26.12.3.3 The mutex_instances Table

Table des matières

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 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

Rechercher dans le manuel MySQL

Traduction non disponible

Le manuel MySQL n'est pas encore traduit en français sur l'infobrol. Seule la version anglaise est disponible pour l'instant.

Document créé le 26/06/2006, dernière modification le 26/10/2018
Source du document imprimé : https://www.gaudry.be/mysql-rf-performance-schema-mutex-instances-table.html

L'infobrol est un site personnel dont le contenu n'engage que moi. Le texte est mis à disposition sous licence CreativeCommons(BY-NC-SA). Plus d'info sur les conditions d'utilisation et sur l'auteur.

Références

Manuel MySQL : https://dev.mysql.com/

Ces références et liens indiquent des documents consultés lors de la rédaction de cette page, ou qui peuvent apporter un complément d'information, mais les auteurs de ces sources ne peuvent être tenus responsables du contenu de cette page.
L'auteur de ce site est seul responsable de la manière dont sont présentés ici les différents concepts, et des libertés qui sont prises avec les ouvrages de référence. N'oubliez pas que vous devez croiser les informations de sources multiples afin de diminuer les risques d'erreurs.

Table des matières Haut