Common wait Events in RAC[转]

http://space.itpub.net/?uid-55472-action-viewspace-itemid-433002

 

 

1. enqueue
Enqueues are local locks that serialize access to various resources. This wait event
indicates a wait for a lock that is held by another session (or sessions) in an
incompatible mode to the requested mode.

2. library cache lock
The library cache lock controls the concurrency between clients of the library cache by
acquiring a lock on the object handle so that one client can prevent other clients from
accessing the same object or the client can maintain a dependency for a long time (no
other client can change the object). This lock is also gotten to locate an object in the
library cache.

3. library cache load lock
This event is used when a database object is loaded. The load lock is always gotten in
Exclusive mode, so that no other process can load the same object. If the load lock is
busy the session will wait on this event until the lock becomes available.

4. library cache pin
Library cache pins are used to manage library cache concurrency. Pinning an object
causes the heaps to be loaded into memory (if not already loaded). PINS can be acquired
in NULL, SHARE or EXCLUSIVE modes and can be considered like a special form of lock.
A wait for a "library cache pin" implies some other session holds that PIN in an
incompatible mode.

5. row cache lock
This event is used to wait for a lock on a data dictionary cache specified by "cache id". If
one is running in shared mode (Parallel Server), the LCK0 is signaled to get the row
cache lock for the foreground waiting on this event. The LCK0 process will get the lock
asynchronously. In exclusive mode the foreground process will try to get the lock.