标签：oracle
Oracle Share Pool内部管理机制
SHARE POOL利用堆（HEAP)的内存管理方式管理，在物理上由多个内存区（EXTENT)组成，内存区又由多个不同大小的CHUNK组成。而CHUNK又有可重用和空闲之分，并且它们分别有LRU LIST、FREE LIST、RESERVED LIST串联起来。
堆管理
Shared Pool是利用堆内存管理方式管理的（KGH:Kernel Generic Heap).从Oracle 9i开始，可以有多个最高级堆（TOP-LEVLE HEAP)，最高级堆可以分成多个副堆，副堆下面还拥有子堆。堆和副对结构基本相同。从物理上讲，一个堆由多个内存区已link list的形式连接组成。一个内存区物理上使用一个Granule，一个内存区由多个chunk组成，所以chunk是heap的最小内存单位。
Chunk的使用情况可由X$KSMSP内部视图查看。每个堆头上则包含了可使用的chunk列表和已使用的chunk列表。通过dump heap命令可以在trace文件中观察heap和extent的关系。
alter system set events ‘immediate trace name heapdump level 2‘;
案例：
SQL> select name,bytes/1024/1024 from v$sgainfo; NAME BYTES/1024/1024 -------------------------------- --------------- Fixed SGA Size 1.27566528 Redo Buffers 5.59765625 Buffer Cache Size 180 Shared Pool Size 104 Large Pool Size 4 Java Pool Size 4 Streams Pool Size 0 Shared IO Pool Size 0 Granule Size 4 Maximum SGA Size 498.875 Startup overhead in Shared Pool 52 NAME BYTES/1024/1024 -------------------------------- --------------- Free SGA Memory Available 200 12 rows selected. [oracle@node1 ~]$ more /u01/app/oracle/diag/rdbms/prod/prod/trace/prod_ora_14983.trc Trace file /u01/app/oracle/diag/rdbms/prod/prod/trace/prod_ora_14983.trc Oracle Database 11g Enterprise Edition Release 11.2.0.1.0 - Production With the Partitioning, OLAP, Data Mining and Real Application Testing options ORACLE_HOME = /u01/app/oracle/product/11.2.0/db_1 System name: Linux Node name: node1 Release: 2.6.18-194.el5 Version: #1 SMP Tue Mar 16 21:52:43 EDT 2010 Machine: i686 Instance name: prod Redo thread mounted by this instance: 1 Oracle process number: 32 Unix process pid: 14983, image: oracle@node1 (TNS V1-V3) *** 2014-07-02 14:58:47.532 *** SESSION ID:(32.758) 2014-07-02 14:58:47.532 *** CLIENT ID:() 2014-07-02 14:58:47.532 *** SERVICE NAME:(SYS$USERS) 2014-07-02 14:58:47.532 *** MODULE NAME:(sqlplus@node1 (TNS V1-V3)) 2014-07-02 14:58:47.532 *** ACTION NAME:() 2014-07-02 14:58:47.532 KGH Latch Directory Information ldir state: 2 last allocated slot: 99 Slot [ 1] Latch: 0x200065ec Index: 1 Flags: 3 State: 2 next: (nil) Slot [ 2] Latch: 0x3e75571c Index: 1 Flags: 3 State: 2 next: (nil) ......(省略） Slot [ 99] Latch: 0x2002616c Index: 1 Flags: 3 State: 2 next: 0x1 ****************************************************** HEAP DUMP heap name="sga heap" desc=0x200010b4 extent sz=0x7ad4 alt=124 het=32767 rec=9 flg=-126 opc=0 parent=(nil) owner=(nil) nex=(nil) xsz=0x0 heap=(nil) fl2=0x60, nex=(nil) ds for latch 1: 0x2002a990 0x2002b5c8 0x2002c200 0x2002ce38 reserved granule count 0 (granule size 4194304) ****************************************************** HEAP DUMP heap name="sga heap(1,0)" desc=0x2002a990 extent sz=0xfc4 alt=124 het=32767 rec=9 flg=-126 opc=0 parent=(nil) owner=(nil) nex=(nil) xsz=0x400000 heap=(nil) fl2=0x20, nex=(nil) latch set 1 of 1 durations enabled for this heap reserved granules for root 0 (granule size 4194304) EXTENT 0 