這個等待事件也算一個常見的等待事件。 在warehouse viewspace blog和 itpub 上有相關的2個文章。 連接配接如下：
cursor: pin S wait on X等待事件模拟
http://warehouse.itpub.net/post/777/493962
cursor: pin S wait on X
http://space.itpub.net/756652/viewspace-348176
一. Mutex 說明
Oracle Mutex 機制 說明
http://blog.csdn.net/xujinyang/article/details/6831253
To improve cursor execution and also hard parsing, a new memory serialization mechanism has been created in 10gR2.
For certain shared-cursor related operations, mutexes are used as a replacement for library cache latches and librarycache pins.
-- mutexes 替代 library cache latches 和 librarycache pins。
Using mutexes is faster, uses less CPU and also allows significantly improved concurrency over the existing latch mechanism.
The use of mutexes for cursor pins can be enabled by setting the init.ora parameter _use_kks_mutex toTRUE.
Btw, things get more fun in 10.2, you can pin cursors without getting library cache pin latch, using KGX mutexes. Mutexes are new thing in 10.2 and they enable shared access to objects in somewhat similar manner than shared latches, that every successful get of particular mutex will increment its value and release will decrement. When the count is zero, no-one has the mutex and it is safe to get it in exclusive mode too. However they are more fine grained than kgl latches and provide better waiting mechanism as far as I understand.
So if your environment supports atomic compare and swap operation (as CMPXCHG on Intel), you might get away without cursor_space_for_time setting for ultrahigh execution rates. Otherwise the atomic mutex operations would be achieved using new KGX latches.
At least on my laptop this feature isn’t enabled by default (from andOracleWorld’s paper I remember that it should become default in 10.2.0.2), but so far you can experiment with it if you set _kks_use_mutex_pin = true and bounce the instance (mutex structures will be stored in shared pool, so you might need to increase SP size).
There are also x$mutex_sleep and x$mutex_sleep_history fixed tables that can show some interesting information if you generate some mutex waits into them.
在Oracle 10.2中，對shared pool中的一些Serialization operation使用更輕量的 KGX mutexes (_use_kks_mutex) 取代library cache pin，進而降低CPU Usage, 是否使用這種muetx機制受到隐含參數_kks_use_mutex_pin的限制。
從10.2.0.2開始該參數default為true,使用這種機制oracle是為了解決library cache bin latch的串行使用問題,但是mutex貌似還不是很穩定,在很多系統中會出現cursor: pin S wait on X等待事件,這個事件和mutex的使用有關,最近一客戶受到cursor: pin S wait on X等待事件的困擾,出現cursor: pin S wait on X等待事件時通常等待比較嚴重,系統會出現hang。
cursor: pin S wait on X
A session waits for this event when it is requesting a shared mutex pin and another session is holding an exclusive mutex pin on the same cursor object.
Wait Time: Microseconds
Parameter Description
P1 Hash value of cursor
P2 Mutex value (top 2 bytes contains SID holding mutex in exclusive mode, and bottom two bytes usually hold the value 0)
P3 Mutex where (an internal code locator) OR'd with Mutex Sleeps
這個事件的出現受到很多因素的影響，在高并發的情況下:
（1）sga自動管理,sga的頻繁擴充和收縮
（2）過渡硬解析,造成library cache中的cursor object被頻繁的reload
（3）bug
_kks_use_mutex_pin 是隐含參數，通過v$parameter 視圖查不到，需要通過如下SQL 來檢視。
SELECT i.ksppinm name,
i.ksppdesc description,
CV.ksppstvl VALUE,
CV.ksppstdf isdefault,
DECODE (BITAND (CV.ksppstvf, 7),
1, 'MODIFIED',
4, 'SYSTEM_MOD',
'FALSE')
ismodified,
DECODE (BITAND (CV.ksppstvf, 2), 2, 'TRUE', 'FALSE') isadjusted
FROM sys.x$ksppi i, sys.x$ksppcv CV
WHERE i.inst_id = USERENV ('Instance')
AND CV.inst_id = USERENV ('Instance')
AND i.indx = CV.indx
AND i.ksppinm LIKE '/_%' ESCAPE '/'
and i.ksppinm like '_kks%'
ORDER BY REPLACE (i.ksppinm, '_', '');
Oracle 參數分類 和 參數的檢視方法
http://blog.csdn.net/xujinyang/article/details/6829538
二. 相關測試
[email protected](rac2)> select * from v$version where rownum<2;
BANNER
----------------------------------------------------------------
Oracle Database 10g Enterprise Edition Release 10.2.0.4.0 - Prod
SESSION 1：
-------------------------
--建立測試表
[email protected](rac2)> create table t as select * from dba_objects;
Table created.
