关于Oracle 数据库等待事件library cache: mutex X

参考自:
WAITEVENT: "library cache: mutex X" (Doc ID 727400.1)

The library cache mutex is acquired for similar purposes that the library cache latches were acquired in prior versions of Oracle.
In 10g,mutexes were introduced for certain operations in the library cache.
Starting with 11g,the library cache latches were replaced by mutexes,hence this new wait event.

Mutexes are a lighter-weight and more granular concurrency mechanism than latches.
Mutexes take advantage of cpu architectures that offer the compare and swap instructions (or similar).
The reason for obtaining a mutex in the first place,is to ensure that certain operations are properly managed for concurrency.
E.g.,if one session is changing a data structure in memory,
then another session must wait to acquire the mutex before it can make a similar change - this prevents unintended changes that would lead to corruptions or crashes if not serialized.

This wait event is present whenever a library cache mutex is held in exclusive mode by a session and other sessions need to wait for it to be released.
There are many different operations in the library cache that will require a mutex,
so its important to recognize which "location" (in Oracle's code) is involved in the wait.
"Location" is useful to Oracle Support engineers for diagnosing the cause for this wait event

awr报告中搜索"Mutex Sleep Summary",定位到Mutex Sleep Summary部分,也能获取一些有价值的东西,如下:

Mutex Sleep Summary

• ordered by number of sleeps desc

Mutex Type	Location	Sleeps	Wait Time (ms)
Library Cache	kglpnal2 91	532	0
Library Cache	kglpin1 4	147	0
kglhdgn2 106	144	kgllkdl1 85	141	kglhdgn1 62	129	kgllkc1 57	95	kglpndl1 95	83	kglget2 2	72	kglpnal1 90	61	kgllkal1 80	44	Cursor Pin	kkslce [KKSCHLPIN2]	41	kglget1 1	22	kksLockDelete [KKSCHLPIN6]	17	Cursor Pin	kksfbc [KKSCHLPIN1]	15	kksfbc [KKSCHLFSP2]	13	kglhdgh1 64	12	kgldtin1 42	11	kglrfcl1 79	9	kglobpn1 71	7	Cursor Parent	kkscsAddChildNode [KKSPRTLOC34]	3	Cursor Parent	kkscsPruneChild [KKSPRTLOC35]	3	hash table	kkscsSearchChildList [KKSHBKLOC2]	1	hash table	kkshGetNextChild [KKSHBKLOC1]	1	0

找到blocker:

SELECT sql_ID,ACTION,BLOCKING_SESSION,BLOCKING_SESSION_STATUS
   FROM v$session 
  WHERE SID=&SID_OF_WAITING_SESSION;
  
 BLOCKING_SESSION列就是mutex x的持有者session
 

11g及其更高的版本:

10g版本:
 SELECT decode(trunc(&&P2/4294967296),trunc(&&P2/65536),trunc(&&P2/4294967296)) SID_HOLDING_MUTEX
 FROM dual;

若是用以上找不到blocker,那就有可能是bug,
请参考:WAITEVENT: "library cache: mutex X" (Doc ID 727400.1)文档中的"KNown Bugs"部分.

另外 的参考文章:

Troubleshooting Databases Hang Due to Heavy Contention for 'library cache: mutex X' Waits (Oracle 11.2 and Later) (Doc ID 2051456.1)