addr=0x3b800000 Chunk 3b800038 sz= 24 R-freeable "reserved stoppe" Chunk 3b800050 sz= 212888 R-free " " Chunk 3b833fe8 sz= 24 R-freeable "reserved stoppe" Chunk 3b834000 sz= 3981312 perm "perm " alo=3290272 EXTENT 1 addr=0x3bc00000 Chunk 3bc00038 sz= 24 R-freeable "reserved stoppe" Chunk 3bc00050 sz= 212888 R-free " " Chunk 3bc33fe8 sz= 24 R-freeable "reserved stoppe" Chunk 3bc34000 sz= 3510272 perm "perm " alo=3510272 Chunk 3bf8d000 sz= 465920 perm "perm " alo=465920 Chunk 3bffec00 sz= 5120 free " " EXTENT 2 addr=0x3c000000 Chunk 3c000038 sz= 24 R-freeable "reserved stoppe" Chunk 3c000050 sz= 212888 R-free " " Chunk 3c033fe8 sz= 24 R-freeable "reserved stoppe" Chunk 3c034000 sz= 3850176 perm "perm " alo=3850176 Chunk 3c3dffc0 sz= 131088 perm "perm " alo=131088 Chunk 3c3fffd0 sz= 48 free " " EXTENT 3 addr=0x3c400000 Chunk 3c400038 sz= 24 R-freeable "reserved stoppe" Chunk 3c400050 sz= 212888 R-free " " Chunk 3c433fe8 sz= 24 R-freeable "reserved stoppe" Chunk 3c434000 sz= 3980368 perm "perm " alo=3980368 Chunk 3c7ffc50 sz= 944 free " " EXTENT 4 addr=0x3c800000 Chunk 3c800038 sz= 24 R-freeable "reserved stoppe" Chunk 3c800050 sz= 212888 R-free " " Chunk 3c833fe8 sz= 24 R-freeable "reserved stoppe" Chunk 3c834000 sz= 3672700 perm "perm " alo=3672700 Chunk 3cbb4a7c sz= 308240 perm "perm " alo=308240 Chunk 3cbffe8c sz= 372 free " "
CHUNK
Chunk是以链条（chain）的方式存在于内存区，每个chunk包含header和body两部分，chunk的状态大体上可以分为free（可立即使用）、recreatable（可再生）、freeable（只有在session或call期间内保存必要的对象状态）、permanent（永久、不可再生）等，其中free和recreatable状态下的chunk可以重复使用。
FREE LIST
Free list主要用于管理空闲的chunk，并且是用bucket管理的。从Oracle 9i开始，一个heap总共255个bucket，bucket所包含的free chunk大小随bucket的编号增加而递增，个bucket下的free chunk已linked list的形式链接。
（1）获得shared pool latch在free list中查找合适大小的空闲chunk。如果在获取shared pool latch时发生了争用，则会出现latch：shared pool等待事件。这时Oracle会一直持有shared pool latch，直到获得所需的内存为止。所在内存碎片化比较严重的shared pool中，进程持有shared pool latch的时间也会相应变长。
（2）如果不存在合适大小的空闲chunk，则在查找到更大的空闲chunk后分割（split）使用，分割后剩下的内存区域则重新登记到free list中。分割内存意味着内存中的碎片开始增多。由于每个cursor所需内存的大小不同，所以shared pool的空闲内存不像buffer cache中的空闲内存一样具有固定大小。
（3）如果检索了free list也没有找到所有合适的空闲chunk，则检索lru list。lru list上的chunk是重建（recreatable）的，而且是当前不使用的（没有处于pin状态）。
（4）如果在lru list上也没有找到合适的chunk，且所请求的内存还没有达到隐含参数_shared_pool_reserved_min_alloc的阈值，则追加分配share pool中剩余的内存空间。
（5）如果以上请求的内存均失败，则出现ORA-4031错误
FREE LISTS: Bucket 0 size=16 Bucket 1 size=20 Bucket 2 size=24 Bucket 3 size=28 Bucket 4 size=32 Bucket 5 size=36 Bucket 6 size=40 Bucket 7 size=44 Bucket 8 size=48 Chunk 3c3fffd0 sz= 48 free " " Chunk 3ebfffd0 sz= 48 free " " Bucket 9 size=52 Bucket 10 size=56 Bucket 11 size=60 Bucket 12 size=64 Bucket 13 size=68 Bucket 14 size=72 Bucket 15 size=76 Bucket 16 size=80 Bucket 17 size=84 Bucket 18 size=88 Chunk 3e7fffa8 sz= 88 free " " Bucket 19 size=92 Bucket 20 size=96 Bucket 21 size=100 Bucket 22 size=104 Bucket 23 size=108 Bucket 24 size=112 Bucket 25 size=116 Bucket 26 size=120 Bucket 27 size=124 ......