--檢視session ID
[email protected](rac2)> select sid from v$mystat where rownum=1;
SID
----------
125
[email protected](rac2)> declare
2 v_string varchar2(100) := 'alter system flush shared_pool';
3 msql varchar2(200);
4 begin
5 loop
6 execute immediate v_string;
7 for i in 1..100 loop
8 msql:='select object_id from t where object_id='||i;
9 execute immediate msql;
10 end loop;
11 end loop;
12 end;
13 /
session 2：
-------------------------
--檢視session ID
[email protected](rac2)> select sid from v$mystat where rownum=1;
SID
----------
130
[email protected](rac2)> declare
2 v_string varchar2(100) := 'alter system flush shared_pool';
3 msql varchar2(200);
4 begin
5 loop
6 execute immediate v_string;
7 for i in 1..100 loop
8 msql:='select object_id from t where object_id='||i;
9 execute immediate msql;
10 end loop;
11 end loop;
12 end;
13 /
session 3：
------------------------
用如下SQL 進行監控，在sqlplus 裡看起來格式有點亂，我放到Toad執行了。
SELECT b.*, sq.sql_text
FROM v$session se,
v$sql sq,
(SELECT a.*, s.sql_text
FROM v$sql s,
(SELECT sid,
event,
wait_class,
p1,
p2raw,
TO_NUMBER (SUBSTR (p2raw, 1, 4), 'xxxx')
sid_hold_mutex_x
FROM v$session_wait
WHERE event LIKE 'cursor%') a
WHERE s.HASH_VALUE = a.p1) b
WHERE se.sid = b.sid AND se.sql_hash_value = sq.hash_value;
通過監控發現兩個session在執行相同的sql,他們在相同的cursor object上互動請求a shared mutex pin或者 an exclusive mutex pin 進而造成等待。
--監視sql reae區的cursor object reload情況
[email protected](rac2)> select namespace ,reloads from v$librarycache;
NAMESPACE RELOADS
--------------- ----------
SQL AREA 790805
TABLE/PROCEDURE 103713
BODY 59
TRIGGER 27
INDEX 94280
CLUSTER 11
OBJECT 0
PIPE 0
JAVA SOURCE 0
JAVA RESOURCE 0
JAVA DATA 0
11 rows selected.
--監視parse情況
[email protected](rac2)> col name format a40
[email protected](rac2)> select s.sid, s.serial#,b.name,a.value
2 from v$sesstat a, v$statname b, v$session s
3 where a.statistic# = b.statistic# and s.sid=a.sid
4 and b.name like '%parse%'
5 and s.sid in (130,125);
sid serial# name value
---------- ---------- ---------------------------------------- ----------
125 41915 parse time cpu 115260
125 41915 parse time elapsed 146605
125 41915 parse count (total) 633792
125 41915 parse count (hard) 602732
125 41915 parse count (failures) 4
130 6074 parse time cpu 69559
130 6074 parse time elapsed 99149
130 6074 parse count (total) 394689
130 6074 parse count (hard) 365538
130 6074 parse count (failures) 0
從這裡看出，硬解析很多，library cache中的cursor object被頻繁的reload。
三. 幾個與mutex 相關的視圖
在第一部分，提到了x$mutex_sleep 和x$mutex_sleep_history。我們在聯機文檔裡看不到相關的說明。
不過可以檢視到v$mutex_sleep 和v$mutex_sleep_history的說明。 但是v$ 比x$ 字典顯示的列要少。
select * from x$mutex_sleep;
select * from v$mutex_sleep;
[email protected](rac2)> desc x$mutex_sleep_history
Name Null? Type
----------------------------------------- -------- ----------------------------
ADDR RAW(4)
INDX NUMBER
INST_ID NUMBER
MUTEX_ADDR RAW(4)
MUTEX_IDENTIFIER NUMBER
SLEEP_TIMESTAMP TIMESTAMP(6)
MUTEX_TYPE VARCHAR2(32)
MUTEX_TYPE_ID NUMBER
GETS NUMBER
SLEEPS NUMBER
REQUESTING_SESSION NUMBER
BLOCKING_SESSION NUMBER
LOCATION_ID NUMBER
LOCATION VARCHAR2(40)
MUTEX_VALUE RAW(4)
P1 NUMBER
P1RAW RAW(4)
P2 NUMBER
P3 NUMBER
P4 NUMBER
P5 VARCHAR2(64)
[email protected](rac2)> desc v$mutex_sleep_history
Name Null? Type
----------------------------------------- -------- ----------------------------
MUTEX_IDENTIFIER NUMBER
SLEEP_TIMESTAMP TIMESTAMP(6)
MUTEX_TYPE VARCHAR2(32)
GETS NUMBER
SLEEPS NUMBER
REQUESTING_SESSION NUMBER
BLOCKING_SESSION NUMBER
LOCATION VARCHAR2(40)
MUTEX_VALUE RAW(4)
P1 NUMBER
P1RAW RAW(4)
P2 NUMBER
P3 NUMBER
P4 NUMBER
P5 VARCHAR2(64)
-------------------------------------------------------------------------------------------------------