1.4.1 Scalability Improvements  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.