13.7.7.2. Faster Locking for Improved Scalability

In MySQL and InnoDB, multiple threads of execution access shared data structures. InnoDB synchronizes these accesses with its own implementation of mutexes and read/write locks. InnoDB has historically protected the internal state of a read/write lock with an InnoDB mutex. On Unix and Linux platforms, the internal state of an InnoDB mutex is protected by a Pthreads mutex, as in IEEE Std 1003.1c (POSIX.1c).

On many platforms, there is a more efficient way to implement mutexes and read/write locks. Atomic operations can often be used to synchronize the actions of multiple threads more efficiently than Pthreads. Each operation to acquire or release a lock can be done in fewer CPU instructions, and thus result in less wasted time when threads are contending for access to shared data structures. This in turn means greater scalability on multi-core platforms.

Beginning with InnoDB 1.0.3, InnoDB implements mutexes and read/write locks with the built-in functions provided by the GNU Compiler Collection (GCC) for atomic memory access instead of using the Pthreads approach previously used. More specifically, an InnoDB that is compiled with GCC version 4.1.2 or later uses the atomic builtins instead of a pthread_mutex_t to implement InnoDB mutexes and read/write locks.

On 32-bit Microsoft Windows, InnoDB has implemented mutexes (but not read/write locks) with hand-written assembler instructions. Beginning with Microsoft Windows 2000, it is possible to use functions for Interlocked Variable Access that are similar to the built-in functions provided by GCC. Beginning with InnoDB 1.0.4, InnoDB makes use of the Interlocked functions on Windows. Unlike the old hand-written assembler code, the new implementation supports read/write locks and 64-bit platforms.

Solaris 10 introduced library functions for atomic operations. Beginning with InnoDB 1.0.4, when InnoDB is compiled on Solaris 10 with a compiler that does not support the built-in functions provided by the GNU Compiler Collection (GCC) for atomic memory access, the library functions are used.

This change improves the scalability of InnoDB on multi-core systems. Note that the user does not have to set any particular parameter or option to take advantage of this new feature. This feature is enabled out-of-the-box on the platforms where it is supported. On platforms where the GCC, Windows, or Solaris functions for atomic memory access are not available, InnoDB uses the traditional Pthreads method of implementing mutexes and read/write locks.

When MySQL starts, InnoDB writes a message to the log file indicating whether atomic memory access is used for mutexes, for mutexes and read/write locks, or neither. If suitable tools are used to build InnoDB and the target CPU supports the atomic operations required, InnoDB uses the built-in functions for mutexing. If, in addition, the compare-and-swap operation can be used on thread identifiers (pthread_t), then InnoDB uses the instructions for read-write locks as well.

Note: If you are building from source, ensure that the build process properly takes advantage of your platform capabilities. If the build is not able to automatically use instructions for atomic memory access where available, consult this Support Tip on the InnoDB website for additional steps.

For more information about the performance implications of locking, see Section 7.10, “Optimizing Locking Operations”.

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