Currently data and BSS root marking are each a single markroot job.
This makes them difficult to load balance, which can draw out mark
termination time if they are large.

Fix this by splitting both in to 256K chunks. While we're putting in
the infrastructure for dynamic roots, we also replace the fixed
sharding of the span roots with sharding in to fixed sizes. In
addition to helping balance root marking, this also paves the way to
parallelizing concurrent scan and to letting assists help with root
marking.

Updates #10345. This fixes the data and BSS aspects of that bug; it
does not partition scanning of large heap objects.

This has negligible effect on either the go1 benchmarks or the garbage
benchmark:

name              old time/op  new time/op  delta
XBenchGarbage-12  4.90ms ± 1%  4.91ms ± 2%   ~     (p=0.058 n=17+16)

name                      old time/op    new time/op    delta
BinaryTree17-12              3.11s ± 4%     3.12s ± 4%    ~     (p=0.512 n=20+20)
Fannkuch11-12                2.53s ± 2%     2.47s ± 2%  -2.28%  (p=0.000 n=20+18)
FmtFprintfEmpty-12          49.1ns ± 1%    50.0ns ± 4%  +1.68%  (p=0.008 n=18+20)
FmtFprintfString-12          170ns ± 0%     172ns ± 1%  +1.05%  (p=0.000 n=14+19)
FmtFprintfInt-12             174ns ± 1%     162ns ± 1%  -6.81%  (p=0.000 n=18+17)
FmtFprintfIntInt-12          284ns ± 1%     277ns ± 1%  -2.42%  (p=0.000 n=20+19)
FmtFprintfPrefixedInt-12     252ns ± 1%     244ns ± 1%  -2.84%  (p=0.000 n=18+20)
FmtFprintfFloat-12           317ns ± 0%     311ns ± 0%  -1.95%  (p=0.000 n=19+18)
FmtManyArgs-12              1.08µs ± 1%    1.11µs ± 1%  +3.43%  (p=0.000 n=18+19)
GobDecode-12                8.56ms ± 1%    8.61ms ± 1%  +0.50%  (p=0.020 n=20+20)
GobEncode-12                6.58ms ± 1%    6.57ms ± 1%    ~     (p=0.792 n=20+19)
Gzip-12                      317ms ± 3%     317ms ± 2%    ~     (p=0.840 n=19+19)
Gunzip-12                   41.6ms ± 0%    41.6ms ± 0%  +0.07%  (p=0.027 n=18+15)
HTTPClientServer-12         62.2µs ± 1%    62.3µs ± 1%    ~     (p=0.283 n=19+20)
JSONEncode-12               16.5ms ± 2%    16.5ms ± 1%    ~     (p=0.857 n=20+19)
JSONDecode-12               58.5ms ± 1%    61.3ms ± 1%  +4.67%  (p=0.000 n=18+17)
Mandelbrot200-12            3.84ms ± 0%    3.84ms ± 0%    ~     (p=0.259 n=17+17)
GoParse-12                  3.70ms ± 2%    3.74ms ± 2%  +0.96%  (p=0.009 n=19+20)
RegexpMatchEasy0_32-12       100ns ± 1%     100ns ± 0%  +0.31%  (p=0.040 n=19+15)
RegexpMatchEasy0_1K-12       340ns ± 1%     340ns ± 1%    ~     (p=0.411 n=17+19)
RegexpMatchEasy1_32-12      82.7ns ± 2%    82.3ns ± 1%    ~     (p=0.456 n=20+19)
RegexpMatchEasy1_1K-12       498ns ± 2%     495ns ± 0%    ~     (p=0.108 n=19+17)
RegexpMatchMedium_32-12      130ns ± 1%     130ns ± 2%    ~     (p=0.405 n=18+19)
RegexpMatchMedium_1K-12     39.4µs ± 2%    39.1µs ± 1%  -0.64%  (p=0.002 n=20+19)
RegexpMatchHard_32-12       2.03µs ± 2%    2.02µs ± 0%    ~     (p=0.561 n=20+17)
RegexpMatchHard_1K-12       61.1µs ± 2%    60.8µs ± 1%    ~     (p=0.615 n=19+18)
Revcomp-12                   532ms ± 2%     531ms ± 1%    ~     (p=0.470 n=19+19)
Template-12                 68.5ms ± 1%    69.1ms ± 1%  +0.87%  (p=0.000 n=17+17)
TimeParse-12                 344ns ± 2%     344ns ± 1%  +0.25%  (p=0.032 n=19+18)
TimeFormat-12                347ns ± 1%     362ns ± 1%  +4.27%  (p=0.000 n=17+19)
[Geo mean]                  62.3µs         62.3µs       -0.04%

name                      old speed      new speed      delta
GobDecode-12              89.6MB/s ± 1%  89.2MB/s ± 1%  -0.50%  (p=0.019 n=20+20)
GobEncode-12               117MB/s ± 1%   117MB/s ± 1%    ~     (p=0.797 n=20+19)
Gzip-12                   61.3MB/s ± 3%  61.2MB/s ± 2%    ~     (p=0.834 n=19+19)
Gunzip-12                  467MB/s ± 0%   466MB/s ± 0%  -0.07%  (p=0.027 n=18+15)
JSONEncode-12              117MB/s ± 2%   117MB/s ± 1%    ~     (p=0.851 n=20+19)
JSONDecode-12             33.2MB/s ± 1%  31.7MB/s ± 1%  -4.47%  (p=0.000 n=18+17)
GoParse-12                15.6MB/s ± 2%  15.5MB/s ± 2%  -0.95%  (p=0.008 n=19+20)
RegexpMatchEasy0_32-12     321MB/s ± 2%   320MB/s ± 1%  -0.57%  (p=0.002 n=17+17)
RegexpMatchEasy0_1K-12    3.01GB/s ± 1%  3.01GB/s ± 1%    ~     (p=0.132 n=17+18)
RegexpMatchEasy1_32-12     387MB/s ± 2%   389MB/s ± 1%    ~     (p=0.423 n=20+19)
RegexpMatchEasy1_1K-12    2.05GB/s ± 2%  2.06GB/s ± 0%    ~     (p=0.129 n=19+17)
RegexpMatchMedium_32-12   7.64MB/s ± 1%  7.66MB/s ± 1%    ~     (p=0.258 n=18+19)
RegexpMatchMedium_1K-12   26.0MB/s ± 2%  26.2MB/s ± 1%  +0.64%  (p=0.002 n=20+19)
RegexpMatchHard_32-12     15.7MB/s ± 2%  15.8MB/s ± 1%    ~     (p=0.510 n=20+17)
RegexpMatchHard_1K-12     16.8MB/s ± 2%  16.8MB/s ± 1%    ~     (p=0.603 n=19+18)
Revcomp-12                 477MB/s ± 2%   479MB/s ± 1%    ~     (p=0.470 n=19+19)
Template-12               28.3MB/s ± 1%  28.1MB/s ± 1%  -0.85%  (p=0.000 n=17+17)
[Geo mean]                 100MB/s        100MB/s       -0.26%

Change-Id: Ib0bfe0145675ce88c5a8791752f7486ac98805b4
Reviewed-on: https://go-review.googlesource.com/16043Reviewed-by: Rick Hudson <rlh@golang.org>