library cache lock和library cache pin到底是什么（续）
Posted: December 16, 2011 | Author: Cui Hua | Filed under: Oracle | Tags: library cache lock and pin | 5 Comments »这篇文章是“library cache lock和library cache pin到底是什么”的姊妹篇，在这篇文章里，我们通过测试得到了如下结论：
1、 针对cursor的library cache lock的lock mode确实是null，无论该cursor所对应的sql是硬解析还是软解析；
2、 MOS上说Oracle说从
3、 sql的软解析时，library cache pin的lock mode始终是S；
4、 sql的硬解析时，library cache pin的lock mode一般是X，但在
这里测试所采用的方法就是event 10049，这个事件在10gR2以后，专门被用来trace library cache lock和library cache pin。但好多朋友不太会用这个事件，我这里以一个实例的方式介绍了如何用10049事件来trace单个sql的library cache lock和library cache pin。
我们先从
Connected to Oracle Database
Connected as SYS
SQL> select name,value,description from sys.all_parameters where name like ‘_kks%’;
NAME VALUE DESCRIPTION
—————————— ———- ————————————————–
_kks_use_mutex_pin FALSE Turning on this will make KKS use mutex for cursor pins.
SQL> select * from scott.emp;
EMPNO ENAME JOB MGR HIREDATE SAL COMM DEPTNO
—– ———- ——— —– ———– ——— ——— ——
7981 CUIHUA 7981
7369 SMITH CLERK 7902
7499 ALLEN SALESMAN 7698
……省略显示部分内容
7800 JAME3 CLERK 7698
13 rows selected
SQL> select hash_value,sql_text from v$sqlarea where sql_text like ‘select * from scott.emp%’;
HASH_VALUE SQL_TEXT
———- ——————————————————————————–
52404428 select * from scott.emp
SQL> select to_char(52404428,’XXXXXXXX’) from dual;
TO_CHAR(52404428,’XXXXXXXX’)
—————————-
31FA0CC
现在我们要来trace针对上述sql的library cache pin和library cache lock，方法我之前已经说了，就是用event 10049，用10049的难点在于如何确定level。
确定10049针对单个sql的level值的算法如下：
首先，10049的level可能会有如下一些值：
#define KGLTRCLCK 0x0010 /* trace lock operations */
#define KGLTRCPIN 0x0020 /* trace pin operations */
#define KGLTRCOBF 0x0040 /* trace object freeing */
#define KGLTRCINV 0x0080 /* trace invalidations */
#define KGLDMPSTK 0x0100 /* DUMP CALL STACK WITH TRACE */
#define KGLDMPOBJ 0x0200 /* DUMP KGL OBJECT WITH TRACE */
#define KGLDMPENQ 0x0400 /* DUMP KGL ENQUEUE WITH TRACE */
#define KGLTRCHSH 0x2000 /* DUMP BY HASH VALUE */
其次，我们是要针对单个sql，所以需要用到这个sql的hash value，以便将10049和这个sql联系起来，即我们一定要用到KGLTRCHSH，也就是0x2000；
另外我们是要trace library cache lock和library cache pin，所以我们一定要用到KGLTRCLCK和KGLTRCPIN，即0x0010和0x0020；
最后就是我们需要把这个sql的hash value的16进制的后两个byte拿出来，作为10049的level的前缀。
从上面结果中我们可以看到，select * from scott.emp的hash value的16进制的后两个byte是0xA0CC。另外KGLTRCHSH | KGLTRCLCK | KGLTRCPIN ＝ 0x2000 | 0x0010 | 0x0020 = 0x2030。按照上述算法，select * from scott.emp的10049的最终level值就是0xa0cc2030，也就是2697732144：
SQL> select to_number(‘a0cc2030′,’XXXXXXXXXXXX’) from dual;
TO_NUMBER(‘A0CC2030’,’XXXXXXXX
——————————
2697732144
现在我们设置好10049后再执行一遍上述sql，以观察
SQL> oradebug setmypid
已处理的语句
SQL> oradebug event 10049 trace name context forever,level 2697732144
已处理的语句
SQL> select * from scott.emp;
EMPNO ENAME JOB MGR HIREDATE SAL COMM DEPTNO
—– ———- ——— —– ———– ——— ——— ——
7981 CUIHUA 7981
7369 SMITH CLERK 7902
7499 ALLEN SALESMAN 7698
……省略显示部分内容
7800 JAME3 CLERK 7698
