Hyperthreading & MySQL InnoDB Thread Concurrency Performance
By default, innodb_thread_concurrency is 0. That's actually the best setting. It means infinite concurrency. It allows the InnoDB storage engine to decide the best number of concurrency tickets to launch and address. Setting it to a nonzero value actually can throttle InnoDB or throttle the OS if not set properly.

I have written many posts in the DBA StackExchange about this setting:


May 26, 2011 : About single threaded versus multithreaded databases performance
Aug 16, 2011 : Why do MySQL threads often show &quot;freeing items&quot; status when the query cache is disabled?
Sep 12, 2011 : Possible to make MySQL use more than one core?
Sep 20, 2011 : Multi cores and MySQL Performance
Apr 26, 2012 : Is the CPU performance relevant for a database server?
Aug 03, 2012 : Slow insert performance in mysql innodb table for solr indexing