Currently, mheap tracks spans in both mSpanLists and mTreaps, but
mSpanLists, while they tend to be smaller, complicate the
implementation. Here we simplify the implementation by removing
free and busy from mheap and renaming freelarge -> free and busylarge
-> busy.

This change also slightly changes the reclamation policy. Previously,
for allocations under 1MB we would attempt to find a small span of the
right size. Now, we just try to find any number of spans totaling the
right size. This may increase heap fragmentation, but that will be dealt
with using virtual memory tricks in follow-up CLs.

For #14045.

Garbage-heavy benchmarks show very little change, except what appears
to be a decrease in STW times and peak RSS.

name                      old STW-ns/GC       new STW-ns/GC       delta
Garbage/benchmem-MB=64-8           263k ±64%           217k ±24%  -17.66%  (p=0.028 n=25+23)

name                      old STW-ns/op       new STW-ns/op       delta
Garbage/benchmem-MB=64-8          9.39k ±65%          7.80k ±24%  -16.88%  (p=0.037 n=25+23)

name                      old peak-RSS-bytes  new peak-RSS-bytes  delta
Garbage/benchmem-MB=64-8           281M ± 0%           249M ± 4%  -11.40%  (p=0.000 n=19+18)

https://perf.golang.org/search?q=upload:20181005.1

Go1 benchmarks perform roughly the same, the most notable regression
being the JSON encode/decode benchmark with worsens by ~2%.

name                     old time/op    new time/op    delta
BinaryTree17-8              3.02s ± 2%     2.99s ± 2%  -1.18%  (p=0.000 n=25+24)
Fannkuch11-8                3.05s ± 1%     3.02s ± 2%  -1.20%  (p=0.000 n=25+25)
FmtFprintfEmpty-8          43.6ns ± 5%    43.4ns ± 3%    ~     (p=0.528 n=25+25)
FmtFprintfString-8         74.9ns ± 3%    73.4ns ± 1%  -2.03%  (p=0.001 n=25+24)
FmtFprintfInt-8            79.3ns ± 3%    77.9ns ± 1%  -1.73%  (p=0.003 n=25+25)
FmtFprintfIntInt-8          119ns ± 6%     116ns ± 0%  -2.68%  (p=0.000 n=25+18)
FmtFprintfPrefixedInt-8     134ns ± 4%     132ns ± 1%  -1.52%  (p=0.004 n=25+25)
FmtFprintfFloat-8           240ns ± 1%     241ns ± 1%    ~     (p=0.403 n=24+23)
FmtManyArgs-8               543ns ± 1%     537ns ± 1%  -1.00%  (p=0.000 n=25+25)
GobDecode-8                6.88ms ± 1%    6.92ms ± 4%    ~     (p=0.088 n=24+22)
GobEncode-8                5.92ms ± 1%    5.93ms ± 1%    ~     (p=0.898 n=25+24)
Gzip-8                      267ms ± 2%     266ms ± 2%    ~     (p=0.213 n=25+24)
Gunzip-8                   35.4ms ± 1%    35.6ms ± 1%  +0.70%  (p=0.000 n=25+25)
HTTPClientServer-8          104µs ± 2%     104µs ± 2%    ~     (p=0.686 n=25+25)
JSONEncode-8               9.67ms ± 1%    9.80ms ± 4%  +1.32%  (p=0.000 n=25+25)
JSONDecode-8               47.7ms ± 1%    48.8ms ± 5%  +2.33%  (p=0.000 n=25+25)
Mandelbrot200-8            4.87ms ± 1%    4.91ms ± 1%  +0.79%  (p=0.000 n=25+25)
GoParse-8                  3.59ms ± 4%    3.55ms ± 1%    ~     (p=0.199 n=25+24)
RegexpMatchEasy0_32-8      90.3ns ± 1%    89.9ns ± 1%  -0.47%  (p=0.000 n=25+21)
RegexpMatchEasy0_1K-8       204ns ± 1%     204ns ± 1%    ~     (p=0.914 n=25+24)
RegexpMatchEasy1_32-8      84.9ns ± 0%    84.6ns ± 1%  -0.36%  (p=0.000 n=24+25)
RegexpMatchEasy1_1K-8       350ns ± 1%     348ns ± 3%  -0.59%  (p=0.007 n=25+25)
RegexpMatchMedium_32-8      122ns ± 1%     121ns ± 0%  -1.08%  (p=0.000 n=25+18)
RegexpMatchMedium_1K-8     36.1µs ± 1%    34.6µs ± 1%  -4.02%  (p=0.000 n=25+25)
RegexpMatchHard_32-8       1.69µs ± 2%    1.65µs ± 1%  -2.38%  (p=0.000 n=25+25)
RegexpMatchHard_1K-8       50.8µs ± 1%    49.4µs ± 1%  -2.69%  (p=0.000 n=25+24)
Revcomp-8                   453ms ± 2%     449ms ± 3%  -0.74%  (p=0.022 n=25+24)
Template-8                 63.2ms ± 2%    63.4ms ± 1%    ~     (p=0.127 n=25+24)
TimeParse-8                 313ns ± 1%     315ns ± 3%    ~     (p=0.924 n=24+25)
TimeFormat-8                294ns ± 1%     292ns ± 2%  -0.65%  (p=0.004 n=23+24)
[Geo mean]                 49.9µs         49.6µs       -0.65%