13 rows selected
SQL> oradebug tracefile_name
d:\oracle\admin\cuihua\udump\cuihua_ora_5808.trc
相应的trace文件（d:\oracle\admin\cuihua\udump\cuihua_ora_5808.trc）的内容为：
*** 2011-06-01 11:59:35.500
KGLTRCLCK kglget hd = 0x33938118 KGL Lock addr = 0x
KGLTRCLCK kglget hd = 0x33938034 KGL Lock addr = 0x
KGLTRCPIN kglpin hd = 0x33938034 KGL Pin addr = 0x
KGLTRCPIN kglpndl hd = 0x33938034 KGL Pin addr = 0x
KGLTRCLCK kgllkdl hd = 0x33938034 KGL Lock addr = 0x
KGLTRCLCK kgllkdl hd = 0x33938118 KGL Lock addr = 0x
hd = 0x33938118所对应的library cache object的name就是select * from scott.emp：
SQL> select sql_text from v$sqlarea where address=’33938118′;
SQL_TEXT
——————————————————————————–
select * from scott.emp
hd = 0x33938034就是hd = 0x33938118的子cursor：
SQL> select kglhdadr,kglhdpar,kglnaobj from x$kglob where kglhdadr=’33938034′;
KGLHDADR KGLHDPAR KGLNAOBJ
——– ——– ——————————————————————————–
33938034 33938118 select * from scott.emp
很明显，从上述trace文件中我们可以得出如下结论：
1、
2、
现在我们来观察
SQL> shutdown immediate
数据库已经关闭。
已经卸载数据库。
ORACLE 例程已经关闭。
SQL> startup
ORACLE 例程已经启动。
Total System Global Area 608174080 bytes
Fixed Size 1250404 bytes
Variable Size 318770076 bytes
Database Buffers 281018368 bytes
Redo Buffers 7135232 bytes
数据库装载完毕。
数据库已经打开。
SQL> select hash_value,sql_text from v$sqlarea where sql_text like ‘select * from scott.emp%’;
HASH_VALUE SQL_TEXT
———- ——————————————————————————–
SQL> oradebug setmypid
已处理的语句
SQL> oradebug event 10049 trace name context forever,level 2697732144
已处理的语句
SQL> select * from scott.emp;
EMPNO ENAME JOB MGR HIREDATE SAL COMM DEPTNO
—– ———- ——— —– ———– ——— ——— ——
7981 CUIHUA 7981
7369 SMITH CLERK 7902
7499 ALLEN SALESMAN 7698
……省略显示部分内容
7800 JAME3 CLERK 7698
13 rows selected
SQL> oradebug tracefile_name
d:\oracle\admin\cuihua\udump\cuihua_ora_5016.trc
相应的trace文件（d:\oracle\admin\cuihua\udump\cuihua_ora_5016.trc）的内容为：
KGLTRCLCK kglget hd = 0x206ECF90 KGL Lock addr = 0x3174E068 mode = N
KGLTRCPIN kglpin hd = 0x206ECF90 KGL Pin addr = 0x
KGLTRCPIN kglpndl hd = 0x206ECF90 KGL Pin addr = 0x
KGLTRCLCK kglget hd = 0x33B19238 KGL Lock addr = 0x3174E618 mode = N
KGLTRCPIN kglpin hd = 0x33B19238 KGL Pin addr = 0x
KGLTRCPIN kglpndl hd = 0x33B19238 KGL Pin addr = 0x
KGLTRCLCK kgllkdl hd = 0x33B19238 KGL Lock addr = 0x3174E618 mode = N
KGLTRCLCK kgllkdl hd = 0x206ECF90 KGL Lock addr = 0x3174E068 mode = N
SQL> select kglhdadr,kglhdpar,kglnaobj from x$kglob where kglhdadr=’33B19238′;
KGLHDADR KGLHDPAR KGLNAOBJ
——– ——– ——————————————————————————–
33B19238 206ECF90 select * from scott.emp
很明显，从上述trace文件中我们可以得出如下结论：
1、
2、
好了，
Connected to Oracle Database
Connected as SYS
MOS上说：从
但实际情况并不完全是这样，详情见后面的测试：
SQL> select name,value,description from sys.all_parameters where name like ‘_kks%’;
NAME VALUE DESCRIPTION
—————————— ———- ————————————————–
_kks_use_mutex_pin TRUE Turning on this will make KKS use mutex for cursor pins.
