session_cached_cursors,cursor_space_for_time,gets,pin

session_cached_cursors,cursor_space_for_time,gets,pin

上周末看到asktom上的一个帖子，其中有人提到session_cached_cursors和cursor_space_for_time对library cache gets&pin的影响，帖子的url为

http://asktom.oracle.com/pls/ask/f?p=4950:8:::::F4950_P8_DISPLAYID:465420331879

 

其中tom还为我们展示了session_cached_cursors和cursor_space_for_time对library cache gets&pin在9i和10g中的影响是不同的。

 

先让我们来了解一下session_cached_cursors，cursor_space_for_time，library cache gets，pin

 

 

session_cached_cursors:

设置pga端的cache list的长度，当session_cached_cursors设置为0时，pga的cache list长度为0，这时候当sga中的cursor关闭的时候它相关的library cache handle的lock位被清0，从v$open_cursor里看不到这个被关闭的cursor，它服从于shared pool的lru机制，当shared pool需要新的buffer空间时，它将会被flush出shared pool。当session_cached_cursors设置为非0值时，pga的cache list长度为session_cached_cursors值的大小，同时pga cache list将会保留一份拷贝，这时候即使sga中的cursor关闭的时候它相关的library cache handle始终被加了null mode lock，当shared pool空间紧张时library cache handle始终将会被保留在shared pool中.而新的应用访问这个cursor的时候会直接去自己的pga cache list里面搜索。

 

cursor_space_for_time：

当设置了session_cached_cursors为非0值后，如果cursor_space_for_time值被设为false，那么当shared pool空间紧张时，虽然library cache handle不会被flush出去，但是它指向的library cached object(lco,其中包含了handle和children handle的地址，权限，类型，状态，指向kgl block的指针，其中kgl block包含了真正的代码段和执行计划等内容）将会被flush出去而不管它相关的cursor有没关闭，如果需要lco的时候将要reloads。

如果cursor_space_for_time值被设为true，那么当cursor在打开状态下，handle指向的lco将不会被flush出shared pool，这样就可以降低reloads出现的频率。不过对于sql共享做的不好的数据库，设置

cursor_space_for_time将会带来一些问题，share pool可能会出现04031的错误。

 

gets:

当试图parse一句sql时，oracle要先获得一个handle，在handle上加载一个lock，gets表示handle request times。

 

pin：

当获得handle后，定位到lco，然后pin住lco使它在被执行的时候不被flush出去。

 

 

既然理解了以上一些概念，那么我们可以通过一些代码演示session_cached_cursors,cursor_space_for_time对pin，gets的影响

 

在9i和10g中两个参数对pin,gets的影响也不一样

 

9i：

 

vi 1.sql

 

set     wrap off
set     linesize 100
set     pagesize 0
set     verify off
select namespace, gets, gethits, pins, pinhits from v$librarycache where namespace = 'SQL AREA' ;
select namespace, gets, gethits, pins, pinhits from v$librarycache where namespace = 'SQL AREA' ;    
select namespace, gets, gethits, pins, pinhits from v$librarycache where namespace = 'SQL AREA' ;
select namespace, gets, gethits, pins, pinhits from v$librarycache where namespace = 'SQL AREA' ;
select namespace, gets, gethits, pins, pinhits from v$librarycache where namespace = 'SQL AREA' ;
select namespace, gets, gethits, pins, pinhits from v$librarycache where namespace = 'SQL AREA' ;
select namespace, gets, gethits, pins, pinhits from v$librarycache where namespace = 'SQL AREA' ;
select namespace, gets, gethits, pins, pinhits from v$librarycache where namespace = 'SQL AREA' ;
select namespace, gets, gethits, pins, pinhits from v$librarycache where namespace = 'SQL AREA' ;

 

执行10次sql

 

 

SQL 9I>show parameter cursor

NAME                                 TYPE        VALUE
------------------------------------ ----------- ------------------------------
cursor_space_for_time                boolean     FALSE
session_cached_cursors               integer     0

 

 

