util: improve BufferPool, eliminate unexpected allocations

[qianbin]

Using sync.Pool to manage bare slices is not the proper way, since it'll bring unexpected allocations. This PR solves this problem with small tricks and makes BufferPool a bit faster.

benchmarks:

$ benchstat before.txt after.txt 
name          old time/op    new time/op    delta
BufferPool-8    55.9ns ± 0%    34.8ns ± 0%   -37.71%  (p=0.008 n=5+5)

name          old alloc/op   new alloc/op   delta
BufferPool-8     24.0B ± 0%      0.0B       -100.00%  (p=0.008 n=5+5)

name          old allocs/op  new allocs/op  delta
BufferPool-8      1.00 ± 0%      0.00       -100.00%  (p=0.008 n=5+5)

[qianbin]

The buffer pool is now mainly for reusing block read/write buffers. After #367 merged, the baseline does not work well as wish. Last two commits simplify BufferPool and highly increase the reuse rate of pooled buffers.

The raw benchmark output files: master.txt, pr.txt

Benchstat:

$ benchstat master.txt pr.txt 
name                        old time/op    new time/op    delta
DefaultBatchWrite-8           19.8ms ± 0%    19.8ms ± 0%     ~     (p=0.690 n=5+5)
FastAllocationBatchWrite-8    6.35ms ± 3%    6.35ms ± 2%     ~     (p=1.000 n=5+5)
DBWrite-8                     2.29µs ± 0%    2.29µs ± 0%     ~     (p=0.302 n=5+5)
DBWriteBatch-8                1.77µs ±10%    1.67µs ± 3%     ~     (p=0.548 n=5+5)
DBWriteUncompressed-8         2.57µs ± 8%    2.56µs ±13%     ~     (p=0.841 n=5+5)
DBWriteBatchUncompressed-8    2.21µs ±24%    1.98µs ±15%     ~     (p=0.548 n=5+5)
DBWriteRandom-8               6.14µs ±10%    5.72µs ±13%     ~     (p=0.310 n=5+5)
DBWriteRandomSync-8           8.42ms ±10%    7.58ms ±10%  -10.05%  (p=0.032 n=5+5)
DBOverwrite-8                 4.96µs ±15%    4.55µs ±10%     ~     (p=0.310 n=5+5)
DBOverwriteRandom-8           9.90µs ± 8%    9.36µs ± 6%     ~     (p=0.151 n=5+5)
DBPut-8                       2.45µs ± 9%    2.36µs ± 1%     ~     (p=1.000 n=5+5)
DBRead-8                       259ns ± 8%     246ns ± 5%     ~     (p=0.056 n=5+5)
DBReadGC-8                     272ns ± 0%     269ns ± 0%   -1.07%  (p=0.029 n=4+4)
DBReadUncompressed-8           242ns ± 0%     225ns ± 8%   -6.67%  (p=0.008 n=5+5)
DBReadTable-8                  226ns ± 0%     218ns ± 1%   -3.25%  (p=0.008 n=5+5)
DBReadReverse-8                345ns ± 0%     344ns ± 0%   -0.50%  (p=0.016 n=5+5)
DBReadReverseTable-8           345ns ± 0%     343ns ± 0%   -0.44%  (p=0.016 n=5+5)
DBSeek-8                      5.66µs ± 1%    5.67µs ± 1%     ~     (p=0.841 n=5+5)
DBSeekRandom-8                8.37µs ± 1%    8.28µs ± 1%   -1.01%  (p=0.032 n=5+5)
DBGet-8                       2.12µs ± 1%    2.16µs ± 0%   +1.64%  (p=0.029 n=4+4)
DBGetRandom-8                 5.48µs ± 2%    5.36µs ± 0%   -2.34%  (p=0.016 n=5+4)
DBReadConcurrent-8            77.2ns ± 2%    81.6ns ± 4%   +5.72%  (p=0.008 n=5+5)
DBReadConcurrent2-8           89.4ns ± 2%    90.6ns ± 2%     ~     (p=0.222 n=5+5)
GetOverlapLevel0-8            51.4ms ± 6%    52.4ms ± 3%     ~     (p=0.548 n=5+5)
GetOverlapNonLevel0-8          653µs ± 2%     669µs ± 1%     ~     (p=0.056 n=5+5)
VersionStagingNonTrivial-8    31.5ms ± 1%    31.4ms ± 0%     ~     (p=0.841 n=5+5)
VersionStagingTrivial-8       3.96ms ± 3%    3.90ms ± 2%     ~     (p=0.151 n=5+5)

