MySQL 8.0 Release Notes
MySQL 8.0 Source Code Documentation
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:
NAMEThe instrument name associated with the mutex.
OBJECT_INSTANCE_BEGINThe address in memory of the instrumented mutex.
LOCKED_BY_THREAD_IDWhen a thread currently has a mutex locked,
LOCKED_BY_THREAD_IDis theTHREAD_IDof the locking thread, otherwise it isNULL.
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_instrumentstable lists the name of the instrumentation point, with the prefixwait/synch/mutex/.When some code creates a mutex, a row is added to the
mutex_instancestable. TheOBJECT_INSTANCE_BEGINcolumn is a property that uniquely identifies the mutex.When a thread attempts to lock a mutex, the
events_waits_currenttable shows a row for that thread, indicating that it is waiting on a mutex (in theEVENT_NAMEcolumn), and indicating which mutex is waited on (in theOBJECT_INSTANCE_BEGINcolumn).When a thread succeeds in locking a mutex:
events_waits_currentshows that the wait on the mutex is completed (in theTIMER_ENDandTIMER_WAITcolumns)The completed wait event is added to the
events_waits_historyandevents_waits_history_longtablesmutex_instancesshows that the mutex is now owned by the thread (in theTHREAD_IDcolumn).
When a thread unlocks a mutex,
mutex_instancesshows that the mutex now has no owner (theTHREAD_IDcolumn isNULL).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 formutex_instances, to see which other thread currently owns a mutex