cursor: pin S wait on X 等待事件

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标题:cursor: pin S wait on X 等待事件


cursor: pin S整体描述

cursor: pin S A session waits on this event when it wants to update a shared mutex pin and another session 
is currently in the process of updating a shared mutex pin for the same cursor object. This wait event should
rarely be seen because a shared mutex pin update is very fast.(Wait Time: Microseconds)

P1 Hash value of cursor

P2 Mutex value 
64 bit platforms
8 bytes are used.
Top 4 bytes hold the session id (if the mutex is held X)
Bottom 4 bytes hold the ref count (if the mutex is held S).

32 bit platforms 
4 bytes are used.
Top 2 bytes hold the session id (if the mutex is held X) 
Bottom 2 bytes hold the ref count (if the mutex is held S).

P3 Mutex where (an internal code locator) OR'd with Mutex Sleeps

SELECT a.*, s.sql_text
  FROM v$sql s,
       (SELECT sid,
               p1 cursor_hash_value,
               p2raw Mutex_value,
               TO_NUMBER (SUBSTR (p2raw, 1, 8), 'xxxxxxxx') hold_mutex_x_sid
          FROM v$session_wait
         WHERE event LIKE 'cursor%') a
 WHERE s.HASH_VALUE = a.cursor_hash_value

cursor: pin S wait on X描述

- In previous versions of Oracle, library cache pin is protected by “library cache pin latch”.
- But in recent versions of Oracle(I believe it’s, 
  library cache pin for the cursor LCO is protected by mutext.
- Mutex is allocated per LCO, so it enables fine-grained access control.

“cursor: pin S wait on X” wait event is mostly related to mutex and hard parse.
- When a process hard parses the SQL statement, it should acquire exclusive 
  library cache pin for the corresponding LCO.
- This means that the process acquires the mutex in exclusive mode.
- Another process which also executes the same query needs to acquire the mutex 
  but it’s being blocked by preceding process. The wait event is “cursor: pin S wait on X”.

--发生cursor: pin S wait on X原因
Frequent Hard Parses
If the frequency of Hard Parsing is extremely high, then contention can occur on this pin.

High Version Counts
When Version counts become excessive, a long chain of versions needs to 
be examined and this can lead to contention on this event

Known bugs
Bug 5907779 - Self deadlock hang on "cursor: pin S wait on X" (typically from DBMS_STATS) [ID 5907779.8]
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