LRU LIST
Lru list主要保存着当前未使用而且可以重建的chunk。当会话在free list中找不到空闲的chunk时，在会在lru list中寻找。
UNPINNED RECREATABLE CHUNKS (lru first): Chunk 3b1f4000 sz= 4096 recreate "CCUR^3de2d7f5 " latch=(nil) Chunk 3a649000 sz= 4096 recreate "CCUR^f5a8106a " latch=(nil) Chunk 383b3000 sz= 4096 recreate "CCUR^2b855a04 " latch=(nil) Chunk 383b1000 sz= 4096 recreate "CCUR^f65cc3ed " latch=(nil) Chunk 3ee4c524 sz= 332 recreate "KGLHD " latch=(nil) Chunk 3af76da8 sz= 4096 recreate "SQLA^13df1501 " latch=(nil) Chunk 3a72e000 sz= 4096 recreate "CCUR^13df1501 " latch=(nil) Chunk 39ff96e0 sz= 332 recreate "KGLHD " latch=(nil) Chunk 3a37e740 sz= 4096 recreate "SQLA^7a9f4a60 " latch=(nil) Chunk 38330000 sz= 4096 recreate "CCUR^7a9f4a60 " latch=(nil) Chunk 39f8732c sz= 332 recreate "KGLHD " latch=(nil) Chunk 3ab0dec4 sz= 4096 recreate "SQLA^8005cf3c " latch=(nil) Chunk 3a670000 sz= 4096 recreate "CCUR^8005cf3c " latch=(nil) Chunk 3ed28b1c sz= 1524 recreate "KGLHD " latch=(nil) Chunk 39cb0d20 sz= 332 recreate "KGLHD " latch=(nil) Chunk 3aaeb214 sz= 4096 recreate "SQLA^63c15ff " latch=(nil) Chunk 3a646000 sz= 4096 recreate "CCUR^63c15ff " latch=(nil) Chunk 39ff9a38 sz= 540 recreate "KQR PO " latch=0x3ca28438 Chunk 3ee4c88c sz= 540 recreate "KQR PO " latch=0x3ca28438 Chunk 3ee4caa8 sz= 540 recreate "KQR PO " latch=0x3ca28438 Chunk 39fabd2c sz= 540 recreate "KQR PO " latch=0x3ca28438 Chunk 39fabf48 sz= 540 recreate "KQR PO " latch=0x3ca28438 Chunk 3ee481f8 sz= 540 recreate "KQR PO " latch=0x3ca28438 Chunk 3ee48414 sz= 540 recreate "KQR PO " latch=0x3ca28438 Chunk 3ee48630 sz= 540 recreate "KQR PO " latch=0x3ca28438 Chunk 3a1a1228 sz= 4096 recreate "KGLS^9b4819a8 " latch=(nil) Chunk 39b4a504 sz= 4096 recreate "KGLS^6ccccfe " latch=(nil) Chunk 3abf0fb4 sz= 4096 recreate "KGLS^8db54da " latch=(nil) Chunk 3aa65508 sz= 4096 recreate "KGLS^e2d7481e " latch=(nil) Chunk 3a2839d0 sz= 4096 recreate "KGLS^8dd845ff " latch=(nil) Chunk 3abd4f78 sz= 4096 recreate "KGLS^f79d229a " latch=(nil)
RESERVED FREE LIST
Oracle 设置了保留内存区域，该区域用 Reserved free list管理。其大小由参数shared_pool_reserved_size决定（最小为5000字节，最大不能超过shared pool的50%）。
RESERVED FREE LISTS: Reserved bucket 0 size=16 Chunk 3dfffc14 sz= 980 R-free " " Chunk 3dbffbe0 sz= 1032 R-free " " Reserved bucket 1 size=4400 Reserved bucket 2 size=8204 Reserved bucket 3 size=8460 Reserved bucket 4 size=8464 Reserved bucket 5 size=8468 Reserved bucket 6 size=8472 Reserved bucket 7 size=9296 Reserved bucket 8 size=9300 Reserved bucket 9 size=12320 Reserved bucket 10 size=12324 Reserved bucket 11 size=16396 Reserved bucket 12 size=32780 Reserved bucket 13 size=65548 Chunk 3b800050 sz= 212888 R-free " " Chunk 3bc00050 sz= 212888 R-free " " Chunk 3c000050 sz= 212888 R-free " " Chunk 3c400050 sz= 212888 R-free " " Chunk 3c800050 sz= 212888 R-free " " Chunk 3cc00050 sz= 212888 R-free " " Chunk 3d3cb8b8 sz= 214832 R-free " " Chunk 3d400050 sz= 212888 R-free " " Chunk 3e000050 sz= 212888 R-free " " Chunk 3e400050 sz= 212888 R-free " " Chunk 3e800050 sz= 212888 R-free " " Chunk 3f000050 sz= 212888 R-free " " Reserved bucket 14 size=1990630 Total reserved free space = 2558612 UNPINNED RECREATABLE CHUNKS (lru first): Chunk 3b1f4000 sz= 4096 recreate "CCUR^3de2d7f5 " latch=(nil) Chunk 3a649000 sz= 4096 recreate "CCUR^f5a8106a " latch=(nil) ......
Oracle对进入reserved free list的对象大小有限制，即只有大于_shared_pool_reserved_min_alloc隐含参数阈值（默认值4400）的cursor才能进入到reserved free list。
本文出自 “天涯客的blog” 博客，请务必保留此出处http://tiany.blog.51cto.com/513694/1433452
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标签：oracle
原文地址：http://tiany.blog.51cto.com/513694/1433452