SQL 9I>@$HOME/1.sql

NAMESPACE             GETS    GETHITS       PINS    PINHITS
--------------- ---------- ---------- ---------- ----------
SQL AREA           2942908    2939105   75218597   75211231
SQL AREA           2942909    2939106   75218600   75211234
SQL AREA           2942910    2939107   75218603   75211237
SQL AREA           2942911    2939108   75218606   75211240
SQL AREA           2942912    2939109   75218609   75211243
SQL AREA           2942913    2939110   75218612   75211246
SQL AREA           2942914    2939111   75218615   75211249
SQL AREA           2942915    2939112   75218618   75211252
SQL AREA           2942916    2939113   75218621   75211255
SQL AREA           2942917    2939114   75218624   75211258

 

可以看到gets每次增加1，pin每次增加3

 

修改session_cached_cursors＝100；

 

SQL 9I>alter session set session_cached_cursors=100;
SQL 9I>@$HOME/1.sql
SQL AREA           2942935    2939123   75218728   75211344
SQL AREA           2942935    2939123   75218730   75211346
SQL AREA           2942935    2939123   75218732   75211348
SQL AREA           2942935    2939123   75218734   75211350
SQL AREA           2942935    2939123   75218736   75211352
SQL AREA           2942935    2939123   75218738   75211354
SQL AREA           2942935    2939123   75218740   75211356
SQL AREA           2942935    2939123   75218742   75211358
SQL AREA           2942935    2939123   75218744   75211360
SQL AREA           2942935    2939123   75218746   75211362

 

gets不再增加，pin增加2

 

SQL 9I>show parameter cursor

NAME                                 TYPE        VALUE
------------------------------------ ----------- ------------------------------
cursor_space_for_time                boolean     TRUE
session_cached_cursors               integer     100

SQL 9I>@$HOME/1.sql

NAMESPACE             GETS    GETHITS       PINS    PINHITS
--------------- ---------- ---------- ---------- ----------
SQL AREA               932        508       3561       3159
SQL AREA               933        509       3563       3161
SQL AREA               934        510       3565       3163
SQL AREA               934        510       3566       3164
SQL AREA               934        510       3567       3165
SQL AREA               934        510       3568       3166
SQL AREA               934        510       3569       3167
SQL AREA               934        510       3570       3168
SQL AREA               934        510       3571       3169
SQL AREA               934        510       3572       3170

 

cursor_space_for_time改为ture后，pin变成每次增加1

 

10g：

SQL 10G>show parameter cursor
cursor_space_for_time                boolean     FALSE
session_cached_cursors               integer     0

SQL 10G>@$HOME/1.sql
SQL AREA              3328         98      23112      21363
SQL AREA              3328         98      23113      21364
SQL AREA              3328         98      23114      21365
SQL AREA              3328         98      23115      21366
SQL AREA              3328         98      23116      21367
SQL AREA              3328         98      23117      21368
SQL AREA              3328         98      23118      21369
SQL AREA              3328         98      23119      21370
SQL AREA              3328         98      23120      21371
SQL AREA              3328         98      23121      21372

 

gets不变化，pin每次增加1

 

SQL 10G>alter session set session_cached_cursors=100;
SQL 10G>@$HOME/1.sql
SQL AREA              3513         98      28456      26335
SQL AREA              3513         98      28457      26336
SQL AREA              3513         98      28458      26337
SQL AREA              3513         98      28459      26338
SQL AREA              3513         98      28460      26339
SQL AREA              3513         98      28461      26340
SQL AREA              3513         98      28462      26341
SQL AREA              3513         98      28463      26342
SQL AREA              3513         98      28464      26343
SQL AREA              3513         98      28465      26344

设置了session set session_cached_cursors后没有变化

 

SQL 10G>show parameter cursor

NAME                                 TYPE        VALUE
------------------------------------ ----------- ------------------------------
cursor_space_for_time                boolean     TRUE
session_cached_cursors               integer     100
SQL 10G>@$HOME/1.sql            

NAMESPACE             GETS    GETHITS       PINS    PINHITS
--------------- ---------- ---------- ---------- ----------
SQL AREA              1038         24       4404       3488
SQL AREA              1038         24       4405       3489
SQL AREA              1038         24       4406       3490
SQL AREA              1038         24       4407       3491
SQL AREA              1038         24       4408       3492
SQL AREA              1038         24       4409       3493
SQL AREA              1038         24       4410       3494
SQL AREA              1038         24       4411       3495
SQL AREA              1038         24       4412       3496
SQL AREA              1038         24       4413       3497

 

设置cursor_space_for_time为true后也没有变化

 

