我们继续把前面的问题展开一下.
其实我们可以从数据库内部监控shared pool的空间碎片情况.
这涉及到一个内部视图x$ksmsp
X$KSMSP的名称含义为: [K]ernal [S]torage [M]emory Management [S]GA Hea[P]
其中每一行都代表着shared pool中的一个chunk
首先记录一下测试环境：
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SQL> select * from v$version;
BANNER
----------------------------------------------------------------
Oracle9i Enterprise Edition Release 9.2.0.3.0 - Production
PL/SQL Release 9.2.0.3.0 - Production
CORE 9.2.0.3.0 Production
TNS for Linux: Version 9.2.0.3.0 - Production
NLSRTL Version 9.2.0.3.0 - Production
我们看一下x$ksmsp的结构:
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SQL> desc x$ksmsp
Name Null? Type
----------------------------------------- -------- ----------------------------
ADDR RAW(4)
INDX NUMBER
INST_ID NUMBER
KSMCHIDX NUMBER
KSMCHDUR NUMBER
KSMCHCOM VARCHAR2(16)
KSMCHPTR RAW(4)
KSMCHSIZ NUMBER
KSMCHCLS VARCHAR2(8)
KSMCHTYP NUMBER
KSMCHPAR RAW(4)
我们关注以下几个字段:
KSMCHCOM是注释字段,每个内存块被分配以后，注释会添加在该字段中.
x$ksmsp.ksmchsiz代表块大小
x$ksmsp.ksmchcls列代表类型，主要有四类，说明如下:
free
Free chunks--不包含任何对象的chunk,可以不受限制的被分配.
recr
Recreatable chunks--包含可以被临时移出内存的对象,在需要的时候，这个对象可以被重新创建.例如,许多存储共享sql代码的内存都是可以重建的.
freeabl
Freeable chunks--包含session周期或调用的对象,随后可以被释放.这部分内存有时候可以全部或部分提前释放.但是注意，由于某些对象是中间过程产生的，这些对象不能临时被移出内存(因为不可重建).
perm
Permanent memory chunks--包含永久对象.通常不能独立释放.
我们可以通过查询x$ksmsp视图来考察shared pool中存在的内存片的数量
不过注意:Oracle的某些版本(如:10.1.0.2)在某些平台上(如:HP-UX PA-RISC 64-bit)查询该视图可能导致过度的CPU耗用,这是由于bug引起的.
我们看一下测试:
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初始启动数据库,x$ksmsp中存在2259个chunk
SQL> select count(*) from x$ksmsp;
COUNT(*)
----------
2259
执行查询:
SQL> select count(*) from dba_objects;
COUNT(*)
----------
10491
此时shared pool中的chunk数量增加
SQL> select count(*) from x$ksmsp;
COUNT(*)
----------
2358
这就是由于shared pool中进行sql解析，请求空间，进而导致请求free空间,分配、分割从而产生了更多，更细碎的内存chunk
由此我们可以看出，如果数据库系统中存在大量的硬解析，不停请求分配free的shred pool内存除了必须的shared pool latch等竞争外，还不可避免的会导致shared pool中产生更多的内存碎片(当然，在内存回收时，你可能看到chunk数量减少的情况)
我们看以下测试:
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首先重新启动数据库:
SQL> startup force;
ORACLE instance started.
Total System Global Area 47256168 bytes
Fixed Size 451176 bytes
Variable Size 29360128 bytes
Database Buffers 16777216 bytes
Redo Buffers 667648 bytes
Database mounted.
Database opened.
创建一张临时表用以保存之前x$ksmsp的状态:
SQL> CREATE GLOBAL TEMPORARY TABLE e$ksmsp ON COMMIT PRESERVE ROWS AS
2 SELECT a.ksmchcom,
3 SUM (a.CHUNK) CHUNK,
4 SUM (a.recr) recr,
5 SUM (a.freeabl) freeabl,
6 SUM (a.SUM) SUM
7 FROM (SELECT ksmchcom, COUNT (ksmchcom) CHUNK,
8 DECODE (ksmchcls, 'recr', SUM (ksmchsiz), NULL) recr,
9 DECODE (ksmchcls, 'freeabl', SUM (ksmchsiz), NULL) freeabl,
10 SUM (ksmchsiz) SUM
11 FROM x$ksmsp GROUP BY ksmchcom, ksmchcls) a
12 where 1 = 0
13 GROUP BY a.ksmchcom;
Table created.
保存当前shared pool状态:
SQL> INSERT INTO E$KSMSP
2 SELECT a.ksmchcom,
3 SUM (a.CHUNK) CHUNK,
4 SUM (a.recr) recr,
5 SUM (a.freeabl) freeabl,
6 SUM (a.SUM) SUM
7 FROM (SELECT ksmchcom, COUNT (ksmchcom) CHUNK,
8 DECODE (ksmchcls, 'recr', SUM (ksmchsiz), NULL) recr,
9 DECODE (ksmchcls, 'freeabl', SUM (ksmchsiz), NULL) freeabl,
10 SUM (ksmchsiz) SUM
11 FROM x$ksmsp
12 GROUP BY ksmchcom, ksmchcls) a
13 GROUP BY a.ksmchcom
14 /
41 rows created.
执行查询:
SQL> select count(*) from dba_objects;
COUNT(*)
----------
10492
比较前后shared pool内存分配的变化:
SQL> select a.ksmchcom,a.chunk,a.sum,b.chunk,b.sum,(a.chunk - b.chunk) c_diff,(a.sum -b.sum) s_diff
2 from
3 (SELECT a.ksmchcom,
4 SUM (a.CHUNK) CHUNK,
5 SUM (a.recr) recr,
6 SUM (a.freeabl) freeabl,
7 SUM (a.SUM) SUM
8 FROM (SELECT ksmchcom, COUNT (ksmchcom) CHUNK,
9 DECODE (ksmchcls, 'recr', SUM (ksmchsiz), NULL) recr,
10 DECODE (ksmchcls, 'freeabl', SUM (ksmchsiz), NULL) freeabl,
11 SUM (ksmchsiz) SUM
12 FROM x$ksmsp
13 GROUP BY ksmchcom, ksmchcls) a
14 GROUP BY a.ksmchcom) a,e$ksmsp b
15 where a.ksmchcom = b.ksmchcom and (a.chunk - b.chunk) <>0
16 /
KSMCHCOM CHUNK SUM CHUNK SUM C_DIFF S_DIFF
---------------- ---------- ---------- ---------- ---------- ---------- ----------
KGL handles 313 102080 302 98416 11 3664
KGLS heap 274 365752 270 360424 4 5328
KQR PO 389 198548 377 192580 12 5968
free memory 93 2292076 90 2381304 3 -89228
library cache 1005 398284 965 381416 40 16868
sql area 287 547452 269 490052 18 57400
6 rows selected.
我们简单分析一下以上结果:
首先free memory的大小减少了89228,这说明sql解析存储占用了一定的内存空间而chunk从90增加为93，这说明内存碎片增加了.
在下面的部分中,我会着手介绍一下KGL handles, KGLS heap这两个非常重要的shared pool中的内存结构.