有同学在T.askmaclean.com上求助Shared Pool里SubPool子分区问题，这里我们来搞清楚_kghdsidx_count 与 subpool 以及subpool中的分区( 实际是duration)的关系：
SQL> select * from v$version;
BANNER
----------------------------------------------------------------
Oracle Database 10g Enterprise Edition Release 10.2.0.5.0 - 64bi
PL/SQL Release 10.2.0.5.0 - Production
CORE 10.2.0.5.0 Production
TNS for Linux: Version 10.2.0.5.0 - Production
NLSRTL Version 10.2.0.5.0 - Production
SQL> set linesize 200 pagesize 1400
SQL> show parameter kgh
NAME TYPE VALUE
------------------------------------ -------------------------------- ------------------------------
_kghdsidx_count integer 7
SQL> oradebug setmypid;
Statement processed.
SQL> oradebug dump heapdump 536870914;
Statement processed.
SQL> oradebug tracefile_name
/s01/admin/G10R25/udump/g10r25_ora_11783.trc
[oracle@vrh8 dbs]$ grep "sga heap" /s01/admin/G10R25/udump/g10r25_ora_11783.trc
HEAP DUMP heap name="sga heap" desc=0x60000058
HEAP DUMP heap name="sga heap(1,0)" desc=0x60036110
FIVE LARGEST SUB HEAPS for heap name="sga heap(1,0)" desc=0x60036110
HEAP DUMP heap name="sga heap(2,0)" desc=0x6003f938
FIVE LARGEST SUB HEAPS for heap name="sga heap(2,0)" desc=0x6003f938
HEAP DUMP heap name="sga heap(3,0)" desc=0x60049160
FIVE LARGEST SUB HEAPS for heap name="sga heap(3,0)" desc=0x60049160
HEAP DUMP heap name="sga heap(4,0)" desc=0x60052988
FIVE LARGEST SUB HEAPS for heap name="sga heap(4,0)" desc=0x60052988
HEAP DUMP heap name="sga heap(5,0)" desc=0x6005c1b0
FIVE LARGEST SUB HEAPS for heap name="sga heap(5,0)" desc=0x6005c1b0
HEAP DUMP heap name="sga heap(6,0)" desc=0x600659d8
FIVE LARGEST SUB HEAPS for heap name="sga heap(6,0)" desc=0x600659d8
HEAP DUMP heap name="sga heap(7,0)" desc=0x6006f200
FIVE LARGEST SUB HEAPS for heap name="sga heap(7,0)" desc=0x6006f200
SQL> alter system set "_kghdsidx_count"=6 scope=spfile;
System altered.
SQL> startup force;
ORACLE instance started.
Total System Global Area 859832320 bytes
Fixed Size 2100104 bytes
Variable Size 746587256 bytes
Database Buffers 104857600 bytes
Redo Buffers 6287360 bytes
Database mounted.
Database opened.
SQL>
SQL> oradebug setmypid;
Statement processed.
SQL> oradebug dump heapdump 536870914;
Statement processed.
SQL> oradebug tracefile_name
/s01/admin/G10R25/udump/g10r25_ora_11908.trc
[oracle@vrh8 dbs]$ grep "sga heap" /s01/admin/G10R25/udump/g10r25_ora_11908.trc
HEAP DUMP heap name="sga heap" desc=0x60000058
HEAP DUMP heap name="sga heap(1,0)" desc=0x600360f0
FIVE LARGEST SUB HEAPS for heap name="sga heap(1,0)" desc=0x600360f0
HEAP DUMP heap name="sga heap(2,0)" desc=0x6003f918
FIVE LARGEST SUB HEAPS for heap name="sga heap(2,0)" desc=0x6003f918
HEAP DUMP heap name="sga heap(3,0)" desc=0x60049140
FIVE LARGEST SUB HEAPS for heap name="sga heap(3,0)" desc=0x60049140
HEAP DUMP heap name="sga heap(4,0)" desc=0x60052968
FIVE LARGEST SUB HEAPS for heap name="sga heap(4,0)" desc=0x60052968
HEAP DUMP heap name="sga heap(5,0)" desc=0x6005c190
FIVE LARGEST SUB HEAPS for heap name="sga heap(5,0)" desc=0x6005c190
HEAP DUMP heap name="sga heap(6,0)" desc=0x600659b8
FIVE LARGEST SUB HEAPS for heap name="sga heap(6,0)" desc=0x600659b8
SQL>
SQL> alter system set "_kghdsidx_count"=2 scope=spfile;
System altered.
SQL>
SQL> startup force;
ORACLE instance started.
Total System Global Area 851443712 bytes
Fixed Size 2100040 bytes
Variable Size 738198712 bytes
Database Buffers 104857600 bytes
Redo Buffers 6287360 bytes
Database mounted.
Database opened.
SQL> oradebug setmypid;
Statement processed.
SQL> oradebug dump heapdump 2;
Statement processed.
SQL> oradebug tracefile_name
/s01/admin/G10R25/udump/g10r25_ora_12003.trc
[oracle@vrh8 ~]$ grep "sga heap" /s01/admin/G10R25/udump/g10r25_ora_12003.trc
HEAP DUMP heap name="sga heap" desc=0x60000058
HEAP DUMP heap name="sga heap(1,0)" desc=0x600360b0
HEAP DUMP heap name="sga heap(2,0)" desc=0x6003f8d
SQL> alter system set cpu_count=16 scope=spfile;
System altered.
SQL> startup force;
ORACLE instance started.
Total System Global Area 851443712 bytes
Fixed Size 2100040 bytes
Variable Size 738198712 bytes
Database Buffers 104857600 bytes
Redo Buffers 6287360 bytes
Database mounted.