但是当设置cursor_space_for_time为true后pl/sql block将会使pin也停止增长

 

看tom的例子

 

ops$tkyte@ORA10G> create or replace procedure p
  2  authid current_user
  3  as
  4      l_ns      varchar2(4000);
  5      l_gets    number;
  6      l_gethits number;
  7      l_pins    number;
  8      l_pinhits number;
  9      l_sgets    number;
10      l_sgethits number;
11      l_spins    number;
12      l_spinhits number;
13  begin
14      for i in 1 .. 1000
15      loop
16          execute immediate
17          'select namespace, gets, gethits, pins, pinhits
18             from v$librarycache
19            where namespace = ''SQL AREA'' '
20          into l_ns, l_gets, l_gethits, l_pins, l_pinhits;
21
22          if ( i in (1,1000) )
23          then
24              if ( i = 1 )
25              then
26                  l_sgets := l_gets; l_sgethits := l_gethits;
27                  l_spins := l_pins; l_spinhits := l_pinhits;
28              end if;
29              dbms_output.put_line
30              ( l_ns || to_char(l_gets,'999,999') ||
31                to_char(l_gethits,'999,999') ||
32                to_char(l_pins,'999,999') ||
33                to_char(l_pinhits,'999,999')  );
34              if ( i = 1000 )
35              then
36                  dbms_output.put_line
37                  ( l_ns || to_char(l_gets-l_sgets,'999,999') ||
38                    to_char(l_gethits-l_sgethits,'999,999') ||
39                    to_char(l_pins-l_spins,'999,999') ||
40                    to_char(l_pinhits-l_spinhits,'999,999')  );
41              end if;
42          end if;
43      end loop;
44  end;
45  /
                                                                                
                                                                             
Procedure created.

sys@ORA10G> alter system set session_cached_cursors=100 scope=spfile;

System altered.

sys@ORA10G> alter system set cursor_space_for_time=TRUE scope=spfile;

System altered.

sys@ORA10G> startup force
ORACLE instance started.

Total System Global Area  171966464 bytes
Fixed Size                   777956 bytes
Variable Size             145760540 bytes
Database Buffers           25165824 bytes
Redo Buffers                 262144 bytes
Database mounted.
Database opened.
sys@ORA10G> @connect /
sys@ORA10G> set termout off
ops$tkyte@ORA10G> @login
ops$tkyte@ORA10G> set termout off
ops$tkyte@ORA10G> REM GET afiedt.buf NOLIST
ops$tkyte@ORA10G> set termout on 
ops$tkyte@ORA10G> exec p
SQL AREA   1,181     373   4,828   4,171
SQL AREA   1,181     373   4,828   4,171
SQL AREA       0       0       0       0

PL/SQL procedure successfully completed.

 

可以看到10g的pl/sql引擎也有了变化。

 

再来看一下设置session_cached_cursors后handle的dump

 

BUCKET 110279:
  LIBRARY OBJECT HANDLE: handle=9df283a8 mutex=0x9df2845c(1)
  name=select namespace, gets, gethits, pins, pinhits from v$librarycache where namespace = 'SQL AREA'
  hash=f1deb637acf7a42dd55d86a8ae3baec7 timestamp=08-30-2005 17:07:35
  namespace=CRSR flags=RON/KGHP/TIM/KEP/PN0/MED/KST/DBN/MTX/[500100d4]
  kkkk-dddd-llll=0001-0001-0001 lock= N pin=0 latch#=7 hpc=0002 hlc=0002

 

而不设置的话

BUCKET 110279:
  LIBRARY OBJECT HANDLE: handle=9df283a8 mutex=0x9df2845c(1)
  name=select namespace, gets, gethits, pins, pinhits from v$librarycache where namespace = 'SQL AREA'
  hash=f1deb637acf7a42dd55d86a8ae3baec7 timestamp=08-30-2005 17:07:35
  namespace=CRSR flags=RON/KGHP/TIM/KEP/PN0/MED/KST/DBN/MTX/[500100d4]
  kkkk-dddd-llll=0001-0001-0001 lock= 0 pin=0 latch#=7 hpc=0002 hlc=0002

 

如果执行alter system flush shared_pool 那么没有加载null mode lock的handle将被flush出shared pool。
http://wzwanghai.spaces.live.com/blog/cns!56626E237AFBD116!187.entry