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latch:shared pool的一点理解

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latch:shared pool的一点理解

原创 Oracle 作者:tolilong 时间:2012-10-18 12:59:57  5400  0
 
 
[@more@]
latch:shared pool
shared pool latch 起到保护堆的作用,为了查找free chunk,检索空闲列,分配适当的chunk,
必要时候分割chunk。这些全部只能在获得shared pool latch之后才能执行。
因此在获得shared pool latch过程中会发生latch:shared pool等待事件.
一般情况下一个实例上只有一个latch:shared pool
harding parse严重时,会发生分割chunk的现象.
 
---==============================================================
测试latch:shard pool等待事件
创建测试脚本:
create or replace procedure tt_pro is
begin
for i in 1..100000 loop
execute immediate ‘select * from tt where object_id=‘||i;
end loop;
end;
/
 
同时在三个会话执行这个脚本,查看v$session_event
select * from (
select event,total_waits,time_waited from v$session_event where sid in(12,192,197) order by 2 desc )where rownum<=10
EVENT TOTAL_WAITS TIME_WAITED
------------------------------ ----------- -----------
latch: shared pool 84996 612
latch: shared pool 57746 500
latch: shared pool 56557 640
cursor: pin S wait on X 9798 9471
cursor: pin S wait on X 8682 8370
library cache: mutex X 471 9
library cache: mutex X 408 12
library cache: mutex X 241 10
library cache lock 231 81
events in waitclass Other 224 675
 
10 rows selected.
 
如果发生latch:shared pool等待事件,不应该加大共享池,加大了共享池,
相应的要管理的chunk数量也要增加,检索空闲列的时候,占有
shared pool latch的时间会增大。
 
---==============================================================
相应的解决方法:
1)oracle9i之后,可以有多个共享池(副池),最多有7个副池分别管理。
oracle在cpu数位4,共享池大小为250M以上的情况下,通过创建与
_kghdsidx_count值相同量的副池来管理共享池。副池也是独立的共享池结构。
2)减少共享的大小,这样就可以减少占有shared pool latch的时间。
但这回导致ora-04031错误
可以利用dbms_shared_pool.keep指定对象永久置于共享池中,来消除问题
3)cursor sharing
 
---===============================================================
下面,测试一下,使用过个共享池的情况下,会不会减少latch:shared pool
确认cpu_count大于4,共享池大小大于250M
SQL> show parameter cpu_count
 
NAME TYPE VALUE
------------------------------------ ---------------------- ------------------------------
cpu_count integer 4
 
SQL> show sga
 
Total System Global Area 626327552 bytes
Fixed Size 2178336 bytes
Variable Size 461374176 bytes
Database Buffers 155189248 bytes
Redo Buffers 7585792 bytes
 
SQL> alter system set "_kghdsidx_count"=4 scope=spfile;
 
System altered.
 
SQL> select name,gets from v$latch_children where name=‘shared pool‘;
 
NAME GETS
-------------------- ----------
shared pool 19
shared pool 19
shared pool 19
shared pool 27058
shared pool 27862
shared pool 31982
shared pool 27345
可以看出,4个shared pool latch 被激活
然后再在三个会话中重新执行tt_pro脚本,看看情况
 
SQL> select * from (
2 select event,total_waits,time_waited from v$session_event where sid in(9,10,199) order by 2 desc )where rownum<=10
3 ;
 
EVENT TOTAL_WAITS TIME_WAITED
------------------------------ ----------- -----------
latch: shared pool 23120 126
cursor: pin S wait on X 18792 18766
latch: shared pool 14996 68
cursor: pin S wait on X 13740 13490
latch: shared pool 9860 70
library cache lock 716 256
library cache lock 705 262
library cache: mutex X 692 35
library cache: mutex X 605 41
library cache load lock 479 2
 
10 rows selected.
相应的latch:shared pool减少很多.
 
---====================================================
 
hard parse引起的library cache争用比shared pool争用多

latch:shared pool的一点理解

标签:大于   int   its   imm   sid   脚本   redo   增加   ESS   

原文地址:https://www.cnblogs.com/imok-blog/p/12936391.html

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