InnoDB checksum performance

Once again Domas is unhappy with some aspect of Innodb performance and doing crazy things with gdb to tune it. I made it faster by changing the checksum code to process one 32-bit word at a time rather than one byte at a time. This will be in a future Google patch and is enabled with the parameter innodb_fast_checksum. This is not compatible with the old checksum so you must dump and reload the database to use it.

I measured the benefit using the insert benchmark from Tokutek on a server that can do a lot of IO. CPU overheads are measured using oprofile. The data below lists the percentage of time for the top 4 functions in mysqld. The checksum is computed in buf_calc_page_new_checksum. By using the fast checksum, the checksum overhead drops from 33.6% to 22.1% for gcc -O2 and from 31.6% to 17.3% for gcc -O3.

Overhead for gcc -O2

Using the original checksum code:

• 33.6% - buf_calc_page_new_checksum
• 10.4% - memcpy
• 4.4% - os_aio_simulated_handle
• 4.3% - rec_get_offsets_func
  

Using the fast checksum code:

• 22.1% - buf_calc_page_new_checksum
• 12.1% - memcpy
• 5.1% - rec_get_offsets_func
• 4.9% - os_aio_simulated_handle
  

Overhead for gcc -O3

Using the original checksum code:

• 31.6% - buf_calc_page_new_checksum
• 12.6% - memcpy
• 5.8% - rec_get_offsets_func
• 2.6 - os_aio_simulated_handle

Using the fast checksum code:

• 17.3% - buf_calc_page_new_checksum
• 13.6% - memcpy
• 6.8% - rec_get_offsets_func
• 2.0% - os_aio_simulated_handle