29.12.3.3 The mutex_instances Table

The mutex_instances table lists all the mutexes seen by the Performance Schema while the server executes. A mutex is a synchronization mechanism used in the code to enforce that only one thread at a given time can have access to some common resource. The resource is said to be “protected” by the mutex.

When two threads executing in the server (for example, two user sessions executing a query simultaneously) do need to access the same resource (a file, a buffer, or some piece of data), these two threads compete against each other, so that the first query to obtain a lock on the mutex causes the other query to wait until the first is done and unlocks the mutex.

The work performed while holding a mutex is said to be in a “critical section,” and multiple queries do execute this critical section in a serialized way (one at a time), which is a potential bottleneck.

The mutex_instances table has these columns:

• NAME

  The instrument name associated with the mutex.

• OBJECT_INSTANCE_BEGIN

  The address in memory of the instrumented mutex.

• LOCKED_BY_THREAD_ID

  When a thread currently has a mutex locked, LOCKED_BY_THREAD_ID is the THREAD_ID of the locking thread, otherwise it is NULL.

The mutex_instances table has these indexes:

• Primary key on (OBJECT_INSTANCE_BEGIN)

• Index on (NAME)

• Index on (LOCKED_BY_THREAD_ID)

TRUNCATE TABLE is not permitted for the mutex_instances table.

For every mutex instrumented in the code, the Performance Schema provides the following information.

• The setup_instruments table lists the name of the instrumentation point, with the prefix wait/synch/mutex/.

• When some code creates a mutex, a row is added to the mutex_instances table. The OBJECT_INSTANCE_BEGIN column is a property that uniquely identifies the mutex.

• When a thread attempts to lock a mutex, the events_waits_current table shows a row for that thread, indicating that it is waiting on a mutex (in the EVENT_NAME column), and indicating which mutex is waited on (in the OBJECT_INSTANCE_BEGIN column).

• When a thread succeeds in locking a mutex:

  • events_waits_current shows that the wait on the mutex is completed (in the TIMER_END and TIMER_WAIT columns)

  • The completed wait event is added to the events_waits_history and events_waits_history_long tables

  • mutex_instances shows that the mutex is now owned by the thread (in the THREAD_ID column).

• When a thread unlocks a mutex, mutex_instances shows that the mutex now has no owner (the THREAD_ID column is NULL).

• When a mutex object is destroyed, the corresponding row is removed from mutex_instances.

By performing queries on both of the following tables, a monitoring application or a DBA can detect bottlenecks or deadlocks between threads that involve mutexes:

• events_waits_current, to see what mutex a thread is waiting for

• mutex_instances, to see which other thread currently owns a mutex