The rwlock_instances table lists all read write lock (rwlock) instances that the Performance Schema sees while the server is executing. A read write is a mechanism for ensuring threads can either share access to common resources, or have exclusive access.
The performance_schema_max_rwlock_instances system variable specifies the maximum number of instrumented rwlock objects.
The rwlock_instances table contains the following columns:
| Column | Description |
|---|---|
NAME |
Instrument name associated with the read write lock |
OBJECT_INSTANCE_BEGIN |
Address in memory of the instrumented lock |
WRITE_LOCKED_BY_THREAD_ID |
THREAD_ID of the locking thread if locked in write (exclusive) mode, otherwise NULL. |
READ_LOCKED_BY_COUNT |
Count of current read locks held |
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https://mariadb.com/kb/en/performance-schema-rwlock_instances-table/