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Linux上shmmax参数的设置及含义
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作者：eygle | 【转载请注】|【云和恩墨 领先的zData数据库一体机 | zCloud PaaS云管平台 | SQM SQL审核平台 | ZDBM 数据库备份一体机】
链接：
shmmax内核参数定义单个共享内存段的最大值，如果该参数设置小于Oracle SGA设置，那么SGA就会被分配多个共享内存段。这在繁忙的系统中可能成为性能负担，带来系统问题。

Linux上该参数的缺省值通常为32M。
[root@neirong root]# more /proc/sys/kernel/shmmax
33554432

可以通过ipcs命令查看此设置下共享内存的分配，我们可以看到Oracle分配了多个共享内存段以满足SGA设置的需要:
[root@neirong root]# ipcs -sa

------ Shared Memory Segments --------
key        shmid      owner      perms      bytes      nattch     status      
0x00000000 884736     oracle    640        4194304    14                      
0x00000000 917505     oracle    640        33554432   14                      
0x00000000 950274     oracle    640        33554432   14                      
0x00000000 983043     oracle    640        33554432   14                      
0x00000000 1015812    oracle    640        33554432   14                      
0x00000000 1048581    oracle    640        33554432   14                      
0x00000000 1081350    oracle    640        33554432   14                      
0x00000000 1114119    oracle    640        33554432   14                      
0x00000000 1146888    oracle    640        33554432   14                      
0x00000000 1179657    oracle    640        33554432   14                      
0x00000000 1212426    oracle    640        33554432   14                      
0x00000000 1245195    oracle    640        33554432   14                      
0x00000000 1277964    oracle    640        33554432   14                      
0x00000000 1310733    oracle    640        33554432   14                      
0x00000000 1343502    oracle    640        33554432   14                      
0x00000000 1376271    oracle    640        33554432   14                      
0x00000000 1409040    oracle    640        33554432   14                      
0x00000000 1441809    oracle    640        33554432   14                      
0x00000000 1474578    oracle    640        33554432   14                      
0x00000000 1507347    oracle    640        33554432   14                      
0x00000000 1540116    oracle    640        33554432   14                      
0x00000000 1572885    oracle    640        33554432   14                      
0x00000000 1605654    oracle    640        33554432   14                      
0x00000000 1638423    oracle    640        33554432   14                      
0x00000000 1671192    oracle    640        33554432   14                      
0x00000000 1703961    oracle    640        33554432   14                      
0x7a9c9900 1736730    oracle    640        4194304    56                      

------ Semaphore Arrays --------
key        semid      owner      perms      nsems     
0xfcf02e10 229376     oracle    640        154       

------ Message Queues --------
key        msqid      owner      perms      used-bytes   messages


使用pmap我们可以看到每个共享内存段的地址空间
[root@neirong root]# ps -ef|grep 3102
oracle    3102     1  0 09:27 ?        00:00:26 ora_dbw0_hsmkt
root      3447  3397  0 10:39 pts/4    00:00:00 grep 3102

