了解X$KSMSP视图:
Shared Pool的空间分配和使用情况,可以通过一个内部视图来观察,这个视图就是X$KSMSP([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。以下是X$KSMSP的结构:
sys@CCDB> desc x$ksmsp
Name Null? Type
------------------------ -------- ----------------
ADDR RAW(8)
INDX NUMBER
INST_ID NUMBER
KSMCHIDX NUMBER
KSMCHDUR NUMBER
KSMCHCOM VARCHAR2(16)
KSMCHPTR RAW(8)
KSMCHSIZ NUMBER
KSMCHCLS VARCHAR2(8)
KSMCHTYP NUMBER
KSMCHPAR RAW(8)
这里需要关注一下以下几个字段。
⑴ x$ksmsp.ksmchcom 是注释字段,每个内存块被分配以后,注释会添加在该字段中。
⑵ x$ksmsp.ksmchsiz 代表块大小。
⑶ x$ksmsp.ksmchcls 列代表类型,主要有4类,具体说明如下:
·free:即Free Chunks,不包含任何对象的Chunk,可以不受限制的被自由分配。
·recr:即Recreatable Chunks,包含可以被临时移出内存的对象,在需要的时候,这个对象可以被重新创建。例如,许多存储共享SQL代码的内存都是可以重建的。
·freeable:即Freeable Chunks,包含session周期或调用的对象,随后可以被释放。这部分内存有时候可以全部或部分提前释放。但是注意,由于某些对象是中间过程产生的,这些对象不能临时被移出内存(因为不可重建)。
·perm:即Permanent Memory Chunks,包含永久对象,通常不能独立释放。
从以上引用的trace文件中,摘出开头一段,可以清楚地看到Oracle对这部分Chunk的记录情况:
HEAP DUMP heap name="sga heap(1,0)" desc=0x2001aae4
extent sz=0xfc4 alt=108 het=32767 rec=9 flg=-126 opc=0
parent=(nil) owner=(nil) nex=(nil) xsz=0x400000
EXTENT 0 addr=0x41000000
Chunk 41000038 sz= 24 R-freeable "reserved stoppe"
Chunk 41000050 sz= 212888 R-free " "
Chunk 41033fe8 sz= 24 R-freeable "reserved stoppe"
Chunk 41034000 sz= 3981312 perm "perm " alo=3955992
EXTENT 1 addr=0x41400000
Chunk 41400038 sz= 24 R-freeable "reserved stoppe"
Chunk 41400050 sz= 212888 R-free " "
Chunk 41433fe8 sz= 24 R-freeable "reserved stoppe"
Chunk 41434000 sz= 2097168 perm "perm " alo=2097168
Chunk 41634010 sz= 1884144 free " "
可以通过查询x$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。
看一下测试,在这个测试数据库中,初始启动数据库,在x$ksmsp视图中存在11361个Chunk:
sys@NEI> select count(*) from x$ksmsp;
COUNT(*)
----------
11361
执行查询:
sys@NEI> select count(*) from dba_objects;
COUNT(*)
----------
50404
此时shared pool中的chunk数量增加:
sys@NEI> select count(*) from x$ksmsp;
COUNT(*)
----------
11428
这就是由于Shared Pool中进行SQL解析,请求空间,进而导致请求free空间分配、分割,从而产生了更多、更细碎的内存Chunk。
由此可以看出,如果数据库系统中存在大量的硬解析,不停请求分配free的Shared Pool内存,除了必需的Shared Pool Latch等竞争外,还不可避免地会导致Shared Pool中产生更多的内存碎片(当然,在内存回收时,你可能看到Chunk数量减少的情况)。
继续进行一点深入的研究,首先重新启动数据库:
sys@NEI> startup force
创建一张临时表用以保存之前x$ksmsp的状态:
sys@NEI> 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:
sys@NEI> 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 group by ksmchcom,ksmchcls) a
12 group by a.ksmchcom;
85 rows created.
执行查询:
sys@NEI> select count(*) from dba_objects;
COUNT(*)
----------
50405
比较前后Shared Pool内存分配的变化:
sys@NEI> 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;KSMCHCOM CHUNK SUM CHUNK SUM C_DIFF S_DIFF
---------------- ---------- ---------- ---------- ---------- ---------- ----------
free memory 219 21339452 208 22231952 11 -892500
Heap0: KGL 656 708884 624 674580 32 34304
trigger inform 6 2620 5 2560 1 60
KGL handles 3491 1709404 3248 1596568 243 112836
modification 14 28840 13 26780 1 2060
KGLS heap 1211 1550260 1150 1467528 61 82732
obj stat memo 296 89984 284 86336 12 3648
KQR SO 380 244880 346 226520 34 18360
kpscad: kpscsco 4 232 3 176 1 56
sql area:PLSQL 50 206980 46 190460 4 16520
PCursor 899 963728 851 912272 48 51456
CCursor 1447 1563120 1368 1478432 79 84688
library cache 1362 130812 1299 124736 63 6076
sql area 1184 4850764 1120 4588516 64 262248
PL/SQL DIANA 340 1392640 328 1343488 12 49152
KQR PO 2138 1074408 2016 1010576 122 63832
PL/SQL MPCODE 575 2476872 552 2376544 23 100328
CURSOR STATS 42 174048 41 169904 1 414418 rows selected.
简单分析一下以上结果:首先free memory的大小减小了892500(增加到另外17个组件中),这说明SQL解析存储占用了一定的内存空间;而从208增加到219,这说明内存碎片增加了,碎片增加是共享池性能下降的开始。
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