SQL> select * from scott.emp;
EMPNO ENAME JOB MGR HIREDATE SAL COMM DEPTNO
———- ———- ——— ———- ——— ———- ———- ———-
7369 SMITH CLERK 7902 17-DEC-80 800 20
7499 ALLEN SALESMAN 7698 20-FEB-81 1600 300 30
……省略显示部分内容
7934 MILLER CLERK 7782 23-JAN-82 1300 10
14 rows selected.
SQL> select hash_value,sql_text from v$sqlarea where sql_text like ‘select * from scott.emp%’;
HASH_VALUE SQL_TEXT
———- ——————————–
52404428 select * from scott.emp
SQL> oradebug setmypid
Statement processed.
SQL> oradebug event 10049 trace name context forever,level 2697732144
Statement processed.
SQL> select * from scott.emp;
EMPNO ENAME JOB MGR HIREDATE SAL COMM DEPTNO
———- ———- ——— ———- ——— ———- ———- ———-
7369 SMITH CLERK 7902 17-DEC-80 800 20
7499 ALLEN SALESMAN 7698 20-FEB-81 1600 300 30
……省略显示部分内容
7934 MILLER CLERK 7782 23-JAN-82 1300 10
14 rows selected.
SQL> oradebug tracefile_name
/u01/app/oracle/admin/testdb/udump/testdb_ora_1237156.trc
$ cat /u01/app/oracle/admin/testdb/udump/testdb_ora_1237156.trc
/u01/app/oracle/admin/testdb/udump/testdb_ora_1237156.trc
Oracle Database
With the Partitioning, OLAP, Data Mining and Real Application Testing options
ORACLE_HOME = /u01/app/oracle/product/
System name: AIX
Node name: P550_03_LD
Release: 3
Version: 5
Machine: 0001DA17D600
Instance name: testdb
Redo thread mounted by this instance: 1
Oracle process number: 15
Unix process pid: 1237156, image: oracle@P550_03_LD (TNS V1-V3)
*** 2011-06-01 13:38:07.949
*** ACTION NAME:() 2011-06-01 13:38:07.944
*** MODULE NAME:(sqlplus@P550_03_LD (TNS V1-V3)) 2011-06-01 13:38:07.944
*** SERVICE NAME:(SYS$USERS) 2011-06-01 13:38:07.944
*** SESSION ID:(146.3) 2011-06-01 13:38:07.944
KGLTRCLCK kgllkal hd = 0x
KGLTRCLCK kglget hd = 0x
KGLTRCLCK kgllkal hd = 0x7000000226ec
KGLTRCLCK kgllkdl hd = 0x7000000226ec
KGLTRCLCK kgllkdl2 hd = 0x7000000226ec
KGLTRCLCK kgllkdl hd = 0x
KGLTRCLCK kgllkdl2 hd = 0x
这里mode=0应该是表示调用kgllkdl2所产生的library cache lock在调用完上述方法后已经释放了。
SQL> select kglhdadr,kglhdpar,kglnaobj from x$kglob where lower(kglhdadr)=’07000000226ec
KGLHDADR KGLHDPAR KGLNAOBJ
—————- —————- ——————————————————————————–
07000000226EC
很明显，从上述trace文件中我们可以得出如下结论：
现在我们来观察
$ sqlplus ‘/ as sysdba’;
SQL*Plus: Release
Copyright (c) 1982, 2010, Oracle. All Rights Reserved.
