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  1. MariaDB Server
  2. MDEV-32340

Improve performance of my_uca_level_booster_simple_prefix_cmp




      The patch for MDEV-27266 introduced a new function my_uca_level_booster_simple_prefix_cmp(). It calculates the length of a simple prefix.
      It efficiently skips equal prefixes consisting of simple mapping when one or two bytes produce one or two weights.

      Whenever it comes to a position with non-simple mapping or with non-equal weight, it breaks and returns the length of the equal prefix.
      The caller function after that initializes two instances of my_uca_scanner and goes to a less efficient comparison loop.

      In cases when my_uca_level_booster_simple_prefix_cmp() returns because of a non-simple mapping found, going to the slower loop is OK.

      But in cases when my_uca_level_booster_simple_prefix_cmp() breaks at some position because:

      • both strings have simple mappings at this position
      • but weights are different
        going to the slower loop is not really necessary. The weights can be compared right inside my_uca_level_booster_simple_prefix_cmp().

      Let's do the following:
      1. Rename the function and change its signature from

      static size_t
      my_uca_level_booster_equal_prefix_length(const MY_UCA_LEVEL_BOOSTER *booster,
                                               const uchar *s, size_t slen,
                                               const uchar *t, size_t tlen)


      static int
      my_uca_level_booster_simple_prefix_cmp(my_uca_scanner *sscanner,
                                             my_uca_scanner *tscanner,
                                             const MY_UCA_LEVEL_BOOSTER *booster)

      so the function returns the difference between two strings as follows:

      • a negative number if the string pointed by sscanner appeared to be slower
      • a positive number if the string pointed by sscanner appeared to be greater
      • 0 if a complex mapping was found

      The caller code will change as follows:

      • The two my_uca_scanner instances will now be initialized before calling my_uca_level_booster_simple_prefix_cmp()
      • On a non-zero value returned from my_uca_level_booster_simple_prefix_cmp(), the caller will exit
      • On zero returned my_uca_level_booster_simple_prefix_cmp() the caller will continue with a slower loop

      The attached patch MDEV-32340.v07.diff is a working prototype.


      Micro-benchmarking can be done using this SQL script:

      -- Warning up
      SET NAMES utf8mb4 COLLATE utf8mb4_uca1400_ai_ci;
      DO BENCHMARK(10000000,strcmp('xxxx','xxxx'));
      -- Benchmarking
      DO BENCHMARK(10000000,strcmp('',''));
      DO BENCHMARK(10000000,strcmp('a','a'));
      DO BENCHMARK(10000000,strcmp('aa','aa'));
      DO BENCHMARK(10000000,strcmp('aaa','aaa'));
      DO BENCHMARK(10000000,strcmp('aaaa','aaaa'));
      DO BENCHMARK(10000000,strcmp('aaaaa','aaaaa'));
      DO BENCHMARK(10000000,strcmp('aaaaaa','aaaaaa'));
      DO BENCHMARK(10000000,strcmp('aaaaaaa','aaaaaaa'));
      DO BENCHMARK(10000000,strcmp('aaaaaaaa','aaaaaaaa'));
      DO BENCHMARK(10000000,strcmp('aaaaaaaaa','aaaaaaaaa'));
      DO BENCHMARK(10000000,strcmp('я','я'));
      DO BENCHMARK(10000000,strcmp('яя','яя'));
      DO BENCHMARK(10000000,strcmp('яяя','яяя'));
      DO BENCHMARK(10000000,strcmp('яяяя','яяяя'));
      DO BENCHMARK(10000000,strcmp('яяяяя','яяяяя'));
      DO BENCHMARK(10000000,strcmp('ss','ß'));
      DO BENCHMARK(10000000,strcmp('ssss','ßß'));
      DO BENCHMARK(10000000,strcmp('ssssss','ßßß'));
      DO BENCHMARK(10000000,strcmp('ắ','ắ'));
      DO BENCHMARK(10000000,strcmp('ắắ','ắắ'));
      DO BENCHMARK(10000000,strcmp('ắắắ','ắắắ'));
      DO BENCHMARK(10000000,strcmp('ắắắắ','ắắắắ'));
      DO BENCHMARK(10000000,strcmp('ắắắắắ','ắắắắắ'));
      DO BENCHMARK(10000000,strcmp('👍','👍'));
      DO BENCHMARK(10000000,strcmp('👍👍','👍👍'));
      DO BENCHMARK(10000000,strcmp('👍👍👍','👍👍👍'));
      DO BENCHMARK(10000000,strcmp('👍👍👍👍','👍👍👍👍'));
      DO BENCHMARK(10000000,strcmp('👍👍👍👍👍','👍👍👍👍👍'));

      Benchmarking shows a very good performance improvement on 1-byte and 2-byte characters with insignificant slowdown on 3-byte and 4-byte characters:

      • Column#1 - the function passed to BENCHMARK()
      • Column#2 - time spent before the change
      • Column#3 - time spent after the change

      # 1-byte characters: Notable improvement
      strcmp('','')                      0.175  0.103
      strcmp('a','a')                    0.233  0.116
      strcmp('aa','aa')                  0.180  0.114
      strcmp('aaa','aaa')                0.238  0.122
      strcmp('aaaa','aaaa')              0.192  0.127
      strcmp('aaaaa','aaaaa')            0.256  0.137
      strcmp('aaaaaa','aaaaaa')          0.207  0.140
      strcmp('aaaaaaa','aaaaaaa')        0.271  0.147 
      strcmp('aaaaaaaa','aaaaaaaa')      0.220  0.152 
      strcmp('aaaaaaaaa','aaaaaaaaa')    0.295  0.162
      # 2-byte characters: Notable improvement
      strcmp('я','я')                    0.184 0.115
      strcmp('яя','яя')                  0.193 0.127
      strcmp('яяя','яяя')                0.208 0.141
      strcmp('яяяя','яяяя')              0.219 0.154
      strcmp('яяяяя','яяяяя')            0.231 0.167
      # Two 1-byte characters vs 2-byte character with expansion: Notable improvement
      strcmp('ss','ß')                   0.185 0.116
      strcmp('ssss','ßß')                0.199 0.127
      strcmp('ssssss','ßßß')             0.208 0.141
      -- 3-byte characters: Insignificant slowdown
      strcmp('ắ','ắ')                    0.306 0.317
      strcmp('ắắ','ắắ')                  0.433 0.450
      strcmp('ắắắ','ắắắ')                0.571 0.582
      strcmp('ắắắắ','ắắắắ')              0.698 0.711
      strcmp('ắắắắắ','ắắắắắ')            0.822 0.839
      -- 4-byte characters: Insignificant slowdown
      strcmp('👍','👍')                  0.331 0.342
      strcmp('👍👍','👍👍')              0.479 0.501
      strcmp('👍👍👍','👍👍👍')          0.639 0.649
      strcmp('👍👍👍👍','👍👍👍👍')      0.787 0.801
      strcmp('👍👍👍👍👍','👍👍👍👍👍')  1.043 1.053


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              Unassigned Unassigned
              bar Alexander Barkov
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