Hi Bar, The purpose of this bit arithmetics (one could say CPU athletics) is to reduce the size of the InnoDB data dictionary cache. If we look at dtype_t and dict_col_t, these are not exactly at any byte boundary at the moment (61 and 84 bits, respectively).
I would simply replace the mbminmaxlen field with two 3-bit fields mbminlen, mbmaxlen. This would use up one more bit and would be faster to extract (no multiplication or division, just simple bit-shifting and masking). Also the code would be simpler, because a number of macros would be removed. I think that a similar patch was done in MySQL, but apparently it was never merged to the main trees. Marko On Thu, May 4, 2017 at 1:34 PM, Alexander Barkov <[email protected]> wrote: > Hello Marko, > > InnoDB uses 5 bits to store the combination of mbminlen and mbmaxlen, > and its calculated as: > > mbmaxlen * 5 + mbminlen > > > Note, combinations with mbminlen=0 and mbmaxlen=0 are not possible. > Also, combinations with mbminlen>mbmaxlen are not possible. > > > This is a summary: > > mbminlen > mbmaxlen 0 1 2 3 4 > 0 (0) (1) (2) (3) (4) > 1 (5) 6 (7) (8) (9) > 2 (10) 11 12 (13) (14) > 3 (15) 16 17 18 (19) > 4 (20) 21 22 23 24 > > > I marked impossible numbers with parentheses. > > In the range 0..24: > - 10 pairs are valid > - 15 pairs are not valid > > It should be safe to use any impossible number to mean > mbmaxlen=5 & mbminlen=1 > > It seems there is no really a need to extend the header to use > separate 3 bits for both mbminlen and mbmaxlen. > > -- Marko Mäkelä, Lead Developer InnoDB MariaDB Corporation
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