Database opened.
SQL> oradebug setmypid;
Statement processed.
SQL> oradebug dump heapdump 2;
Statement processed.
SQL> oradebug tracefile_name
/s01/admin/G10R25/udump/g10r25_ora_12065.trc
[oracle@vrh8 ~]$ grep "sga heap" /s01/admin/G10R25/udump/g10r25_ora_12065.trc
HEAP DUMP heap name="sga heap" desc=0x60000058
HEAP DUMP heap name="sga heap(1,0)" desc=0x600360b0
HEAP DUMP heap name="sga heap(2,0)" desc=0x6003f8d8
SQL> show parameter sga_target
NAME TYPE VALUE
------------------------------------ -------------------------------- ------------------------------
sga_target big integer 0
SQL> alter system set sga_target=1000M scope=spfile;
System altered.
SQL> startup force;
ORACLE instance started.
Total System Global Area 1048576000 bytes
Fixed Size 2101544 bytes
Variable Size 738201304 bytes
Database Buffers 301989888 bytes
Redo Buffers 6283264 bytes
Database mounted.
Database opened.
SQL> alter system set sga_target=1000M scope=spfile;
System altered.
SQL> startup force;
ORACLE instance started.
Total System Global Area 1048576000 bytes
Fixed Size 2101544 bytes
Variable Size 738201304 bytes
Database Buffers 301989888 bytes
Redo Buffers 6283264 bytes
Database mounted.
Database opened.
SQL>
SQL>
SQL> oradebug setmypid;
Statement processed.
SQL> oradebug dump heapdump 2;
Statement processed.
SQL> oradebug tracefile_name
/s01/admin/G10R25/udump/g10r25_ora_12148.trc
SQL>
SQL> Disconnected from Oracle Database 10g Enterprise Edition Release 10.2.0.5.0 - 64bit Production
With the Partitioning, OLAP, Data Mining and Real Application Testing options
[oracle@vrh8 dbs]$ grep "sga heap" /s01/admin/G10R25/udump/g10r25_ora_12148.trc
HEAP DUMP heap name="sga heap" desc=0x60000058
HEAP DUMP heap name="sga heap(1,0)" desc=0x60036690
HEAP DUMP heap name="sga heap(1,1)" desc=0x60037ee8
HEAP DUMP heap name="sga heap(1,2)" desc=0x60039740
HEAP DUMP heap name="sga heap(1,3)" desc=0x6003af98
HEAP DUMP heap name="sga heap(2,0)" desc=0x6003feb8
HEAP DUMP heap name="sga heap(2,1)" desc=0x60041710
HEAP DUMP heap name="sga heap(2,2)" desc=0x60042f68
_enable_shared_pool_durations:该参数控制是否启用10g中特有的shared pool duration特性，当我们设置sga_target为0时该参数为false；
同时在10.2.0.5前若cursor_space_for_time设置为true时该参数也为false，不过在10.2.0.5以后cursor_space_for_time参数被废弃
SQL> alter system set "_enable_shared_pool_durations"=false scope=spfile;
System altered.
SQL>
SQL> startup force;
ORACLE instance started.
Total System Global Area 1048576000 bytes
Fixed Size 2101544 bytes
Variable Size 738201304 bytes
Database Buffers 301989888 bytes
Redo Buffers 6283264 bytes
Database mounted.
Database opened.
SQL> oradebug setmypid;
Statement processed.
SQL> oradebug dump heapdump 2;
Statement processed.
SQL> oradebug tracefile_name
/s01/admin/G10R25/udump/g10r25_ora_12233.trc
SQL>
SQL> Disconnected from Oracle Database 10g Enterprise Edition Release 10.2.0.5.0 - 64bit Production
With the Partitioning, OLAP, Data Mining and Real Application Testing options\
[oracle@vrh8 dbs]$ grep "sga heap" /s01/admin/G10R25/udump/g10r25_ora_12233.trc
HEAP DUMP heap name="sga heap" desc=0x60000058
HEAP DUMP heap name="sga heap(1,0)" desc=0x60036690
HEAP DUMP heap name="sga heap(2,0)" desc=0x6003feb8
结论：
1.
_kghdsidx_count 控制了 shared pool subpool的数量， _kghdsidx_count的最大允许值是7 即最多 7个 shared pool subpool
2.
为什么会在一个 subpool中还有4个 sub partition 如：
sga heap(1,0) sga heap(1,1) sga heap(1,2) sga heap(1,3)
这不是因为 cpu的数目 也不是因为_kghdsidx_count， 而是因为 在10g 中AUTO SGA 引入了 shared pool duration的概念，
duration 分成4类：
- Session duration
- Instance duration (never freed)
- Execution duration (freed fastest)
- Free memory
引入了 shared pool duration的目的是
在10gR1中Shared Pool的shrink收缩操作存在一些缺陷，造成缺陷的原因是在该版本中Buffer Cache还没有能力共享使用一个granule，这是因为Buffer Cache的granule的尾部由granule header和Metadata(可能是buffer header或者RAC中的Lock Elements)拼接组成，在其尾部不容许存在空洞。另一个原因是当时的shared pool允许不同生命周期duration(以后会介绍)的chunk存放在同一个granule中，这造成共享池无法完全释放granule。到10gR2中通过对Buffer Cache Granule结构的修改允许在granule header和buffer及Metadata(buffer header或LE)存在缝隙，同时shared pool中不同duration的chunk将不在共享同一个granule，通过以上改进buffer cache与shared pool间的内存交换变得可行。此外在10gr2中streams pool也开始支持内存交换(实际根据不同的streams pool duration存在限制)
reference : http://www.oracledatabase12g.com/archives/understanding-automatic-sga-memory-management.html