name                     old speed      new speed      delta
GobDecode-8               112MB/s ± 1%   110MB/s ± 4%  -1.00%  (p=0.036 n=24+24)
GobEncode-8               130MB/s ± 1%   129MB/s ± 1%    ~     (p=0.894 n=25+24)
Gzip-8                   72.7MB/s ± 2%  73.0MB/s ± 2%    ~     (p=0.208 n=25+24)
Gunzip-8                  549MB/s ± 1%   545MB/s ± 1%  -0.70%  (p=0.000 n=25+25)
JSONEncode-8              201MB/s ± 1%   198MB/s ± 3%  -1.29%  (p=0.000 n=25+25)
JSONDecode-8             40.7MB/s ± 1%  39.8MB/s ± 5%  -2.23%  (p=0.000 n=25+25)
GoParse-8                16.2MB/s ± 4%  16.3MB/s ± 1%    ~     (p=0.211 n=25+24)
RegexpMatchEasy0_32-8     354MB/s ± 1%   356MB/s ± 1%  +0.47%  (p=0.000 n=25+21)
RegexpMatchEasy0_1K-8    5.00GB/s ± 0%  4.99GB/s ± 1%    ~     (p=0.588 n=24+24)
RegexpMatchEasy1_32-8     377MB/s ± 1%   378MB/s ± 1%  +0.39%  (p=0.000 n=25+25)
RegexpMatchEasy1_1K-8    2.92GB/s ± 1%  2.94GB/s ± 3%  +0.65%  (p=0.008 n=25+25)
RegexpMatchMedium_32-8   8.14MB/s ± 1%  8.22MB/s ± 1%  +0.98%  (p=0.000 n=25+24)
RegexpMatchMedium_1K-8   28.4MB/s ± 1%  29.6MB/s ± 1%  +4.19%  (p=0.000 n=25+25)
RegexpMatchHard_32-8     18.9MB/s ± 2%  19.4MB/s ± 1%  +2.43%  (p=0.000 n=25+25)
RegexpMatchHard_1K-8     20.2MB/s ± 1%  20.7MB/s ± 1%  +2.76%  (p=0.000 n=25+24)
Revcomp-8                 561MB/s ± 2%   566MB/s ± 3%  +0.75%  (p=0.021 n=25+24)
Template-8               30.7MB/s ± 2%  30.6MB/s ± 1%    ~     (p=0.131 n=25+24)
[Geo mean]                120MB/s        121MB/s       +0.48%

https://perf.golang.org/search?q=upload:20181004.6

Change-Id: I97f9fee34577961a116a8ddd445c6272253f0f95
Reviewed-on: https://go-review.googlesource.com/c/139837
Run-TryBot: Michael Knyszek <mknyszek@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Austin Clements <austin@google.com>