runtime: allocate black during GC
Currently we allocate white for most of concurrent marking. This is based on the classical argument that it produces less floating garbage, since allocations during GC may not get linked into the heap and allocating white lets us reclaim these. However, it's not clear how often this actually happens, especially since our write barrier shades any pointer as soon as it's installed in the heap regardless of the color of the slot. On the other hand, allocating black has several advantages that seem to significantly outweigh this downside. 1) It naturally bounds the total scan work to the live heap size at the start of a GC cycle. Allocating white does not, and thus depends entirely on assists to prevent the heap from growing faster than it can be scanned. 2) It reduces the total amount of scan work per GC cycle by the size of newly allocated objects that are linked into the heap graph, since objects allocated black never need to be scanned. 3) It reduces total write barrier work since more objects will already be black when they are linked into the heap graph. This gives a slight overall improvement in benchmarks. name old time/op new time/op delta XBenchGarbage-12 2.24ms ± 0% 2.21ms ± 1% -1.32% (p=0.000 n=18+17) name old time/op new time/op delta BinaryTree17-12 2.60s ± 3% 2.53s ± 3% -2.56% (p=0.000 n=20+20) Fannkuch11-12 2.08s ± 1% 2.08s ± 0% ~ (p=0.452 n=19+19) FmtFprintfEmpty-12 45.1ns ± 2% 45.3ns ± 2% ~ (p=0.367 n=19+20) FmtFprintfString-12 131ns ± 3% 129ns ± 0% -1.60% (p=0.000 n=20+16) FmtFprintfInt-12 122ns ± 0% 121ns ± 2% -0.86% (p=0.000 n=16+19) FmtFprintfIntInt-12 187ns ± 1% 186ns ± 1% ~ (p=0.514 n=18+19) FmtFprintfPrefixedInt-12 189ns ± 0% 188ns ± 1% -0.54% (p=0.000 n=16+18) FmtFprintfFloat-12 256ns ± 0% 254ns ± 1% -0.43% (p=0.000 n=17+19) FmtManyArgs-12 769ns ± 0% 763ns ± 0% -0.72% (p=0.000 n=18+18) GobDecode-12 7.08ms ± 2% 7.00ms ± 1% -1.22% (p=0.000 n=20+20) GobEncode-12 5.88ms ± 0% 5.88ms ± 1% ~ (p=0.406 n=18+18) Gzip-12 214ms ± 0% 214ms ± 1% ~ (p=0.103 n=17+18) Gunzip-12 37.6ms ± 0% 37.6ms ± 0% ~ (p=0.563 n=17+17) HTTPClientServer-12 77.2µs ± 3% 76.9µs ± 2% ~ (p=0.606 n=20+20) JSONEncode-12 15.1ms ± 1% 15.2ms ± 2% ~ (p=0.138 n=19+19) JSONDecode-12 53.3ms ± 1% 53.1ms ± 1% -0.33% (p=0.000 n=19+18) Mandelbrot200-12 4.04ms ± 1% 4.04ms ± 1% ~ (p=0.075 n=19+18) GoParse-12 3.30ms ± 1% 3.29ms ± 1% -0.57% (p=0.000 n=18+16) RegexpMatchEasy0_32-12 69.5ns ± 1% 69.9ns ± 3% ~ (p=0.822 n=18+20) RegexpMatchEasy0_1K-12 237ns ± 1% 237ns ± 0% ~ (p=0.398 n=19+18) RegexpMatchEasy1_32-12 69.8ns ± 2% 69.5ns ± 1% ~ (p=0.090 n=20+16) RegexpMatchEasy1_1K-12 371ns ± 1% 372ns ± 1% ~ (p=0.178 n=19+20) RegexpMatchMedium_32-12 108ns ± 2% 108ns ± 3% ~ (p=0.124 n=20+19) RegexpMatchMedium_1K-12 33.9µs ± 2% 34.2µs ± 4% ~ (p=0.309 n=20+19) RegexpMatchHard_32-12 1.75µs ± 2% 1.77µs ± 4% +1.28% (p=0.018 n=19+18) RegexpMatchHard_1K-12 52.7µs ± 1% 53.4µs ± 4% +1.23% (p=0.013 n=15+18) Revcomp-12 354ms ± 1% 359ms ± 4% +1.27% (p=0.043 n=20+20) Template-12 63.6ms ± 2% 63.7ms ± 2% ~ (p=0.654 n=20+18) TimeParse-12 313ns ± 1% 316ns ± 2% +0.80% (p=0.014 n=17+20) TimeFormat-12 332ns ± 0% 329ns ± 0% -0.66% (p=0.000 n=16+16) [Geo mean] 51.7µs 51.6µs -0.09% Change-Id: I2214a6a0e4f544699ea166073249a8efdf080dc0 Reviewed-on: https://go-review.googlesource.com/21323Reviewed-by:Rick Hudson <rlh@golang.org> Run-TryBot: Austin Clements <austin@google.com> TryBot-Result: Gobot Gobot <gobot@golang.org>
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