> For the complete documentation index, see [llms.txt](https://mariadb.com/docs/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://mariadb.com/docs/server/reference/system-tables/performance-schema/performance-schema-tables/performance-schema-mutex_instances-table.md).

# Performance Schema mutex\_instances Table

## Description

The `mutex_instances` table lists all mutexes that the Performance Schema seeing while the server is executing.

A mutex is a code mechanism for ensuring that threads can only access resources one at a time. A second thread attempting to access a resource will find it protected by a mutex, and will wait for it to be unlocked.

The [performance\_schema\_max\_mutex\_instances](/docs/server/reference/system-tables/performance-schema/performance-schema-system-variables.md#performance_schema_max_mutex_instances) system variable specifies the maximum number of instrumented mutex instances.

| Column                  | Description                                                                          |
| ----------------------- | ------------------------------------------------------------------------------------ |
| NAME                    | Instrument name associated with the mutex.                                           |
| OBJECT\_INSTANCE\_BEGIN | Memory address of the instrumented mutex.                                            |
| LOCKED\_BY\_THREAD\_ID  | The THREAD\_ID of the locking thread if a thread has a mutex locked, otherwise NULL. |

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