latch 相關效能問題診斷： latch: row cache objects等待事件導致CPU負載高

Oracle 11.2.0.1.0 ,  EBS R12.1.3 

現象：   DB環境效能不穩定，CPU持續100%，後臺很多Active Session。

分析：
1.   生成AWR報告，檢視等待事件，發現 latch: row cache objects 等待事件比較嚴重。

Top 5 Timed Foreground Events
Event      Waits     Time(s)    Avg wait (ms)     % DB time     Wait Class
DB CPU     44,158                     37.91  
latch: row cache objects  101,681,447   25,442   0   21.84   Concurrency
library cache lock 1,636 14,850 9077 12.75 Concurrency
enq: TX - row lock contention 164 9,053 55200 7.77 Application
enq: SQ - contention 468 3,617 7728 3.11 Configuration

2.  檢視AWR報告中Latch Statistics下的 Latch Miss Sources  
Latch Miss Sources
-only latches with sleeps are shown
-ordered by name, sleeps desc
Latch Name           Where          NoWaitMisses       Sleeps        Waiter Sleeps
row cache objects     kqrpre: find obj     0            47,385,304      29,486,952
row cache objects      kqreqd          0             13,710,373      22,722,332
row cache objects       kqreqd: reget      0            13,121,647      22,009,939
undo global data      ktusm_stealext_2     0            4,990           2,025

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