maria-developers team mailing list archive

[Alexander Barkov <bar@xxxxxxxxxxx>]

Hi Marko,


On 05/04/2017 03:01 PM, Marko Mäkelä wrote:
> 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.

Thanks for clarification.

So it's about in-memory format. Then the things are even simpler.

For some reasons I thought that the problem is with disk format, which
is not desirable to change.


> 
> Marko
> 
> On Thu, May 4, 2017 at 1:34 PM, Alexander Barkov <bar@xxxxxxxxxxx
> <mailto:bar@xxxxxxxxxxx>> 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