The rwlock_instances table lists all the rwlock instances (read write locks) seen by the Performance Schema while the server executes. An rwlock is a synchronization mechanism used in the code to enforce that threads at a given time can have access to some common resource following certain rules. The resource is said to be “protected” by the rwlock. The access is either shared (many threads can have a read lock at the same time) or exclusive (only one thread can have a write lock at a given time).

Depending on how many threads are requesting a lock, and the nature of the locks requested, access can be either granted in shared mode, granted in exclusive mode, or not granted at all, waiting for other threads to finish first.

The rwlock_instances table has these columns:

TRUNCATE TABLE is not permitted for the rwlock_instances table.

By performing queries on both of the following tables, a monitoring application or a DBA may detect some bottlenecks or deadlocks between threads that involve locks:

There is a limitation: The rwlock_instances can be used only to identify the thread holding a write lock, but not the threads holding a read lock.