I consider innodb_thread_concurrency a vestigial tail of the “built-in InnoDB” that ships by default with MySQL 5.0 or 5.1, and should generally be set to 0 with recent versions of XtraDB or the InnoDB Plugin.Can this be? I cannot wait for innodb_thread_concurrency to be made obsolete. I run a lot of CPU-bound benchmarks on 8 and 16 core servers and I always set it to 0 in my benchmark framework. A few times I have repeated tests with it set to a non-zero value to understand whether that helps and it has never helped. Alas, this will also make my FLIFO patch obsolete.
I agree with Baron that it should be set to 0 with the InnoDB plugin and XtraDB. This is a big deal that has not received enough attention. InnoDB and XtraDB have gotten much better at supporting highly-concurrent workloads on many-core servers. For me highly-concurrent means 100 to 1000 concurrent transactions and many-core means 8 and 16 core servers.
This is not an easy workload to support. MySQL is getting much better at it. A lot of work remains to be done. MySQL 5.5 has even more improvements and several problems have yet to be fixed in InnoDB. But this is a huge deal. Maybe we can have a going away party for innodb_thread_concurrency at the conference?