Connected to:
Oracle Database
With the Partitioning, OLAP, Data Mining and Real Application Testing options
SQL> shutdown immediate
Database closed.
Database dismounted.
ORACLE instance shut down.
SQL> startup
ORACLE instance started.
Total System Global Area 314572800 bytes
Fixed Size 2096032 bytes
Variable Size 96470112 bytes
Database Buffers 209715200 bytes
Redo Buffers 6291456 bytes
Database mounted.
Database opened.
SQL> oradebug setmypid
Statement processed.
SQL> oradebug event 10049 trace name context forever,level 2697732144
Statement processed.
SQL> select * from scott.emp;
EMPNO ENAME JOB MGR HIREDATE SAL COMM DEPTNO
———- ———- ——— ———- ——— ———- ———- ———-
7369 SMITH CLERK 7902 17-DEC-80 800 20
7499 ALLEN SALESMAN 7698 20-FEB-81 1600 300 30
……省略显示部分内容
7934 MILLER CLERK 7782 23-JAN-82 1300 10
14 rows selected.
SQL> oradebug tracefile_name
/u01/app/oracle/admin/testdb/udump/testdb_ora_1536246.trc
$ cat /u01/app/oracle/admin/testdb/udump/testdb_ora_1536246.trc
/u01/app/oracle/admin/testdb/udump/testdb_ora_1536246.trc
Oracle Database
With the Partitioning, OLAP, Data Mining and Real Application Testing options
ORACLE_HOME = /u01/app/oracle/product/
System name: AIX
Node name: P550_03_LD
Release: 3
Version: 5
Machine: 0001DA17D600
Instance name: testdb
Redo thread mounted by this instance: 1
Oracle process number: 15
Unix process pid: 1536246, image: oracle@P550_03_LD (TNS V1-V3)
*** ACTION NAME:() 2011-06-01 13:42:44.913
*** MODULE NAME:(sqlplus@P550_03_LD (TNS V1-V3)) 2011-06-01 13:42:44.913
*** SERVICE NAME:(SYS$USERS) 2011-06-01 13:42:44.913
*** SESSION ID:(159.3) 2011-06-01 13:42:44.913
DBRM(kskinitrm) cpu_count : old(0) -> new(2)
kwqmnich: current time:: 5: 42: 44
kwqmnich: instance no 0 check_only flag 1
kwqmnich: initialized job cache structure
*** 2011-06-01 13:44:13.657
KGLTRCLCK kgllkal hd = 0x7000000225ccfa8 KGL Lock addr = 0x
KGLTRCLCK kglget hd = 0x7000000225ccfa8 KGL Lock addr = 0x
KGLTRCPIN kglpin hd = 0x7000000225ccfa8 KGL Pin addr = 0x
KGLTRCPIN kglpndl hd = 0x7000000225ccfa8 KGL Pin addr = 0x
KGLTRCLCK kgllkal hd = 0x7000000225abf18 KGL Lock addr = 0x
KGLTRCLCK kglget hd = 0x7000000225abf18 KGL Lock addr = 0x
KGLTRCPIN kglpin hd = 0x7000000225abf18 KGL Pin addr = 0x
KGLTRCPIN kglpndl hd = 0x7000000225abf18 KGL Pin addr = 0x
KGLTRCLCK kgllkdl hd = 0x7000000225abf18 KGL Lock addr = 0x
KGLTRCLCK kgllkdl2 hd = 0x7000000225abf18 KGL Lock addr = 0x
KGLTRCLCK kgllkdl hd = 0x7000000225ccfa8 KGL Lock addr = 0x
KGLTRCLCK kgllkdl2 hd = 0x7000000225ccfa8 KGL Lock addr = 0x
SQL> select kglhdadr,kglhdpar,kglnaobj from x$kglob where lower(kglhdadr)=’07000000225abf18′;
KGLHDADR KGLHDPAR KGLNAOBJ
—————- —————- ——————————————————————————–
07000000225ABF18 07000000225CCFA8 select * from scott.emp
很明显，从上述trace文件中我们可以得出如下结论：
1、
2、
library cache lock和library cache pin到底是什么
Posted: December 16, 2011 | Author: Cui Hua | Filed under: Oracle | Tags: library cache lock and pin | 3 Comments »可能有很多朋友从来就没有搞清楚过到底什么是library cache lock和library cache pin，它们到底是enqueue还是latch？它们的作用是什么？
这里我尝试对上述问题做一番解释，这些解释可能是有问题的，因为里面包含了我的一些猜测。
最近连续写了一些基于我的猜测、没有确凿证据的文章，这也许不太合适。
我们通常说的library cache lock和library cache pin是enqueue，不是latch，它们是两种DDL lock。但需要我们注意的是，在11gR1之前，Oracle中又存在名为library cache lock和library cache pin的latch。
是不是感觉很混乱？没关系，我们一点一点往下看。很抱歉，我这里引用了大量英文，因为我觉得如果翻译出来就失去了原先的味道。
1、 作为enqueue，library cache lock和library cache pin的作用是什么？
