17.8.4 Configuring Thread Concurrency for InnoDB (original) (raw)

17.8.4 Configuring Thread Concurrency for InnoDB

InnoDB uses operating systemthreads to process requests from user transactions. (Transactions may issue many requests toInnoDB 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.

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. 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 parameterinnodb_thread_concurrency. Once the number of executing threads reaches this limit, additional threads sleep for a number of microseconds, set by the configuration parameterinnodb_thread_sleep_delay, before being placed into the queue.

You can set the configuration optioninnodb_adaptive_max_sleep_delay to the highest value you would allow forinnodb_thread_sleep_delay, andInnoDB automatically adjustsinnodb_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 forinnodb_thread_concurrency and the implied default limit on the number of concurrent threads has been changed in various releases of MySQL andInnoDB. The default value ofinnodb_thread_concurrency is0, 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 innodb_thread_concurrency is 0, the value ofinnodb_thread_sleep_delay is ignored.

When there is a limit on the number of threads (wheninnodb_thread_concurrency is > 0), InnoDB reduces context switching overhead by permitting multiple requests made during the execution of a_single SQL statement_ to enterInnoDB without observing the limit set byinnodb_thread_concurrency. Since an SQL statement (such as a join) may comprise multiple row operations within InnoDB,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 observeinnodb_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, andinnodb_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 optioninnodb_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 asinnodb_adaptive_hash_index.

For general performance information about MySQL thread handling, see Section 7.1.12.1, “Connection Interfaces”.