There’s one bottleneck in MySQL/InnoDB that ultimately sucks. It sucked in 4.0, sucked in 5.0, sucks in 5.1 with newest InnoDB plugin. Opening tables has been a bottleneck on machines that have thousands of tables all the time (as LOCK_open is being held during the process), and while there was a table being opened, everything else would stall on the machine.
It can simply take hours on such systems just to open tables – and the major portion of time spent is randomly diving into InnoDB tables to populate index statistics. It obviously sounds like low hanging fruit – as statistics aren’t needed while you are opening a table, they’re needed just for querying the table.
So, I threw in few thousand tables to my machine, and tried opening them with ten connections. Standard InnoDB code was opening 13.5 tables a second. After spending few minutes and moving (this is pure prototype, not suitable for production) statistic collection post ha_innodb::open(), I noticed performance increase.
Tables were opened at 105-a-second speed. A bit better, ~8x better.
Merry Christmas, MySQL!