[root@neirong root]# pmap 3102
ora_dbw0_hsmkt[3102]
08048000 (37308 KB)    r-xp (68:06 1525072)  /opt/oracle/product/9.2.0/bin/oracle
0a4b7000 (8804 KB)     rw-p (68:06 1525072)  /opt/oracle/product/9.2.0/bin/oracle
0ad50000 (380 KB)      rw-p (00:00 0)        
50000000 (4096 KB)     rw-s (00:04 884736)   /SYSV00000000
51000000 (32768 KB)    rw-s (00:04 917505)   /SYSV00000000
53000000 (32768 KB)    rw-s (00:04 950274)   /SYSV00000000
55000000 (32768 KB)    rw-s (00:04 983043)   /SYSV00000000
57000000 (32768 KB)    rw-s (00:04 1015812)  /SYSV00000000
59000000 (32768 KB)    rw-s (00:04 1048581)  /SYSV00000000
5b000000 (32768 KB)    rw-s (00:04 1081350)  /SYSV00000000
5d000000 (32768 KB)    rw-s (00:04 1114119)  /SYSV00000000
5f000000 (32768 KB)    rw-s (00:04 1146888)  /SYSV00000000
61000000 (32768 KB)    rw-s (00:04 1179657)  /SYSV00000000
63000000 (32768 KB)    rw-s (00:04 1212426)  /SYSV00000000
65000000 (32768 KB)    rw-s (00:04 1245195)  /SYSV00000000
67000000 (32768 KB)    rw-s (00:04 1277964)  /SYSV00000000
69000000 (32768 KB)    rw-s (00:04 1310733)  /SYSV00000000
6b000000 (32768 KB)    rw-s (00:04 1343502)  /SYSV00000000
6d000000 (32768 KB)    rw-s (00:04 1376271)  /SYSV00000000
6f000000 (32768 KB)    rw-s (00:04 1409040)  /SYSV00000000
71000000 (32768 KB)    rw-s (00:04 1441809)  /SYSV00000000
73000000 (32768 KB)    rw-s (00:04 1474578)  /SYSV00000000
75000000 (32768 KB)    rw-s (00:04 1507347)  /SYSV00000000
77000000 (32768 KB)    rw-s (00:04 1540116)  /SYSV00000000
79000000 (32768 KB)    rw-s (00:04 1572885)  /SYSV00000000
7b000000 (32768 KB)    rw-s (00:04 1605654)  /SYSV00000000
7d000000 (32768 KB)    rw-s (00:04 1638423)  /SYSV00000000
7f000000 (32768 KB)    rw-s (00:04 1671192)  /SYSV00000000
81000000 (32768 KB)    rw-s (00:04 1703961)  /SYSV00000000
83000000 (4 KB)        r--s (00:04 1736730)  /SYSV7a9c9900
83001000 (644 KB)      rw-s (00:04 1736730)  /SYSV7a9c9900
830a2000 (4 KB)        r--s (00:04 1736730)  /SYSV7a9c9900
830a3000 (3444 KB)     rw-s (00:04 1736730)  /SYSV7a9c9900
b6ec2000 (44 KB)       r-xp (68:03 32811)    /lib/libnss_files-2.3.2.so
b6ecd000 (4 KB)        rw-p (68:03 32811)    /lib/libnss_files-2.3.2.so
b6ece000 (512 KB)      rw-p (68:03 40360)    /dev/zero
b6f4e000 (1140 KB)     rw-p (00:00 0)        
b706b000 (1224 KB)     r-xp (68:03 114692)   /lib/tls/libc-2.3.2.so
b719d000 (12 KB)       rw-p (68:03 114692)   /lib/tls/libc-2.3.2.so
b71a0000 (12 KB)       rw-p (00:00 0)        
b71a3000 (72 KB)       r-xp (68:03 32795)    /lib/libnsl-2.3.2.so
b71b5000 (4 KB)        rw-p (68:03 32795)    /lib/libnsl-2.3.2.so
b71b6000 (8 KB)        rw-p (00:00 0)        
b71b8000 (52 KB)       r-xp (68:03 114696)   /lib/tls/libpthread-0.60.so
b71c5000 (4 KB)        rw-p (68:03 114696)   /lib/tls/libpthread-0.60.so
b71c6000 (8 KB)        rw-p (00:00 0)        
b71c8000 (132 KB)      r-xp (68:03 114694)   /lib/tls/libm-2.3.2.so
b71e9000 (4 KB)        rw-p (68:03 114694)   /lib/tls/libm-2.3.2.so
b71ea000 (8 KB)        r-xp (68:03 32791)    /lib/libdl-2.3.2.so
b71ec000 (4 KB)        rw-p (68:03 32791)    /lib/libdl-2.3.2.so
b71ed000 (2940 KB)     r-xp (68:06 491677)   /opt/oracle/product/9.2.0/lib/libjox9.so
b74cc000 (1088 KB)     rw-p (68:06 491677)   /opt/oracle/product/9.2.0/lib/libjox9.so
b75dc000 (12 KB)       rw-p (00:00 0)        
b75df000 (4 KB)        r-xp (68:06 491685)   /opt/oracle/product/9.2.0/lib/libskgxn9.so
b75e0000 (8 KB)        rw-p (68:06 491685)   /opt/oracle/product/9.2.0/lib/libskgxn9.so
b75e2000 (4 KB)        r-xp (68:06 491567)   /opt/oracle/product/9.2.0/lib/libskgxp9.so
b75e3000 (4 KB)        ---p (68:06 491567)   /opt/oracle/product/9.2.0/lib/libskgxp9.so
b75e4000 (4 KB)        rw-p (68:06 491567)   /opt/oracle/product/9.2.0/lib/libskgxp9.so
b75e5000 (4 KB)        r-xp (68:06 491688)   /opt/oracle/product/9.2.0/lib/libodmd9.so
b75e6000 (4 KB)        rw-p (68:06 491688)   /opt/oracle/product/9.2.0/lib/libodmd9.so
b75e7000 (4 KB)        r-xp (68:03 101245)   /etc/libcwait.so
b75e8000 (4 KB)        rw-p (68:03 101245)   /etc/libcwait.so
b75ea000 (4 KB)        rw-p (00:00 0)        
b75eb000 (84 KB)       r-xp (68:03 32778)    /lib/ld-2.3.2.so
b7600000 (4 KB)        rw-p (68:03 32778)    /lib/ld-2.3.2.so
bfff8000 (32 KB)       rwxp (00:00 0)        
mapped: 881332 KB       writable/private: 12056 KB      shared: 827392 KB


