Know GCS AND GES structure size in shared pool

RAC环境中共享池很大一部分被gcs和ges资源所占用,一般来说这些资源对象都是永久的(perm)的,所以我们无法期待LRU或flush shared_pool操作能够清理这些资源。

在使用大缓存(large buffer cache)的RAC实例环境中,查询v$sgastat内存动态性能视图时总是能发现’gcs resources’、’gcs shadows’、’ ges resource’、’ges enqueues ‘这些组件占用了共享池中的大量内存,为了避免shared pool出现著名的ORA-04031错误,Oracle推荐在RAC环境中设置较大的shared_pool_size初始化参数,此外显示地设置较大的GCS和GES资源结构的初始化分配数(INITIAL_ALLOCATION)也有利于避免ORA-4031。

这些控制GES和GCS资源结构初始化分配数量的参数主要包括:

  • _gcs_resources  number of gcs resources to be allocated GCS Resources Number of GCS resource structures determined by
    _gcs_resources parameter
    Stored in segmented array
    Externalized in X$KJBR
    Number of free GCS resource structures in X$KJBRFX
  • _gcs_shadow_locks number of pcm shadow locks to be allocated GCS Enqueues (Shadows/Clients) Number of GCS enqueue structures determined by  _gcs_shadow_locks parameter Stored in segmented array
    Externalized in X$KJBL
    Number of free GCS enqueue structures in X$KJBLFX
  • _lm_ress number of resources configured for cluster database LM_RESS controls the number of resources that can be locked by each lock manager instance. These resources include lock resources allocated for DML, DDL (data dictionary locks), data dictionary, and library cache locks plus the file and log management locks. Stored in heap
    Externalized in X$KJIRFT
  • _lm_locks number of enqueues configured for cluster database Stored in segmented array
    Externalized in X$KJILKFT

为了更好地在RAC环境中设置shared_pool_size共享池的大小(手动设置该参数并不会disable AMM or ASMM),我们很有必要评估大量初始化分配的全局资源本身将占用shared pool多大的空间。

我们可以通过v$resource_limit视图了解这些GES、GCS全局资源的分配情况:

SQL> select * from v$version;

BANNER
--------------------------------------------------------------------------------
Oracle Database 11g Enterprise Edition Release 11.2.0.2.0 - 64bit Production
PL/SQL Release 11.2.0.2.0 - Production
CORE    11.2.0.2.0      Production
TNS for Linux: Version 11.2.0.2.0 - Production
NLSRTL Version 11.2.0.2.0 - Production

SQL> select * from global_name;

GLOBAL_NAME
--------------------------------------------------------------------------------
www.askmaclean.com



SQL> select * from v$resource_limit where resource_name in ('gcs_resources', 'gcs_shadows','ges_ress','ges_locks'); 

RESOURCE_NAME                  CURRENT_UTILIZATION MAX_UTILIZATION INITIAL_ALLOCATION        LIMIT_VALUE
------------------------------ ------------------- --------------- ------------------------- ------------------
ges_ress                                      7223            7486    1000000                 UNLIMITED
ges_locks                                     4944            5027    1000000                 UNLIMITED
gcs_resources                                 4021            4021     114466                    114466
gcs_shadows                                   3925            3925     114466                    114466

可以通过v$sgastat视图了解这些全局资源占用了多少空间:

select *
  from v$sgastat
 where name in
       ('ges resource ', 'ges enqueues', 'gcs resources', 'gcs shadows');

POOL         NAME                            BYTES
------------ -------------------------- ----------
shared pool  gcs resources                16483232
shared pool  gcs shadows                  11904560
shared pool  ges enqueues                 47809680
shared pool  ges resource                288405768

单个gcs_resources结构大约占用120 bytes
单个gcs_shadows 结构大约占用72 bytes
单个ges_resource 结构大约占用288 bytes

我们可以使用一下初步估算GES、GCS资源结构将至少占用多大的共享池资源:

‘gcs_resources’ = initial_allocation * 120 bytes = “_gcs_resources parameter” * 120 bytes
‘gcs_shadows’ = initial_allocation * 72 bytes = “_gcs_shadow_locks parameter” * 72 bytes
‘ges_resource’= initial_allocation * 288 bytes = “_lm_ress parameter ” * 288 bytes

注意这里计算出的仅仅是理论的最小值,实际值因为内存分配的机制所以必然会远大于计算值

如上例中 gcs resources = 114466 * 120 =13735920 << 实际值的16483232
gcs_shadows = 114466 * 72 = 8241552 << 实际值的11904560
ges_resource = 1000000 * 288 = 288000000 < 实际的288405768

一般来说我们将计算值 * 160% 后可以得出一个较为客观的估算值。

注意以上公式只是为我们在RAC环境中调优共享池的大小提供参考的依据。当我们观察v$resource_limit视图并认为需要提高GES、GSC资源的初始化分配数目时,可以参照上述方式估算出必要的shared_pool_size或sga_target大小。

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