Both library cache lock and library cache pin are provided to access objects in the library cache. Library cache lock manages concurrency between processes, whereas library cache pin manages cache coherence. In order to access an object in library cache, a process must first lock the library cache object handle, and then pin the object data heap itself. Requests for both library cache lock and library cache pin will wait until granted. This is a possible source of contention, because there is no NOWAIT request mode.
By acquiring a library cache lock on the library cache object handle, a process can prevent other processes from accessing the object, or even finding out what type it is. It can even maintain a dependency on an object without preventing other processes from accessing the object. Acquiring a library cache lock is also the only way to locate an object in cache—a process locates and locks an object in a single operation.
If the process wants to actually examine or modify the object, then it must acquire a library cache pin on the object data heap itself (after acquiring a library cache lock on the library cache object handle). Pinning the object causes information about the object and its data heaps to be loaded into memory if they were not already there. This information is guaranteed to remain in memory at least until the pin is released. Locks and pins are externalized in X$KGLLK and X$KGLPN, respectively.
2、 作为enqueue，library cache lock和library cache pin有哪几种lock mode？
a) Library cache lock有三种lock mode，分别是share、exclusive和null。A process acquires a share library cache lock if it intends only to read the object. For example, it wants to reference the object during compilation. A process acquires an exclusive library cache lock if it intends to create or modify the object. For example, it wants to drop the object from the database. Null library cache locks are a special case. They are acquired on objects that are to be executed like child cursor, procedure, function, package, or type body. You can use them to maintain an interest on an object for a long period of time (session persistency), and to detect if the object becomes invalid. You can break null library cache lock at any time. This is used as a mechanism to notify a session that an executable object is no longer valid. If a null library cache lock is broken, and thus the object is invalidated, then it is an indication to the user who was holding the null library cache lock that the object needs to be recompiled. A null library cache lock is acquired during the parse phase of SQL statement execution and is held as long as the shared SQL area for that statement remains in the shared pool. A null library cache lock does not prevent any DDL operation, and can be broken to allow conflicting DDL operations, hence the term “breakable parse lock.” A null library cache lock on an object is broken when there is an exclusive library cache pin on the object.
b) Library cache pin有两种lock mode，分别是share和exclusive。 When a process pins an object data heap that is not in memory, the process can determine whether the data heap is to be loaded in the PGA or SGA. An object must be pinned in Exclusive mode if it is to be modified. However, the process first will always pin the object in Share mode, examine it for errors and security checks, and then, if necessary, (such as needing modification) pin it in Exclusive mode. An object is never pinned in Exclusive mode if only read access is required. This is because all dependent transient objects (cursors) are invalidated (null locks broken) when an object is unpinned from Exclusive mode. The effect would be unnecessary recompilation and reparsing of all dependent packages, procedures, and functions.
