10.3.4 The rwlock_instances Table

The rwlock_instances table lists all the rwlock (read write lock) instances 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), exclusive (only one thread can have a write lock at a given time), or shared-exclusive (a thread can have a write lock while permitting inconsistent reads by other threads). Shared-exclusive access is otherwise known as an sxlock and optimizes concurrency and improves scalability for read-write workloads.

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

The rwlock_instances table has these columns:

• NAME

  The instrument name associated with the lock.

• OBJECT_INSTANCE_BEGIN

  The address in memory of the instrumented lock.

• WRITE_LOCKED_BY_THREAD_ID

  When a thread currently has an rwlock locked in exclusive (write) mode, WRITE_LOCKED_BY_THREAD_ID is the THREAD_ID of the locking thread, otherwise it is NULL.

• READ_LOCKED_BY_COUNT

  When a thread currently has an rwlock locked in shared (read) mode, READ_LOCKED_BY_COUNT is incremented by 1. This is a counter only, so it cannot be used directly to find which thread holds a read lock, but it can be used to see whether there is a read contention on an rwlock, and see how many readers are currently active.

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:

• events_waits_current, to see what rwlock a thread is waiting for

• rwlock_instances, to see which other thread currently owns an rwlock

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.