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 4655aad 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/21324
Reviewed-by: Rick Hudson <[email protected]>
Run-TryBot: Austin Clements <[email protected]>
TryBot-Result: Gobot Gobot <[email protected]>

src/runtime/mgc.go

@@ -1602,27 +1602,8 @@ func gcMark(start_time int64) {
 
 	cachestats()
 
-	// Compute the reachable heap size at the beginning of the
-	// cycle. This is approximately the marked heap size at the
-	// end (which we know) minus the amount of marked heap that
-	// was allocated after marking began (which we don't know, but
-	// is approximately the amount of heap that was allocated
-	// since marking began).
-	allocatedDuringCycle := memstats.heap_live - work.initialHeapLive
-	if memstats.heap_live < work.initialHeapLive {
-		// This can happen if mCentral_UncacheSpan tightens
-		// the heap_live approximation.
-		allocatedDuringCycle = 0
-	}
-	if work.bytesMarked >= allocatedDuringCycle {
-		memstats.heap_reachable = work.bytesMarked - allocatedDuringCycle
-	} else {
-		// This can happen if most of the allocation during
-		// the cycle never became reachable from the heap.
-		// Just set the reachable heap approximation to 0 and
-		// let the heapminimum kick in below.
-		memstats.heap_reachable = 0
-	}
+	// Update the reachable heap stat.
+	memstats.heap_reachable = work.bytesMarked
 
 	// Trigger the next GC cycle when the allocated heap has grown
 	// by triggerRatio over the reachable heap size. Assume that