3、 作为latch，library cache lock和library cache pin的作用是什么？
这是一个很纠结的问题，既然已经有了作为enqueue的library cache lock和library cache pin，为什么在11gR1以前，Oracle里还有同名latch，而且明显这些同名latch是在被使用：
Connected to Oracle Database
Connected as ipra
SQL> select name,level#,gets,misses,sleeps,immediate_gets,immediate_misses from v$latch where name like ‘library%’;
NAME LEVEL# GETS MISSES SLEEPS IMMEDIATE_GETS IMMEDIATE_MISSES
————————————————– ———- ———- ———- ———- ————– —————-
library cache 5 9221760 1608 800 2596 76766
library cache lock 6 13548247 582 6 0 0
library cache lock allocation 3 208273 0 0 0 0
library cache pin 6 4207462 193 0 2 0
library cache hash chains 9 0 0 0 0 0
library cache pin allocation 3 57276 0 0 0 0
library cache load lock 5 24848 0 0 1 0
7 rows selected
从结果里我们可以看到，对于
那么library cache lock latch、library cache pin latch以及大家最耳熟能详的library cache latch等等，这些latch是做什么用的呢？
也许我们可以从下面的一段文字中找到答案：
The library cache latches serialize access to the objects in the library cache. Access to library cache objects always occurs through library cache locks. Because locking an object is not an atomic instruction, a library cache latch is acquired before the library cache lock request and is released after it. For most operations, the library cache latches are used, and therefore they can become a point of contention.
If an object is not in memory, then a library cache lock cannot be acquired on it. In order to prevent multiple processes to request the load of the same object simultaneously, another latch must be acquired before the load request. This is the library cache load lock latch. The library cache load lock latch is taken and held until a library cache load lock is allocated, then the latch is released. Loading of the object is performed under the library cache load lock and not under the library cache load lock latch as it may take quite a long time.
这里提到了几点值得我们关注：
a) Oracle使用上述library cache latches（包括library cache latch、library cache lock latch、library cache pin latch、library cache pin allocation latch、library cache load lock latch）的目的是控制并发访问library cache object所需要的相关的enqueue或者是为了控制并发访问library cache中的相关的内存结构，比如用相关的library cache lock latch控制并发获得library cache lock。这里我猜测Oracle用library cache lock latch控制并发获得library cache lock，用library cache pin latch控制并发获得library cache pin，用library cache load lock latch控制并发获得library cache load lock，用library cache latch去控制并发访问library cache object handle中的某些结构，如library cache object handle中的flag中的special status flag （special status flags are protected by the library cache latch. Examples of these flags indicate that: The object is valid; The object is authorized; The object has compilation errors）。
b) Library cache load lock是另外一种enqueue。The session tries to find the library cache load lock for the database object so that it can load the object. The library cache load lock is always obtained in Exclusive mode, so that no other process can load the same object. If the library cache load lock is busy the session will wait on this event until the lock becomes available.
