MariaDB XtraDB Performance Tuning
InnoDB in Percona Server 5.0.91 and MySQL 5.5.x has three options


innodb_read_io_threads
innodb_write_io_threads
innodb_io_capacity


These options were introduced in MySQL 5.1.38 InnoDB Plugin

Tuning these will engage more CPUs within InnoDB/XtraDB

I have discussed this in past posts in the DBA StackExchange along with other InnoDB tuning suggestions


Multi cores and MySQL Performance
About single threaded versus multithreaded databases performance
Possible to make MySQL use more than one core?
Query runs a long time in some newer MySQL versions


Click here for the XtraDB features that the InnoDB Plugin does not have.