0% -3.41% (p=0.000 n=19+19) RegexpMatchEasy1_32 82.5ns ± 0% 81.7ns ± 0% -0.98% (p=0.000 n=18+18) RegexpMatchEasy1_1K 505ns ± 0% 494ns ± 1% -2.16% (p=0.000 n=18+18) RegexpMatchMedium_32 137ns ± 1% 137ns ± 1% -0.24% (p=0.048 n=20+18) RegexpMatchMedium_1K 41.6µs ± 0% 41.3µs ± 1% -0.57% (p=0.004 n=18+20) RegexpMatchHard_32 2.11µs ± 0% 2.11µs ± 1% +0.20% (p=0.037 n=17+19) RegexpMatchHard_1K 63.9µs ± 2% 63.3µs ± 0% -0.99% (p=0.000 n=20+17) Revcomp 560ms ± 1% 522ms ± 0% -6.87% (p=0.000 n=18+16) Template 75.0ms ± 0% 75.1ms ± 1% +0.18% (p=0.013 n=18+19) TimeParse 358ns ± 1% 364ns ± 0% +1.74% (p=0.000 n=20+15) TimeFormat 360ns ± 0% 372ns ± 0% +3.55% (p=0.000 n=20+18) Change-Id: If8a9bfae6c128d15a4f405e02bcfa50129df82a2 Reviewed-on: https://go-review.googlesource.com/10314 Reviewed-by: Russ Cox <rsc@golang.org> Run-TryBot: Austin Clements <austin@google.com> TryBot-Result: Gobot Gobot <gobot@golang.org> |
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