好了，现在我们来验证一下，还是上述
SQL> select name,level#,gets,misses,sleeps,immediate_gets,immediate_misses from v$latch where name like ‘library%’;
NAME LEVEL# GETS MISSES SLEEPS IMMEDIATE_GETS IMMEDIATE_MISSES
————————————————– ———- ———- ———- ———- ————– —————-
library cache 5 9222166 1608 800 2596 76766
library cache lock 6 13548760 582 6 0 0
library cache lock allocation 3 208287 0 0 0 0
library cache pin 6 4207656 193 0 2 0
library cache hash chains 9 0 0 0 0 0
library cache pin allocation 3 57278 0 0 0 0
library cache load lock 5 24848 0 0 1 0
7 rows selected
从结果里我们可以看到，library cache lock latch的gets从13548247递增到了13548760，library cache pin latch的gets从4207462递增到了4207656，但library cache load lock latch的gets还是保持24848不变。
现在我们来让library cache load lock latch的gets发生变化，这是非常容易的事情，我们只需要执行一个需要硬解析的sql就可以了：
SQL> select * from scott.emp_temp;
EMPNO ENAME JOB MGR HIREDATE SAL COMM DEPTNO ISINSPECT
—– ———- ——— —– ———– ——— ——— —— ———-
SQL> select name,level#,gets,misses,sleeps,immediate_gets,immediate_misses from v$latch where name like ‘library%’;
NAME LEVEL# GETS MISSES SLEEPS IMMEDIATE_GETS IMMEDIATE_MISSES
————————————————– ———- ———- ———- ———- ————– —————-
library cache 5 9223549 1608 800 2596 76766
library cache lock 6 13550296 582 6 0 0
library cache lock allocation 3 208348 0 0 0 0
library cache pin 6 4208118 193 0 2 0
library cache hash chains 9 0 0 0 0 0
library cache pin allocation 3 57294 0 0 0 0
library cache load lock 5 24856 0 0 1 0
7 rows selected
由于我们执行了一个需要硬解析的sql，导致Oracle需要获得library cache load lock以便load相关信息到这个sql的子cursor的heap 6中，而要获得library cache load lock，必须先持有library cache load lock latch。从上述结果中我们可以看到，此时library cache load lock latch的gets已经发生了变化，从24848递增到了24856。
接下来我们再来看一看上述library cache latches的子latch情况：
SQL> show parameter cpu_count
NAME TYPE VALUE
———————————— ———– ——————————
cpu_count integer 2
这里cpu的个数为2，显然上述library cache latches的子latch应该为3：
SQL> select name,level#,gets,misses,sleeps,immediate_gets,immediate_misses from v$latch_children where name like ‘library%’;
NAME LEVEL# GETS MISSES SLEEPS IMMEDIATE_GETS IMMEDIATE_MISSES
————————————————– ———- ———- ———- ———- ————– —————-
library cache 5 3274551 1301 94 187 0
library cache 5 2218356 116 80 933 0
library cache 5 3731320 191 626 1476 76766
library cache lock 6 5339737 362 3 0 0
library cache lock 6 6223353 194 3 0 0
library cache lock 6 1987799 26 0 0 0
library cache pin 6 1484918 184 0 0 0
library cache pin 6 891695 3 0 2 0
library cache pin 6 1831837 6 0 0 0
library cache pin allocation 3 23177 0 0 0 0
library cache pin allocation 3 8272 0 0 0 0
library cache pin allocation 3 25849 0 0 0 0
library cache lock allocation 3 75900 0 0 0 0
library cache lock allocation 3 28229 0 0 0 0
library cache lock allocation 3 104237 0 0 0 0
library cache hash chains 9 0 0 0 0 0
library cache hash chains 9 0 0 0 0 0
library cache hash chains 9 0 0 0 0 0
18 rows selected
注意，结果里并没有library cache load lock latch，说明library cache load lock latch没有children，它是一个solitary类型的latch。
从
这里需要我们了解的是：
a) 从
b) Mutex和latch是互相独立，没有任何关系的：Latches and mutexes are independent mechanisms i.e. a process can hold a latch and a mutex at the same time. In the case of process death, latches are always cleaned up before mutexes. There is no generic mutex deadlock detection (unlike latches). There is no mutex/latch hierarchy.
从11gR1开始，Oracle用mutex替换了library cache latches，并引了一个新的等待事件：library cache: mutex *，我们来看一下这个知识点：
Connected to Oracle Database
Connected as nbs
SQL> select name,level#,gets,misses,sleeps,immediate_gets,immediate_misses from v$latch where name like ‘library%’;
NAME LEVEL# GETS MISSES SLEEPS IMMEDIATE_GETS IMMEDIATE_MISSES
—————————————————————- ———- ———- ———- ———- ————– —————-
library cache load lock 5 0 0 0 0 0
SQL> select name,level#,gets,misses,sleeps,immediate_gets,immediate_misses from v$latch_children where name like ‘library%’;
NAME LEVEL# GETS MISSES SLEEPS IMMEDIATE_GETS IMMEDIATE_MISSES
—————————————————————- ———- ———- ———- ———- ————– —————-
从结果里我们可以看到，在
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