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runtime: allow 5% mutator assist over 25% background mark
Currently, both the background mark worker and the goal GC CPU are both fixed at 25%. The trigger controller's goal is to achieve the goal CPU usage, and with the previous commit it can actually achieve this. But this means there are *no* assists, which sounds ideal but actually causes problems for the trigger controller. Since the controller can't lower CPU usage below the background mark worker CPU, it saturates at the CPU goal and no longer gets feedback, which translates into higher variability in heap growth. This commit fixes this by allowing assists 5% CPU beyond the 25% fixed background mark. This avoids saturating the trigger controller, since it can now get feedback from both sides of the CPU goal. This leads to low variability in both CPU usage and heap growth, at the cost of reintroducing a low rate of mark assists. We also experimented with 20% background plus 5% assist, but 25%+5% clearly performed better in benchmarks. Updates #14951. Updates #14812. Updates #18534. Combined with the previous CL, this significantly improves tail mutator utilization in the x/bechmarks garbage benchmark. On a sample trace, it increased the 99.9%ile mutator utilization at 10ms from 26% to 59%, and at 5ms from 17% to 52%. It reduced the 99.9%ile zero utilization window from 2ms to 700µs. It also helps the mean mutator utilization: it increased the 10s mutator utilization from 83% to 94%. The minimum mutator utilization is also somewhat improved, though there is still some unknown artifact that causes a miniscule fraction of mutator assists to take 5--10ms (in fact, there was exactly one 10ms mutator assist in my sample trace). This has no significant effect on the throughput of the github.com/dr2chase/bent benchmarks-50. This has little effect on the go1 benchmarks (and the slight overall improvement makes up for the slight overall slowdown from the previous commit): name old time/op new time/op delta BinaryTree17-12 2.40s ± 0% 2.41s ± 1% +0.26% (p=0.010 n=18+19) Fannkuch11-12 2.95s ± 0% 2.93s ± 0% -0.62% (p=0.000 n=18+15) FmtFprintfEmpty-12 42.2ns ± 0% 42.3ns ± 1% +0.37% (p=0.001 n=15+14) FmtFprintfString-12 67.9ns ± 2% 67.2ns ± 3% -1.03% (p=0.002 n=20+18) FmtFprintfInt-12 75.6ns ± 3% 76.8ns ± 2% +1.59% (p=0.000 n=19+17) FmtFprintfIntInt-12 123ns ± 1% 124ns ± 1% +0.77% (p=0.000 n=17+14) FmtFprintfPrefixedInt-12 148ns ± 1% 150ns ± 1% +1.28% (p=0.000 n=20+20) FmtFprintfFloat-12 212ns ± 0% 211ns ± 1% -0.67% (p=0.000 n=16+17) FmtManyArgs-12 499ns ± 1% 500ns ± 0% +0.23% (p=0.004 n=19+16) GobDecode-12 6.49ms ± 1% 6.51ms ± 1% +0.32% (p=0.008 n=19+19) GobEncode-12 5.47ms ± 0% 5.43ms ± 1% -0.68% (p=0.000 n=19+20) Gzip-12 220ms ± 1% 216ms ± 1% -1.66% (p=0.000 n=20+19) Gunzip-12 38.8ms ± 0% 38.5ms ± 0% -0.80% (p=0.000 n=19+20) HTTPClientServer-12 78.5µs ± 1% 78.1µs ± 1% -0.53% (p=0.008 n=20+19) JSONEncode-12 12.2ms ± 0% 11.9ms ± 0% -2.38% (p=0.000 n=17+19) JSONDecode-12 52.3ms ± 0% 53.3ms ± 0% +1.84% (p=0.000 n=19+20) Mandelbrot200-12 3.69ms ± 0% 3.69ms ± 0% -0.19% (p=0.000 n=19+19) GoParse-12 3.17ms ± 1% 3.19ms ± 1% +0.61% (p=0.000 n=20+20) RegexpMatchEasy0_32-12 73.7ns ± 0% 73.2ns ± 1% -0.66% (p=0.000 n=17+20) RegexpMatchEasy0_1K-12 238ns ± 0% 239ns ± 0% +0.32% (p=0.000 n=17+16) RegexpMatchEasy1_32-12 69.1ns ± 1% 69.2ns ± 1% ~ (p=0.669 n=19+13) RegexpMatchEasy1_1K-12 365ns ± 1% 367ns ± 1% +0.49% (p=0.000 n=19+19) RegexpMatchMedium_32-12 104ns ± 1% 105ns ± 1% +1.33% (p=0.000 n=16+20) RegexpMatchMedium_1K-12 33.6µs ± 3% 34.1µs ± 4% +1.67% (p=0.001 n=20+20) RegexpMatchHard_32-12 1.67µs ± 1% 1.62µs ± 1% -2.78% (p=0.000 n=18+17) RegexpMatchHard_1K-12 50.3µs ± 2% 48.7µs ± 1% -3.09% (p=0.000 n=19+18) Revcomp-12 384ms ± 0% 386ms ± 0% +0.59% (p=0.000 