Monty e600f9aebb MDEV-35750 Change MEM_ROOT allocation sizes to reduse calls to malloc() and avoid memory fragmentation ...

This commit updates default memory allocations size used with MEM_ROOT
objects to minimize the number of calls to malloc().

Changes:
- Updated MEM_ROOT block sizes in sql_const.h
- Updated MALLOC_OVERHEAD to also take into account the extra memory
  allocated by my_malloc()
- Updated init_alloc_root() to only take MALLOC_OVERHEAD into account as
  buffer size, not MALLOC_OVERHEAD + sizeof(USED_MEM).
- Reset mem_root->first_block_usage if and only if first block was used.
- Increase MEM_ROOT buffers sized used by my_load_defaults, plugin_init,
  Create_tmp_table, allocate_table_share, TABLE and TABLE_SHARE.
  This decreases number of malloc calls during queries.
- Use a small buffer for THD->main_mem_root in THD::THD. This avoids
  multiple malloc() call for new connections.

I tried the above changes on a complex select query with 12 tables.
The following shows the number of extra allocations that where used
to increase the size of the MEM_ROOT buffers.

Original code:
- Connection to MariaDB:   9 allocations
- First query run:       146 allocations
- Second query run:       24 allocations

Max memory allocated for thd when using with heap table:  61,262,408
Max memory allocated for thd when using Aria tmp table:      419,464

After changes:
Connection to MariaDB:     0 allocations
- First run:              25 allocations
- Second run:              7 allocations

Max memory allocated for thd when using with heap table:  61,347,424
Max memory allocated for thd when using Aria table:          529,168

The new code uses slightly more memory, but avoids memory fragmentation
and is slightly faster thanks to much fewer calls to malloc().

Reviewed-by: Sergei Golubchik <serg@mariadb.org>

2025-01-05 16:40:11 +02:00

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