The rwlock_instances
Table
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:
NAME
The instrument name associated with the lock.
OBJECT_INSTANCE_BEGIN
The address in memory of the lock that was instrumented.
WRITE_LOCKED_BY_THREAD_ID
When a thread currently has an
rwlock
locked in exclusive (write) mode,WRITE_LOCKED_BY_THREAD_ID
is theTHREAD_ID
of the locking thread, otherwise it isNULL
.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 anrwlock
, and see how many readers are currently active.
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 whatrwlock
a thread is waiting forrwlock_instances
, to see which other thread currently owns anrwlock
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.