Commits on Apr 5, 2021

1. *: Sample some first iter reads based on size read …

   PR cockroachdb#1098 stopped the first iteration of the read sampling
   loop in a newly-created iterator from sampling. While
   this had the desired effect on Cockroach's benchmarks,
   this had a pretty significant regression on Pebble's own
   YCSB benchamrks, as a lot of them have very short-lived
   iterators.
   
   This change tries to find a middle ground by sampling some
   of the first reads, based on how many bytes were read. H/t
   to Sumeer for coming up with this idea.
   
   Another change is to move `1 << 4` off of the readBytesPeriod
   onto the option, allowing us to control read sampling in
   both directions by tweaking the option. Previously, read
   sampling could only be made less frequent or disabled,
   but not made more frequent.
   
   Pebble ycsb benchmarks (old = before this change):
   
   ```
   name                old ops/sec  new ops/sec  delta
   ycsb/A/values=1024   80.6k ± 1%   80.2k ± 2%      ~     (p=0.421 n=5+5)
   ycsb/B/values=1024    393k ± 5%    383k ±12%      ~     (p=0.548 n=5+5)
   ycsb/C/values=1024    675k ± 5%    918k ±12%   +35.87%  (p=0.008 n=5+5)
   ycsb/D/values=1024    217k ± 4%    208k ±13%      ~     (p=0.151 n=5+5)
   ycsb/E/values=1024   65.5k ± 4%   62.9k ±14%      ~     (p=0.310 n=5+5)
   ycsb/F/values=1024   26.9k ± 2%   26.8k ± 2%      ~     (p=0.841 n=5+5)
   ycsb/A/values=64      816k ± 6%    840k ± 5%      ~     (p=0.310 n=5+5)
   ycsb/B/values=64      962k ± 0%   1138k ± 7%   +18.25%  (p=0.016 n=4+5)
   ycsb/C/values=64     1.38M ± 6%   2.17M ±27%   +57.57%  (p=0.008 n=5+5)
   ycsb/D/values=64      835k ± 0%    936k ± 7%   +12.08%  (p=0.016 n=4+5)
   ycsb/E/values=64      205k ± 1%    216k ± 5%    +5.31%  (p=0.032 n=4+5)
   ycsb/F/values=64      239k ± 1%    239k ± 1%      ~     (p=0.421 n=5+5)
   
   name                old read     new read     delta
   ycsb/A/values=1024    170G ± 0%    170G ± 1%      ~     (p=0.421 n=5+5)
   ycsb/B/values=1024    118G ± 3%    119G ± 6%      ~     (p=0.548 n=5+5)
   ycsb/C/values=1024   46.1G ± 5%   57.9G ± 4%   +25.68%  (p=0.008 n=5+5)
   ycsb/D/values=1024    131G ± 4%    129G ±11%      ~     (p=0.690 n=5+5)
   ycsb/E/values=1024   64.7G ± 3%   64.2G ± 5%      ~     (p=1.000 n=5+5)
   ycsb/F/values=1024    169G ± 1%    169G ± 0%      ~     (p=1.000 n=5+5)
   ycsb/A/values=64     69.6G ± 0%   71.2G ± 4%      ~     (p=0.730 n=4+5)
   ycsb/B/values=64     12.5G ± 1%   39.5G ± 5%  +216.43%  (p=0.016 n=4+5)
   ycsb/C/values=64     1.53G ±18%   2.82G ± 5%   +84.57%  (p=0.008 n=5+5)
   ycsb/D/values=64     28.6G ± 1%   39.8G ± 9%   +39.37%  (p=0.016 n=4+5)
   ycsb/E/values=64     15.8G ±13%   15.8G ± 8%      ~     (p=0.841 n=5+5)
   ycsb/F/values=64      162G ± 1%    163G ± 1%      ~     (p=0.690 n=5+5)
   
   name                old write    new write    delta
