14.8.5 Configuring Thread Concurrency for InnoDB
InnoDB uses operating system threads to process requests from user transactions. (Transactions may issue many requests to
InnoDB before they commit or roll back.) On modern operating systems and servers with multi-core processors, where context switching is efficient, most workloads run well without any limit on the number of concurrent threads. Scalability improvements in MySQL 5.5 and up reduce the need to limit the number of concurrently executing threads inside
In situations where it is helpful to minimize context switching between threads,
InnoDB can use a number of techniques to limit the number of concurrently executing operating system threads (and thus the number of requests that are processed at any one time). When
InnoDB receives a new request from a user session, if the number of threads concurrently executing is at a pre-defined limit, the new request sleeps for a short time before it tries again. A request that cannot be rescheduled after the sleep is put in a first-in/first-out queue and eventually is processed. Threads waiting for locks are not counted in the number of concurrently executing threads.
You can limit the number of concurrent threads by setting the configuration parameter
innodb_thread_concurrency. Once the number of executing threads reaches this limit, additional threads sleep for a number of microseconds, set by the configuration parameter
innodb_thread_sleep_delay, before being placed into the queue.
Previously, it required experimentation to find the optimal value for
innodb_thread_sleep_delay, and the optimal value could change depending on the workload. In MySQL 5.6.3 and higher, you can set the configuration option
innodb_adaptive_max_sleep_delay to the highest value you would allow for
InnoDB automatically adjusts
innodb_thread_sleep_delay up or down depending on the current thread-scheduling activity. This dynamic adjustment helps the thread scheduling mechanism to work smoothly during times when the system is lightly loaded and when it is operating near full capacity.
The default value for
innodb_thread_concurrency and the implied default limit on the number of concurrent threads has been changed in various releases of MySQL and
InnoDB. The default value of
0, so that by default there is no limit on the number of concurrently executing threads.
InnoDB causes threads to sleep only when the number of concurrent threads is limited. When there is no limit on the number of threads, all contend equally to be scheduled. That is, if
0, the value of
innodb_thread_sleep_delay is ignored.
When there is a limit on the number of threads (when
innodb_thread_concurrency is > 0),
InnoDB reduces context switching overhead by permitting multiple requests made during the execution of a single SQL statement to enter
InnoDB without observing the limit set by
innodb_thread_concurrency. Since an SQL statement (such as a join) may comprise multiple row operations within
InnoDB assigns a specified number of “tickets” that allow a thread to be scheduled repeatedly with minimal overhead.
When a new SQL statement starts, a thread has no tickets, and it must observe
innodb_thread_concurrency. Once the thread is entitled to enter
InnoDB, it is assigned a number of tickets that it can use for subsequently entering
InnoDB to perform row operations. If the tickets run out, the thread is evicted, and
innodb_thread_concurrency is observed again which may place the thread back into the first-in/first-out queue of waiting threads. When the thread is once again entitled to enter
InnoDB, tickets are assigned again. The number of tickets assigned is specified by the global option
innodb_concurrency_tickets, which is 5000 by default. A thread that is waiting for a lock is given one ticket once the lock becomes available.
The correct values of these variables depend on your environment and workload. Try a range of different values to determine what value works for your applications. Before limiting the number of concurrently executing threads, review configuration options that may improve the performance of
InnoDB on multi-core and multi-processor computers, such as
For general performance information about MySQL thread handling, see Section 220.127.116.11, “Connection Interfaces”.