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>