SHOW INNODB STATUS: SEMAPHORES

A summary from the mySQL Performace Blog

Innodb takes averages over time and displays its numbers which are hard to figure out what a true average is from some sort of polling method.


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SEMAPHORES
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OS WAIT ARRAY INFO: reservation count 28655958, signal count 25483315
Mutex spin waits 1468249294, rounds 5838311115, OS waits 20658052
RW-shared spins 4676021, OS waits 1740480; RW-excl spins 13056306, OS waits 2135090


• reservation count - counter for the sync array when slots are added to it


• signal count - counter for the sync array for when threads are signal for use


• OS Waits - used for exclusive and shared locks, and relates to context switching on the OS side.


• spin waits / rounds - internal innodb lock that is faster then an OS wait but wastes CPU cycles
  

Tuning

To help with context switching a new variable was added in the 5.0 tree called innodb_sync_spin_loops playing with this might reduce unneed context switches since it's used to ballance between CPU and spins.