name                        old alloc/op   new alloc/op   delta
DefaultBatchWrite-8           74.2MB ± 0%    74.2MB ± 0%     ~     (all equal)
FastAllocationBatchWrite-8    28.7MB ± 0%    28.7MB ± 0%     ~     (all equal)
DBWrite-8                      73.0B ± 0%     73.0B ± 0%     ~     (all equal)
DBWriteBatch-8                 13.6B ±10%     12.6B ±11%     ~     (p=0.111 n=5+5)
DBWriteUncompressed-8          73.2B ± 2%     72.0B ± 0%     ~     (p=0.167 n=5+5)
DBWriteBatchUncompressed-8     14.0B ±29%     11.4B ± 5%  -18.57%  (p=0.048 n=5+5)
DBWriteRandom-8                 160B ± 3%      138B ± 7%  -13.87%  (p=0.008 n=5+5)
DBWriteRandomSync-8             231B ± 6%      222B ±12%     ~     (p=0.341 n=5+5)
DBOverwrite-8                   125B ± 2%      106B ± 1%  -14.77%  (p=0.008 n=5+5)
DBOverwriteRandom-8             192B ± 5%      164B ± 6%  -14.88%  (p=0.008 n=5+5)
DBPut-8                        74.0B ± 0%     73.0B ± 0%   -1.35%  (p=0.008 n=5+5)
DBRead-8                       16.0B ± 0%     15.6B ± 4%     ~     (p=0.444 n=5+5)
DBReadGC-8                     16.0B ± 0%     16.0B ± 0%     ~     (all equal)
DBReadUncompressed-8           15.0B ± 0%     14.0B ± 0%   -6.67%  (p=0.000 n=5+4)
DBReadTable-8                  16.0B ± 0%     15.0B ± 0%   -6.25%  (p=0.000 n=5+4)
DBReadReverse-8                64.0B ± 0%     64.0B ± 0%     ~     (all equal)
DBReadReverseTable-8           64.0B ± 0%     64.0B ± 0%     ~     (all equal)
DBSeek-8                      1.76kB ± 0%    1.76kB ± 0%   -0.14%  (p=0.008 n=5+5)
DBSeekRandom-8                1.78kB ± 1%    1.75kB ± 0%   -1.49%  (p=0.008 n=5+5)
DBGet-8                         801B ± 1%      800B ± 1%     ~     (p=0.952 n=5+5)
DBGetRandom-8                 1.06kB ± 1%    1.02kB ± 0%   -3.21%  (p=0.000 n=5+4)
DBReadConcurrent-8             11.0B ± 0%     13.4B ± 4%  +21.82%  (p=0.016 n=4+5)
DBReadConcurrent2-8            35.0B ± 3%     35.0B ± 0%     ~     (p=1.000 n=5+5)
GetOverlapLevel0-8            5.09MB ±14%    5.15MB ±12%     ~     (p=1.000 n=5+5)
GetOverlapNonLevel0-8         1.01MB ± 2%    1.01MB ± 1%     ~     (p=1.000 n=5+5)
VersionStagingNonTrivial-8     805kB ± 0%     805kB ± 0%     ~     (p=0.841 n=5+5)
VersionStagingTrivial-8       1.21MB ± 1%    1.21MB ± 1%     ~     (p=0.690 n=5+5)

