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26.12.3.3 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 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.
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 theTHREAD_ID
of 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_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 the
mutex_instances
table. TheOBJECT_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 theEVENT_NAME
column), and indicating which mutex is waited on (in theOBJECT_INSTANCE_BEGIN
column).When a thread succeeds in locking a mutex:
events_waits_current
shows that the wait on the mutex is completed (in theTIMER_END
andTIMER_WAIT
columns)The completed wait event is added to the
events_waits_history
andevents_waits_history_long
tablesmutex_instances
shows that the mutex is now owned by the thread (in theTHREAD_ID
column).
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 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
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Document créé le 26/06/2006, dernière modification le 26/10/2018
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