lock_sga和pre_page_sga参数

原标题:使用lock_sga和pre_page_sga参数保证SGA常驻物理内存

转自:http://space.itpub.net/519536/viewspace-623128

通过修改lock_sga和pre_page_sga参数可以保证SGA不被换出到虚拟内存，进而可以提高SGA的使用效率。通过这个小文儿给大家展示一下这两个参数的修改过程，不要太乐观，修改过程是存在“小坎坷”的。
当lock_sga参数设置为TRUE时（默认值是FALSE），可以保证整个SGA被锁定在物理内存中，这样可以防止SGA被换出到虚拟内存。只要设置lock_sga为“TRUE”便可保证SGA被锁定在物理内存中，这里之所以顺便将pre_page_sga参数也设置为“TRUE”，是因为这样可以保证在启动数据库时把整个SGA读入到物理内存中，以便提高系统的效率（虽然会增加系统的启动时间）。
修改过程如下：
1.查看lock_sga和pre_page_sga参数的默认值
sys@ora10g> show parameter sga
NAME            TYPE                 VALUE
--------------- -------------------- -----------------
lock_sga        boolean              FALSE
pre_page_sga    boolean              FALSE
sga_max_size    big integer          5G
sga_target      big integer          5G
2.注意：两个参数都是静态参数。确认之。
sys@ora10g> alter system set lock_sga=true;
alter system set lock_sga=true
                 *
ERROR at line 1:
ORA-02095: specified initialization parameter cannot be modified
sys@ora10g> alter system set pre_page_sga=true;
alter system set pre_page_sga=true
                 *
ERROR at line 1:
ORA-02095: specified initialization parameter cannot be modified
3.使用“scope=spfile”选项修改之，成功。
sys@ora10g> alter system set lock_sga=true scope=spfile;
System altered.
sys@ora10g> alter system set pre_page_sga=true scope=spfile;
System altered.
4.重新启动Oracle使spfile的修改生效
sys@ora10g> shutdown immediate;
Database closed.
Database dismounted.
ORACLE instance shut down.
sys@ora10g> startup;
ORA-27102: out of memory
Linux-x86_64 Error: 12: Cannot allocate memory

“小坎坷”出现了，想一想，这里为什么会启动失败呢？
原因很简单，Linux操作系统对每一个任务在物理内存中能够锁住的最大值做了限制！需要手工进行调整。
5.“ORA-27102”及“Cannot allocate memory”问题处理
1）使用“ulimit -a”命令获得“max locked memory”的默认大小
ora10g@ secDB /home/oracle$ ulimit -a
core file size          (blocks, -c) 0
data seg size           (kbytes, -d) unlimited
scheduling priority             (-e) 0
file size               (blocks, -f) unlimited
pending signals                 (-i) 266239
max locked memory       (kbytes, -l) 32
max memory size         (kbytes, -m) unlimited
open files                      (-n) 65536
pipe size            (512 bytes, -p) 8
POSIX message queues     (bytes, -q) 819200
real-time priority              (-r) 0
stack size              (kbytes, -s) 10240
cpu time               (seconds, -t) unlimited
max user processes              (-u) 16384
virtual memory          (kbytes, -v) unlimited
file locks                      (-x) unlimited
可见，一个任务可以锁住的物理内存最大值是32kbytes，这么小的值根本无法满足我们SGA的5G大小需求。
2）将其修改为无限大
（1）oracle用户是无法完成这个修改任务的
ora10g@ secDB /home/oracle$ ulimit -l unlimited
-bash: ulimit: max locked memory: cannot modify limit: Operation not permitted
（2）切换到root用户
ora10g@ secDB /home/oracle$ su - root
Password:
（3）在root用户下尝试修改，成功。
[root@secDB ~]# ulimit -l unlimited
[root@secDB ~]# ulimit -a
core file size          (blocks, -c) 0
data seg size           (kbytes, -d) unlimited
scheduling priority             (-e) 0
file size               (blocks, -f) unlimited
pending signals                 (-i) 266239
max locked memory       (kbytes, -l) unlimited
max memory size         (kbytes, -m) unlimited
open files                      (-n) 1024
pipe size            (512 bytes, -p) 8
POSIX message queues     (bytes, -q) 819200
real-time priority              (-r) 0
stack size              (kbytes, -s) 10240
cpu time               (seconds, -t) unlimited
max user processes              (-u) 2047
virtual memory          (kbytes, -v) unlimited
file locks                      (-x) unlimited
6.调整完操作系统的限制后，我们再次尝试启动数据库。成功！
[root@secDB ~]# su - oracle
ora10g@ secDB /home/oracle$ sqlplus / as sysdba
SQL*Plus: Release 10.2.0.3.0 - Production on Sun Dec 20 22:21:40 2009
Copyright (c) 1982, 2006, Oracle.  All Rights Reserved.
Connected to an idle instance.
NotConnected@> startup;
ORACLE instance started.
Total System Global Area 5368709120 bytes
Fixed Size                  2080320 bytes
Variable Size             905970112 bytes
Database Buffers         4445962240 bytes
Redo Buffers               14696448 bytes
Database mounted.
Database opened.
7.lock_sga和pre_page_sga参数在Oracle 10gR2官方文档中的描述，供参考。
http://download.oracle.com/docs/cd/B19306_01/server.102/b14237/initparams097.htm#REFRN10084
LOCK_SGA
Property Description
Parameter type Boolean
Default value false
Modifiable No
Range of values true | false
Basic No
LOCK_SGAlocks the entire SGA into physical memory. It is usually advisable to lock the SGA into real (physical) memory, especially if the use of virtual memory would include storing some of the SGA using disk space. This parameter is ignored on platforms that do not support it.
http://download.oracle.com/docs/cd/B19306_01/server.102/b14237/initparams168.htm#REFRN10174
PRE_PAGE_SGA
Property Description
Parameter type Boolean
Default value false
Modifiable No
Range of values true | false
PRE_PAGE_SGAdetermines whether Oracle reads the entire SGA into memory at instance startup. Operating system page table entries are then prebuilt for each page of the SGA. This setting can increase the amount of time necessary for instance startup, but it is likely to decrease the amount of time necessary for Oracle to reach its full performance capacity after startup.
Note:
This setting does not prevent your operating system from paging or swapping the SGA after it is initially read into memory.
PRE_PAGE_SGAcan increase the process startup duration, because every process that starts must access every page in the SGA. The cost of this strategy is fixed; however, you might simply determine that 20,000 pages must be touched every time a process starts. This approach can be useful with some applications, but not with all applications. Overhead can be significant if your system frequently creates and destroys processes by, for example, continually logging on and logging off.
The advantage thatPRE_PAGE_SGAcan afford depends on page size. For example, if the SGA is 80 MB in size and the page size is 4 KB, then 20,000 pages must be touched to refresh the SGA (80,000/4 = 20,000).
If the system permits you to set a 4 MB page size, then only 20 pages must be touched to refresh the SGA (80,000/4,000 = 20). The page size is operating system-specific and generally cannot be changed. Some operating systems, however, have a special implementation for shared memory whereby you can change the page size.
8.小结
通过修改lock_sga和pre_page_sga参数值为“TRUE”可以有效的将整个SGA锁定在物理内存中，这样可以有效的提高系统的性能，推荐酌情进行调整。
注意：不同的操作系统对这lock_sga参数的支持情况是不同的，如果操作系统不支持这种锁定，lock_sga参数将被忽略。