n=19+19) Template-12 61.1ms ± 1% 60.5ms ± 1% -1.02% (p=0.000 n=19+20) TimeParse-12 307ns ± 0% 303ns ± 1% -1.23% (p=0.000 n=19+15) TimeFormat-12 323ns ± 0% 323ns ± 0% -0.12% (p=0.011 n=15+20) [Geo mean] 47.1µs 47.0µs -0.20% https://perf.golang.org/search?q=upload:20171030.4 It slightly improve the performance the x/benchmarks: name old time/op new time/op delta Garbage/benchmem-MB=1024-12 2.29ms ± 3% 2.22ms ± 2% -2.97% (p=0.000 n=18+18) Garbage/benchmem-MB=64-12 2.24ms ± 2% 2.21ms ± 2% -1.64% (p=0.000 n=18+18) HTTP-12 12.6µs ± 1% 12.6µs ± 1% ~ (p=0.690 n=19+17) JSON-12 11.3ms ± 2% 11.3ms ± 1% ~ (p=0.163 n=17+18) and fixes some of the heap size bloat caused by the previous commit: name old peak-RSS-bytes new peak-RSS-bytes delta Garbage/benchmem-MB=1024-12 1.88G ± 2% 1.77G ± 2% -5.52% (p=0.000 n=20+18) Garbage/benchmem-MB=64-12 248M ± 8% 226M ± 5% -8.93% (p=0.000 n=20+20) HTTP-12 47.0M ±27% 47.2M ±12% ~ (p=0.512 n=20+20) JSON-12 206M ±11% 206M ±10% ~ (p=0.841 n=20+20) https://perf.golang.org/search?q=upload:20171030.5 Combined with the change to add a soft goal in the previous commit, the achieves a decent performance improvement on the garbage benchmark: name old time/op new time/op delta Garbage/benchmem-MB=1024-12 2.40ms ± 4% 2.22ms ± 2% -7.40% (p=0.000 n=19+18) Garbage/benchmem-MB=64-12 2.23ms ± 1% 2.21ms ± 2% -1.06% (p=0.000 n=19+18) HTTP-12 12.5µs ± 1% 12.6µs ± 1% ~ (p=0.330 n=20+17) JSON-12 11.1ms ± 1% 11.3ms ± 1% +1.87% (p=0.000 n=16+18) https://perf.golang.org/search?q=upload:20171030.6 Change-Id: If04ddb57e1e58ef2fb9eec54c290eb4ae4bea121 Reviewed-on: https://go-review.googlesource.com/59971 Run-TryBot: Austin Clements <austin@google.com> TryBot-Result: Gobot Gobot <gobot@golang.org> Reviewed-by: Rick Hudson <rlh@golang.org>
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Austin Clements
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03eb9483e3
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@ -879,13 +879,19 @@ func gcSetTriggerRatio(triggerRatio float64) {
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// gcGoalUtilization is the goal CPU utilization for
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// marking as a fraction of GOMAXPROCS.
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const gcGoalUtilization = 0.25
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const gcGoalUtilization = 0.30
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// gcBackgroundUtilization is the fixed CPU utilization for background
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// marking. It must be <= gcGoalUtilization. The difference between
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// gcGoalUtilization and gcBackgroundUtilization will be made up by
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// mark assists. The scheduler will aim to use within 50% of this
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// goal.
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//
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// Setting this to < gcGoalUtilization avoids saturating the trigger
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// feedback controller when there are no assists, which allows it to
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// better control CPU and heap growth. However, the larger the gap,
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// the more mutator assists are expected to happen, which impact
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// mutator latency.
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const gcBackgroundUtilization = 0.25
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// gcCreditSlack is the amount of scan work credit that can can
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