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 will compete against each other, so that the first query to obtain a lock on the mutex will cause 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.
mutex_instances table has
The instrument name associated with the mutex.
The address in memory of the mutex that was instrumented.
When a thread currently has a mutex locked,
LOCKED_BY_THREAD_ID is the
THREAD_ID of the locking thread,
otherwise it is
For every mutex instrumented in the code, the Performance Schema provides the following information.
lists the name of the instrumentation point, with the
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
shows a row for that thread, indicating that it is waiting
on a mutex (in the
and indicating which mutex is waited on (in the
When a thread succeeds in locking a mutex:
When a thread unlocks a mutex,
mutex_instances shows that
the mutex now has no owner (the
THREAD_ID column is
When a mutex object is destroyed, the corresponding row is
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: