cursor: pin S

OTN 解释如下：

cursor: pin SA 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)
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

今天收集性能报告Top 5

Top 5 Timed Foreground Events
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
                                                           Avg
                                                          wait   % DB
Event                                 Waits     Time(s)   (ms)   time Wait Class
------------------------------ ------------ ----------- ------ ------ ----------
DB CPU                                          205,412          99.8
db file sequential read              68,063         487      7     .2 User I/O
log file sync                       139,316         233      2     .1 Commit
cursor: pin S                     1,163,270         127      0     .1 Concurrenc
SQL*Net message from dblink          97,575          46      0     .0 Network

Oracle10gR2中引用的mutexes机制一定程度的替代了library cache pin，其结构更简单，相当于，每个child cursor下面都有一个mutexes这样的简单内存结构，当有session中要执行该SQL而需要pin cursor操作的时候，session需要以shared模式set这个内存位+1，表示session获得该mutex的shared mode lock.可以有很多session同时具有这个mutex的shared mode lock；但是在同一时间，只能有一个session在操作这个mutext +1或者-1。+1 -1的操作是排它性的原子操作。如果因为session并行太多，则导致某个session在等待其他session的mutext +1/-1操作,则该session要等待cursor: pin S等待事件。

当看到系统有很多session等待cursor: pin S事件的时候，要么是CPU不够快，要么是某个SQL的并行执行次数太多了而导致在child cursor上的mutex操作争用。如果是前者，则可以升级硬件。如果是因为SQL的并行太多，则要么想办法降低该SQL执行次数，要么将该SQL复制成N个其它的SQL。

    select /*SQL 1*/ename from t where deptno=?
    select /*SQL 2*/ename from t where deptno=?
    select /*SQL …*/ename from t where deptno=?
    select /*SQL N*/ename from t where deptno=?

这样就有了N个SQL Cursor,N个Mutex内存结构，就将争用分散开来，类似partition的作用了。