   ycsb/A/values=1024    199G ± 0%    198G ± 1%      ~     (p=0.310 n=5+5)
   ycsb/B/values=1024    133G ± 3%    134G ± 7%      ~     (p=0.548 n=5+5)
   ycsb/C/values=1024   46.1G ± 5%   57.9G ± 4%   +25.68%  (p=0.008 n=5+5)
   ycsb/D/values=1024    145G ± 4%    142G ±11%      ~     (p=0.690 n=5+5)
   ycsb/E/values=1024   68.9G ± 3%   68.2G ± 6%      ~     (p=1.000 n=5+5)
   ycsb/F/values=1024    203G ± 1%    203G ± 1%      ~     (p=1.000 n=5+5)
   ycsb/A/values=64      100G ± 0%    103G ± 4%      ~     (p=0.190 n=4+5)
   ycsb/B/values=64     16.2G ± 1%   43.7G ± 4%  +169.23%  (p=0.016 n=4+5)
   ycsb/C/values=64     1.48G ±18%   2.73G ± 5%   +84.28%  (p=0.008 n=5+5)
   ycsb/D/values=64     33.7G ± 0%   45.6G ± 8%   +35.19%  (p=0.016 n=4+5)
   ycsb/E/values=64     17.0G ±13%   17.1G ± 7%      ~     (p=0.690 n=5+5)
   ycsb/F/values=64      192G ± 1%    193G ± 1%      ~     (p=0.690 n=5+5)
   
   name                old r-amp    new r-amp    delta
   ycsb/A/values=1024    14.4 ± 5%    15.1 ±11%      ~     (p=0.421 n=5+5)
   ycsb/B/values=1024    7.13 ± 1%    7.07 ± 2%      ~     (p=0.841 n=5+5)
   ycsb/C/values=1024    4.32 ±13%    2.77 ± 3%   -35.80%  (p=0.008 n=5+5)
   ycsb/D/values=1024    8.81 ± 2%    9.01 ± 4%      ~     (p=0.310 n=5+5)
   ycsb/E/values=1024    13.1 ± 3%    13.5 ± 7%      ~     (p=0.690 n=5+5)
   ycsb/F/values=1024    0.00         0.00           ~     (all equal)
   ycsb/A/values=64      6.27 ± 3%    6.43 ± 2%      ~     (p=0.230 n=5+5)
   ycsb/B/values=64      4.20 ± 0%    3.73 ± 1%   -11.14%  (p=0.008 n=5+5)
   ycsb/C/values=64      3.09 ± 1%    2.13 ± 1%   -31.06%  (p=0.000 n=5+4)
   ycsb/D/values=64      4.44 ± 1%    4.17 ± 1%    -6.08%  (p=0.008 n=5+5)
   ycsb/E/values=64      3.26 ± 4%    3.21 ± 2%      ~     (p=0.135 n=5+5)
   ycsb/F/values=64      0.00         0.00           ~     (all equal)
   
   name                old w-amp    new w-amp    delta
   ycsb/A/values=1024    7.58 ± 1%    7.57 ± 2%      ~     (p=0.413 n=5+5)
   ycsb/B/values=1024    10.4 ± 1%    10.7 ± 6%      ~     (p=0.198 n=5+5)
   ycsb/C/values=1024    0.00         0.00           ~     (all equal)
   ycsb/D/values=1024    20.6 ± 1%    21.0 ± 4%      ~     (p=0.151 n=5+5)
   ycsb/E/values=1024    32.3 ± 3%    33.5 ± 9%      ~     (p=0.421 n=5+5)
   ycsb/F/values=1024    11.6 ± 2%    11.7 ± 2%      ~     (p=0.651 n=5+5)
   ycsb/A/values=64      3.34 ± 1%    3.29 ± 0%    -1.57%  (p=0.016 n=5+4)
   ycsb/B/values=64      4.57 ± 1%   10.41 ±10%  +127.98%  (p=0.008 n=5+5)
   ycsb/C/values=64      0.00         0.00           ~     (all equal)
   ycsb/D/values=64      10.9 ± 1%    13.1 ± 3%   +20.40%  (p=0.008 n=5+5)
   ycsb/E/values=64      21.9 ± 8%    21.4 ± 5%      ~     (p=0.690 n=5+5)
   ycsb/F/values=64      10.9 ± 0%    10.9 ± 1%      ~     (p=0.524 n=5+5)
   ```

   

   itsbilal committed Apr 5, 2021
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