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The Go programming language
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Currently the time spent in scanobject is proportional to the size of the object being scanned. Since scanobject is non-preemptible, large objects can cause significant goroutine (and even whole application) delays through several means: 1. If a GC assist picks up a large object, the allocating goroutine is blocked for the whole scan, even if that scan well exceeds that goroutine's debt. 2. Since the scheduler does not run on the P performing a large object scan, goroutines in that P's run queue do not run unless they are stolen by another P (which can take some time). If there are a few large objects, all of the Ps may get tied up so the scheduler doesn't run anywhere. 3. Even if a large object is scanned by a background worker and other Ps are still running the scheduler, the large object scan doesn't flush background credit until the whole scan is done. This can easily cause all allocations to block in assists, waiting for credit, causing an effective STW. Fix this by splitting large objects into 128 KB "oblets" and scanning at most one oblet at a time. Since we can scan 1–2 MB/ms, this equates to bounding scanobject at roughly 100 µs. This improves assist behavior both because assists can no longer get "unlucky" and be stuck scanning a large object, and because it causes the background worker to flush credit and unblock assists more frequently when scanning large objects. This also improves GC parallelism if the heap consists primarily of a small number of very large objects by letting multiple workers scan a large objects in parallel. Fixes #10345. Fixes #16293. This substantially improves goroutine latency in the benchmark from issue #16293, which exercises several forms of very large objects: name old max-latency new max-latency delta SliceNoPointer-12 154µs ± 1% 155µs ± 2% ~ (p=0.087 n=13+12) SlicePointer-12 314ms ± 1% 5.94ms ±138% -98.11% (p=0.000 n=19+20) SliceLivePointer-12 1148ms ± 0% 4.72ms ±167% -99.59% (p=0.000 n=19+20) MapNoPointer-12 72509µs ± 1% 408µs ±325% -99.44% (p=0.000 n=19+18) ChanPointer-12 313ms ± 0% 4.74ms ±140% -98.49% (p=0.000 n=18+20) ChanLivePointer-12 1147ms ± 0% 3.30ms ±149% -99.71% (p=0.000 n=19+20) name old P99.9-latency new P99.9-latency delta SliceNoPointer-12 113µs ±25% 107µs ±12% ~ (p=0.153 n=20+18) SlicePointer-12 309450µs ± 0% 133µs ±23% -99.96% (p=0.000 n=20+20) SliceLivePointer-12 961ms ± 0% 1.35ms ±27% -99.86% (p=0.000 n=20+20) MapNoPointer-12 448µs ±288% 119µs ±18% -73.34% (p=0.000 n=18+20) ChanPointer-12 309450µs ± 0% 134µs ±23% -99.96% (p=0.000 n=20+19) ChanLivePointer-12 961ms ± 0% 1.35ms ±27% -99.86% (p=0.000 n=20+20) This has negligible effect on all metrics from the garbage, JSON, and HTTP x/benchmarks. It shows slight improvement on some of the go1 benchmarks, particularly Revcomp, which uses some multi-megabyte buffers: name old time/op new time/op delta BinaryTree17-12 2.46s ± 1% 2.47s ± 1% +0.32% (p=0.012 n=20+20) Fannkuch11-12 2.82s ± 0% 2.81s ± 0% -0.61% (p=0.000 n=17+20) FmtFprintfEmpty-12 50.8ns ± 5% 50.5ns ± 2% ~ (p=0.197 n=17+19) FmtFprintfString-12 131ns ± 1% 132ns ± 0% +0.57% (p=0.000 n=20+16) FmtFprintfInt-12 117ns ± 0% 116ns ± 0% -0.47% (p=0.000 n=15+20) FmtFprintfIntInt-12 180ns ± 0% 179ns ± 1% -0.78% (p=0.000 n=16+20) FmtFprintfPrefixedInt-12 186ns ± 1% 185ns ± 1% -0.55% (p=0.000 n=19+20) FmtFprintfFloat-12 263ns ± 1% 271ns ± 0% +2.84% (p=0.000 n=18+20) FmtManyArgs-12 741ns ± 1% 742ns ± 1% ~ (p=0.190 n=19+19) GobDecode-12 7.44ms ± 0% 7.35ms ± 1% -1.21% (p=0.000 n=20+20) GobEncode-12 6.22ms ± 1% 6.21ms ± 1% ~ (p=0.336 n=20+19) Gzip-12 220ms ± 1% 219ms ± 1% ~ (p=0.130 n=19+19) Gunzip-12 37.9ms ± 0% 37.9ms ± 1% ~ (p=1.000 n=20+19) HTTPClientServer-12 82.5µs ± 3% 82.6µs ± 3% ~ (p=0.776 n=20+19) JSONEncode-12 16.4ms ± 1% 16.5ms ± 2% +0.49% (p=0.003 n=18+19) JSONDecode-12 53.7ms ± 1% 54.1ms ± 1% +0.71% (p=0.000 n=19+18) Mandelbrot200-12 4.19ms ± 1% 4.20ms ± 1% ~ (p=0.452 n=19+19) GoParse-12 3.38ms ± 1% 3.37ms ± 1% ~ (p=0.123 n=19+19) RegexpMatchEasy0_32-12 72.1ns ± 1% 71.8ns ± 1% ~ (p=0.397 n=19+17) RegexpMatchEasy0_1K-12 242ns ± 0% 242ns ± 0% ~ (p=0.168 n=17+20) RegexpMatchEasy1_32-12 72.1ns ± 1% 72.1ns ± 1% ~ (p=0.538 n=18+19) RegexpMatchEasy1_1K-12 385ns ± 1% 384ns ± 1% ~ (p=0.388 n=20+20) RegexpMatchMedium_32-12 112ns ± 1% 112ns ± 3% ~ (p=0.539 n=20+20) RegexpMatchMedium_1K-12 34.4µs ± 2% 34.4µs ± 2% ~ (p=0.628 n=18+18) RegexpMatchHard_32-12 1.80µs ± 1% 1.80µs ± 1% ~ (p=0.522 n=18+19) RegexpMatchHard_1K-12 54.0µs ± 1% 54.1µs ± 1% ~ (p=0.647 n=20+19) Revcomp-12 387ms ± 1% 369ms ± 5% -4.89% (p=0.000 n=17+19) Template-12 62.3ms ± 1% 62.0ms ± 0% -0.48% (p=0.002 n=20+17) TimeParse-12 314ns ± 1% 314ns ± 0% ~ (p=1.011 n=20+13) TimeFormat-12 358ns ± 0% 354ns ± 0% -1.12% (p=0.000 n=17+20) [Geo mean] 53.5µs 53.3µs -0.23% Change-Id: I2a0a179d1d6bf7875dd054b7693dd12d2a340132 Reviewed-on: https://go-review.googlesource.com/23540 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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The Go Programming Language
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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.