library cache latch等待事件

产生library cache latch原因

The library cache latches protect the cached SQL statements and objects' definitions held 
in the library cache within the shared pool. The library cache latch must be acquired 
in order to add a new statement to the library cache. During a parse, Oracle searches 
the library cache for a matching statement. If one is not found, then Oracle will parse 
the SQL statement, obtain the library cache latch and insert the new SQL.

每一个sql被执行之前，先要到library cache中根据hash_value查找parent cursor,这就需要先获得library cache latch；找到parent cursor后，就会去查找对应的child cursor,当发现无法找到时，就会释放library cache latch，获得share pool latch分配空间给硬解析后的产生的执行计划；然后再次获得library cache latch进行把执行计划放入share pool,转入library cache pin+lock(null模式)开始执行sql.library cache latch 的个数有限(与CPU_COUNT参数相关)，当数据库中出现大量硬解析的时候，某一个sql无法得到library cache latch就会开始spin,达到spin count后还没得到，就会开始sleep,达到sleep时间后,醒来还再次试图过的library cache latch得不到就在spin再得不到又sleep…依此类推.
综上可知:在sql执行的过程中可以看出在出现High Versions Count和Hard Parse的情况下都有可能出现library cache latch等待.
关于Hard Parse见:shared pool latch 等待事件
关于High Versions Count见:关于High Versions Count总结

1._KGL_LATCH_COUNT控制library cache latches数量

The hidden parameter _KGL_LATCH_COUNT controls the number of library cache latches. 
The default value should be adequate, but if contention for the library cache latch cannot  be resolved, 
it one may consider increasing this value. The default value for _KGL_LATCH_COUNT is the next prime number 
after CPU_COUNT. This value cannot exceed 67. 

2.Library cache: mutex X
在10g及其以后版本中,很多latch使用mutex代替,我们常见的Library cache: mutex X is similar to library cache wait in earlier version.(_kks_use_mutex_pin=false可以禁止mutex)