MySQL 5.5 modifications improve performance on SMP systems to increase scalability on multi-core systems. The changes affect InnoDB locking and memory management.

MySQL 5.5 incorporates changes in InnoDB that improve the performance of RW-locks by using atomic CPU instructions (on platforms where they are available), rather than less scalable mutexes. It is also possible for InnoDB memory allocation to be disabled and replaced by the normal malloc library, or by a different library that implements malloc such as tcmalloc on Linux or mtalloc on Solaris.

The reimplementation of RW-locks requires atomic instructions. A status variable, Innodb_have_atomic_builtins, shows whether the server was built with atomic instructions.