kgl simulator,共享池simulator以及缓冲simulator相关解释

从9i开始学习的朋友一定习惯了v$sgastat视图中 一系列的sim项,主要包括各种 kglsim,sim cache segment trace等等。如下例:

SQL> select * from v$sgastat where lower(name) like '%sim%' order by name;

shared pool  kglsim count of pinned he        1920
shared pool  kglsim free heap list              96
shared pool  kglsim free obj list               96
shared pool  kglsim hash table                8208
shared pool  kglsim hash table bkts        4194304
shared pool  kglsim heap                    782208
shared pool  kglsim main lru count           76800
shared pool  kglsim main lru size           151040
shared pool  kglsim object batch           1163520
shared pool  kglsim pin list arr               288
shared pool  kglsim recovery area             1920
shared pool  kglsim sga                      30552
shared pool  kglsim size of pinned mem        3776

shared pool  ksim client list                  168
shared pool  log_simultaneous_copies           800
shared pool  sim cache nbufs                   640
shared pool  sim cache sizes                   640
shared pool  sim kghx free lists                 8
shared pool  sim lru segments                 1280
shared pool  sim segment hits                 1280
shared pool  sim segment num bufs              640
shared pool  sim state object                   40
shared pool  sim trace buf                    5144
shared pool  sim trace buf context             200
shared pool  sim_knlasg                       1280
shared pool  simulator hash buckets         131328

shared pool  simulator hash latch             5120
shared pool  simulator latch/bucket st        2816

28 rows selected.

有时你也会发现存在sim相关的latch,

SQL> select name from v$latch where name like '%sim%';

sim partition latch
simulator hash latch
simulator lru latch

显然Oracle实例中存在着这许多的模拟器(simulator),他们是 干啥的?

这些模拟器帮助Oracle实例获知当拥有更大的内存容量(涉及buffer_cache,shared_pool_size等)时可以节省多少时间和功。这些信息被汇总到几个常用视图中,如 V$SHARED_POOL_ADVICE, V$MTTR_TARGET_ADVICE, V$DB_CACHE_ADVICE等等。模拟器特性从9i开始被引入,也就是这些视图第一次出现的时候。

MMAN进程也利用这些信息作为SGA动态调整的基本依据(在sga_target或memory_target启用的情况下)。

KGL simulator会跟踪共享池内存分配并扮演模拟器引擎的角色。

KGL是 kernel general Library cache manager,它管理library cache中的对象 包括游标,PL/SQL存储过程,表的定义等等。

KGL simulator 评估当缓存大于目前情况的获益。 其通过将flush out出library cache的object的hash code保存至kgl simulator的 hash 列表中,以造成object似乎仍在缓存中的假象,方便其统计相关的数据。

当有对象被读入library cache中时,Oracle检查kgl simulator中的hash列表,若命中说明历史hash列表中的对象对当前实例仍有用,即更大的内存空间对实例来说是有益的。

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Comments

  1. admin says:

    EXCESSIVE GETS FOR SHARED POOL SIMULATOR LATCH causing hang/performance problem
    Applies to:

    Oracle Server – Enterprise Edition – Version: 10.2.0.1 to 11.1.0.6 – Release: 10.2 to 11.1
    Oracle Server – Standard Edition – Version: 10.2.0.1 to 11.1.0.6 [Release: 10.2 to 11.1]
    Information in this document applies to any platform.
    ***Checked for relevance on 04-Aug-2010***

    Oracle Real Application Cluster instances
    Symptoms

    This issue can cause hangs or Performance issues.

    Performance Issue Symptoms
    AWR/statspack shows high number of shared pool simulator latch gets. For example 577 million “shared pool simulator” latch gets in one hour

    Hang Issue Symptoms
    Hanganalyze or Systemstate dumps with short stacks shows If kglsim_scan_lru or kghscan_sim_lru in the stack

    Changes

    None
    Cause

    This is caused by Unpublished bug 6966286
    Solution

    Check if the fix for unpublished bug 6966286 is available. Customer can also use workaround of setting _library_cache_advice = false.

  2. admin says:

    Bug 6879763 – Excessive gets for “shared pool simulator” latch
    Description

    With library cache advice enabled very high gets of the
    “shared pool simulator” latch may be seen compared to
    the number of gets of the “shared pool” latch.

    Workaround:
    Set “_library_cache_advice” = false at instance level,
    or
    use STATISTICS_LEVEL=BASIC (which implicity disables library
    cache advice but also disables other metrics also)
    Note: This fix introduces the problem described in bug 6966286.If this patch is needed ensure that you also have the fix for bug 6966286 .

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