Documentation Home
MySQL 9.3 Reference Manual
Related Documentation Download this Manual
PDF (US Ltr) - 40.8Mb
PDF (A4) - 40.9Mb
Man Pages (TGZ) - 261.1Kb
Man Pages (Zip) - 368.3Kb
Info (Gzip) - 4.1Mb
Info (Zip) - 4.1Mb


29.12.3.3 The mutex_instances Table

Themutex_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.

Themutex_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 theTHREAD_ID of the locking thread, otherwise it isNULL.

Themutex_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 themutex_instances table.

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

  • Thesetup_instruments table lists the name of the instrumentation point, with the prefixwait/synch/mutex/.

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

  • When a thread attempts to lock a mutex, theevents_waits_current table shows a row for that thread, indicating that it is waiting on a mutex (in theEVENT_NAME column), and indicating which mutex is waited on (in theOBJECT_INSTANCE_BEGIN column).

  • When a thread succeeds in locking a mutex:

  • When a thread unlocks a mutex,mutex_instances shows that the mutex now has no owner (theTHREAD_ID column isNULL).

  • When a mutex object is destroyed, the corresponding row is removed frommutex_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: