runtime: eliminate floating garbage estimate
Currently when we compute the trigger for the next GC, we do it based on an estimate of the reachable heap size at the start of the GC cycle, which is itself based on an estimate of the floating garbage. This was introduced by 4655aadd to fix a bad feedback loop that allowed the heap to grow to many times the true reachable size. However, this estimate gets easily confused by rapidly allocating applications, and, worse it's different than the heap size the trigger controller uses to compute the trigger itself. This results in the trigger controller often thinking that GC finished before it started. Since this would be a pretty great outcome from it's perspective, it sets the trigger for the next cycle as close to the next goal as possible (which is limited to 95% of the goal). Furthermore, the bad feedback loop this estimate originally fixed seems not to happen any more, suggesting it was fixed more correctly by some other change in the mean time. Finally, with the change to allocate black, it shouldn't even be theoretically possible for this bad feedback loop to occur. Hence, eliminate the floating garbage estimate and simply consider the reachable heap to be the marked heap. This harms overall throughput slightly for allocation-heavy benchmarks, but significantly improves mutator availability. Fixes #12204. This brings the average trigger in this benchmark from 0.95 (the cap) to 0.7 and the active GC utilization from ~90% to ~45%. Updates #14951. This makes the trigger controller much better behaved, so it pulls the trigger lower if assists are consuming a lot of CPU like it's supposed to, increasing mutator availability. name old time/op new time/op delta XBenchGarbage-12 2.21ms ± 1% 2.28ms ± 3% +3.29% (p=0.000 n=17+17) Some of this slow down we paid for in earlier commits. Relative to the start of the series to switch to allocate-black (the parent of "count black allocations toward scan work"), the garbage benchmark is 2.62% slower. name old time/op new time/op delta BinaryTree17-12 2.53s ± 3% 2.53s ± 3% ~ (p=0.708 n=20+19) Fannkuch11-12 2.08s ± 0% 2.08s ± 0% -0.22% (p=0.002 n=19+18) FmtFprintfEmpty-12 45.3ns ± 2% 45.2ns ± 3% ~ (p=0.505 n=20+20) FmtFprintfString-12 129ns ± 0% 131ns ± 2% +1.80% (p=0.000 n=16+19) FmtFprintfInt-12 121ns ± 2% 121ns ± 2% ~ (p=0.768 n=19+19) FmtFprintfIntInt-12 186ns ± 1% 188ns ± 3% +0.99% (p=0.000 n=19+19) FmtFprintfPrefixedInt-12 188ns ± 1% 188ns ± 1% ~ (p=0.947 n=18+16) FmtFprintfFloat-12 254ns ± 1% 255ns ± 1% +0.30% (p=0.002 n=19+17) FmtManyArgs-12 763ns ± 0% 770ns ± 0% +0.92% (p=0.000 n=18+18) GobDecode-12 7.00ms ± 1% 7.04ms ± 1% +0.61% (p=0.049 n=20+20) GobEncode-12 5.88ms ± 1% 5.88ms ± 0% ~ (p=0.641 n=18+19) Gzip-12 214ms ± 1% 215ms ± 1% +0.43% (p=0.002 n=18+19) Gunzip-12 37.6ms ± 0% 37.6ms ± 0% +0.11% (p=0.015 n=17+18) HTTPClientServer-12 76.9µs ± 2% 78.1µs ± 2% +1.44% (p=0.000 n=20+18) JSONEncode-12 15.2ms ± 2% 15.1ms ± 1% ~ (p=0.271 n=19+18) JSONDecode-12 53.1ms ± 1% 53.3ms ± 0% +0.49% (p=0.000 n=18+19) Mandelbrot200-12 4.04ms ± 1% 4.03ms ± 0% -0.33% (p=0.005 n=18+18) GoParse-12 3.29ms ± 1% 3.28ms ± 1% ~ (p=0.146 n=16+17) RegexpMatchEasy0_32-12 69.9ns ± 3% 69.5ns ± 1% ~ (p=0.785 n=20+19) RegexpMatchEasy0_1K-12 237ns ± 0% 237ns ± 0% ~ (p=1.000 n=18+18) RegexpMatchEasy1_32-12 69.5ns ± 1% 69.2ns ± 1% -0.44% (p=0.020 n=16+19) RegexpMatchEasy1_1K-12 372ns ± 1% 371ns ± 2% ~ (p=0.086 n=20+19) RegexpMatchMedium_32-12 108ns ± 3% 107ns ± 1% -1.00% (p=0.004 n=19+14) RegexpMatchMedium_1K-12 34.2µs ± 4% 34.0µs ± 2% ~ (p=0.380 n=19+20) RegexpMatchHard_32-12 1.77µs ± 4% 1.76µs ± 3% ~ (p=0.558 n=18+20) RegexpMatchHard_1K-12 53.4µs ± 4% 52.8µs ± 2% -1.10% (p=0.020 n=18+20) Revcomp-12 359ms ± 4% 377ms ± 0% +5.19% (p=0.000 n=20+18) Template-12 63.7ms ± 2% 62.9ms ± 2% -1.27% (p=0.005 n=18+20) TimeParse-12 316ns ± 2% 313ns ± 1% ~ (p=0.059 n=20+16) TimeFormat-12 329ns ± 0% 331ns ± 0% +0.39% (p=0.000 n=16+18) [Geo mean] 51.6µs 51.7µs +0.18% Change-Id: I1dce4640c8205d41717943b021039fffea863c57 Reviewed-on: https://go-review.googlesource.com/21324Reviewed-by:Rick Hudson <rlh@golang.org> Run-TryBot: Austin Clements <austin@google.com> TryBot-Result: Gobot Gobot <gobot@golang.org>
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