name                        old allocs/op  new allocs/op  delta
DefaultBatchWrite-8             53.0 ± 0%      53.0 ± 0%     ~     (all equal)
FastAllocationBatchWrite-8      42.0 ± 0%      42.0 ± 0%     ~     (all equal)
DBWrite-8                       3.00 ± 0%      3.00 ± 0%     ~     (all equal)
DBWriteBatch-8                  0.00           0.00          ~     (all equal)
DBWriteUncompressed-8           3.00 ± 0%      3.00 ± 0%     ~     (all equal)
DBWriteBatchUncompressed-8      0.00           0.00          ~     (all equal)
DBWriteRandom-8                 3.00 ± 0%      3.00 ± 0%     ~     (all equal)
DBWriteRandomSync-8             3.00 ± 0%      3.00 ± 0%     ~     (all equal)
DBOverwrite-8                   3.00 ± 0%      3.00 ± 0%     ~     (all equal)
DBOverwriteRandom-8             3.40 ±18%      3.00 ± 0%     ~     (p=0.444 n=5+5)
DBPut-8                         3.00 ± 0%      3.00 ± 0%     ~     (all equal)
DBRead-8                        0.00           0.00          ~     (all equal)
DBReadGC-8                      0.00           0.00          ~     (all equal)
DBReadUncompressed-8            0.00           0.00          ~     (all equal)
DBReadTable-8                   0.00           0.00          ~     (all equal)
DBReadReverse-8                 0.00           0.00          ~     (all equal)
DBReadReverseTable-8            0.00           0.00          ~     (all equal)
DBSeek-8                        21.0 ± 0%      21.0 ± 0%     ~     (all equal)
DBSeekRandom-8                  24.6 ± 2%      23.0 ± 0%   -6.50%  (p=0.008 n=5+5)
DBGet-8                         13.0 ± 0%      13.0 ± 0%     ~     (all equal)
DBGetRandom-8                   19.0 ± 0%      17.0 ± 0%  -10.53%  (p=0.000 n=5+4)
DBReadConcurrent-8              0.00           0.00          ~     (all equal)
DBReadConcurrent2-8             0.00           0.00          ~     (all equal)
GetOverlapLevel0-8              27.6 ± 2%      28.0 ± 0%     ~     (p=0.556 n=5+4)
GetOverlapNonLevel0-8           3.00 ± 0%      3.00 ± 0%     ~     (all equal)
VersionStagingNonTrivial-8      34.0 ± 0%      34.0 ± 0%     ~     (all equal)
VersionStagingTrivial-8         36.0 ± 0%      36.0 ± 0%     ~     (all equal)

name                        old speed      new speed      delta
DBWrite-8                   50.7MB/s ± 0%  50.6MB/s ± 0%     ~     (p=0.333 n=5+5)
DBWriteBatch-8              65.9MB/s ± 9%  69.5MB/s ± 3%     ~     (p=0.548 n=5+5)
DBWriteUncompressed-8       45.3MB/s ± 8%  45.8MB/s ±12%     ~     (p=0.841 n=5+5)
DBWriteBatchUncompressed-8  54.2MB/s ±24%  58.9MB/s ±14%     ~     (p=0.548 n=5+5)
DBWriteRandom-8             19.0MB/s ±10%  20.4MB/s ±12%     ~     (p=0.310 n=5+5)
DBWriteRandomSync-8         10.0kB/s ± 0%  16.0kB/s ±38%     ~     (p=0.238 n=4+5)
DBOverwrite-8               23.6MB/s ±15%  25.6MB/s ±10%     ~     (p=0.310 n=5+5)
DBOverwriteRandom-8         11.8MB/s ± 8%  12.4MB/s ± 6%     ~     (p=0.151 n=5+5)
DBPut-8                     47.6MB/s ± 8%  49.1MB/s ± 1%     ~     (p=1.000 n=5+5)
DBRead-8                     448MB/s ± 8%   472MB/s ± 5%     ~     (p=0.056 n=5+5)
DBReadGC-8                   427MB/s ± 0%   432MB/s ± 0%   +1.08%  (p=0.029 n=4+4)
DBReadUncompressed-8         480MB/s ± 0%   517MB/s ± 8%   +7.57%  (p=0.008 n=5+5)
DBReadTable-8                514MB/s ± 0%   532MB/s ± 1%   +3.36%  (p=0.008 n=5+5)
DBReadReverse-8              336MB/s ± 0%   338MB/s ± 0%   +0.50%  (p=0.016 n=5+5)
DBReadReverseTable-8         337MB/s ± 0%   338MB/s ± 0%   +0.44%  (p=0.016 n=5+5)
DBReadConcurrent-8          1.50GB/s ± 2%  1.42GB/s ± 4%   -5.39%  (p=0.008 n=5+5)
DBReadConcurrent2-8         1.30GB/s ± 2%  1.28GB/s ± 1%     ~     (p=0.222 n=5+5)

[tianchaijz]

Could you check this allocator? https://github.com/xtaci/smux/blob/master/alloc.go

[qianbin]

Could you check this allocator? https://github.com/xtaci/smux/blob/master/alloc.go

It's limited to 64KB?