_library_cache_advice和latch:shared pool、latch:shared pool simulator

版本10.2.0.4和11.1.0.6中"_library_cache_advice"=TRUE的情况下可能出现高latch:shared pool、latch: shared pool simulator等latch争用等待事件，默认情况下_library_cache_advice受到参数"statistics_level"的影响为TRUE，当_library_cache_advice=TRUE时他启用library cache simulator特性。   该library cache simulator特性负责估算shared pool LRU的表现，simulator模拟器收集heap内存堆大小以及load载入、pin、unpin的次数信息；通过这些数据来估算出若我们有更大的shared pool，我们可以由更大的共享池来缓存更多的SQL、PLSQL在共享池中，以此来节约加载时间。若我们设置更小的shared pool size，则又会对加载时间有何等的影响？   题外话：另一个对ASMM 下shared pool有作用的参数：

_memory_broker_shrink_heaps：

• If 0, will not try to shrink shared pool or Java pool
• If greater than zero, will wait this many seconds after failed shrink request to ask again

  禁用library cache simulator设置"_library_cache_advice"=false"可能"(具体仍需要诊断)解决高latch:shared pool、latch: shared pool simulator、Library Cache - Mutex X具体等的问题，禁用library cache simulator会导致AWR中"shared pool advisory"和 "java pool advisory"2个环节不可用，但是这些特性实际可有可无。 但是"_library_cache_advice"=false"时且启用了ASMM(sga_target>0)的情况，注意为shared_pool_size设置一个合理的最小值！   分别在10.2.0.4和11.1.0.6上进行了针对解析的压力测试：

10204 no change baseline	Executes/second = 3,610, DB Time = 12,349s, DB CPU = 8,938s, latch:library cache wait = 598s, avg.wait = 34ms
10204 - _library_cache_advice=off	Executes/second = 3,843, DB Time = 16,208s, DB CPU = 9,402s, latch:library cache wait = 616s, avg. wait = 50ms
11106- no change -baseline	Executes/second = 3,529, DB Time = 14,148s, DB CPU = 9,286s, library cache: mutex X wait = 2,725s, avg. wait = 1ms
11106 -session_cache=500, instantiation=150	Executes/second = 3,436, DB Time = 13,396s, DB CPU = 9,040s, library cache: mutex X wait = 2,383s avg. wait = 1ms
11106 - _library_cache_advice=off	Executes/second = 6,059, DB Time = 75,134s, DB CPU = 17,321s, library cache: mutex X wait = 38,892s,avg. wait = 1ms

    针对高latch:shared pool、latch: shared pool simulator、Library Cache - Mutex X解析类等待事件，解决的思路包括：

1. 升级到最新的Patch set + PSU
2. 考虑cursor_sharing=FORCE
3. 注意即使_optim_peek_user_binds=false，若你的SQL本身还是有硬绑定的自由变量，则dc_histogram仍可能是硬解析争用的焦点
4. 设置较大的 session_cachced_cursor和instantiation
5. 设置library_cache_advice=false
6. 关闭11g中的ACS自适应游标特性
7. 关闭11g中的cardinality feedback特性
8. 使用MSSM，或者 ASMM下 _memory_broker_shrink_heaps=0 +　_enable_shared_pool_durations＝false