cyb70289 commented on issue #6986:
URL: https://github.com/apache/arrow/pull/6986#issuecomment-616915079


   @pitrou @wesm 
   Oops, I only checked case "BitmapReader" from benchmark 
[arrow-bit-util-benchmark](https://github.com/apache/arrow/blob/master/cpp/src/arrow/util/bit_util_benchmark.cc).
 Obviously it's not enough.
   
   I compared all cases just now and see huge performance drop from below 4 
tests:
   
   Before this patch:
   ```bash
   BenchmarkBitmapAnd/32768/1                 563496 ns       563260 ns         
1243 bytes_per_second=55.4806M/s
   BenchmarkBitmapAnd/131072/1               2219810 ns      2218984 ns         
 318 bytes_per_second=56.3321M/s
   BenchmarkBitmapAnd/32768/2                 561738 ns       561467 ns         
1265 bytes_per_second=55.6577M/s
   BenchmarkBitmapAnd/131072/2               2246229 ns      2245119 ns         
 305 bytes_per_second=55.6763M/s
   ```
   
   After this patch:
   ```bash
   BenchmarkBitmapAnd/32768/1                1653467 ns      1652680 ns         
 422 bytes_per_second=18.9087M/s
   BenchmarkBitmapAnd/131072/1               6665501 ns      6661561 ns         
 105 bytes_per_second=18.7644M/s
   BenchmarkBitmapAnd/32768/2                1670793 ns      1670246 ns         
 423 bytes_per_second=18.7098M/s
   BenchmarkBitmapAnd/131072/2               6702369 ns      6698957 ns         
 103 bytes_per_second=18.6596M/s
   ```
   
   Before reverting this patch, I would like to understand why it happens.
   
   BTW: we definitely need continuous benchmark tools to detect these things 
early.


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