为了避免多个共享内存段，我们可以修改shmmax内核参数，使SGA存在于一个共享内存段中。
通过修改/proc/sys/kernel/shmmax参数可以达到此目的。
[root@neirong root]# echo 1073741824 > /proc/sys/kernel/shmmax
[root@neirong root]# more /proc/sys/kernel/shmmax
1073741824


这里设为1G。
对于shmmax文件的修改，系统重新启动后会复位。可以通过修改 /etc/sysctl.conf 使更改永久化。
在该文件内添加以下一行
这个更改在系统重新启动后生效
kernel.shmmax = 1073741824
重起数据库使更改生效:
SQL> shutdown immediate;
Database closed.
Database dismounted.
ORACLE instance shut down.
SQL> !
[oracle@neirong oracle]$ ipcs -sa

------ Shared Memory Segments --------
key        shmid      owner      perms      bytes      nattch     status      

------ Semaphore Arrays --------
key        semid      owner      perms      nsems     

------ Message Queues --------
key        msqid      owner      perms      used-bytes   messages    

[oracle@neirong oracle]$ exit
exit

SQL> startup
ORACLE instance started.

Total System Global Area  839980852 bytes
Fixed Size                   452404 bytes
Variable Size             201326592 bytes
Database Buffers          637534208 bytes
Redo Buffers                 667648 bytes
Database mounted.
Database opened.
SQL> ! ipcs -sa

------ Shared Memory Segments --------
key        shmid      owner      perms      bytes      nattch     status      
0x7a9c9900 1769472    oracle    640        859832320  35                      

------ Semaphore Arrays --------
key        semid      owner      perms      nsems     
0xfcf02e10 360448     oracle    640        154       

------ Message Queues --------
key        msqid      owner      perms      used-bytes   messages


此时进程的pmap映射显示为：
[oracle@neirong bdump]$ pmap 4178
ora_lgwr_hsmkt[4178]
08048000 (37308 KB)    r-xp (68:06 1525072)  /opt/oracle/product/9.2.0/bin/oracle
0a4b7000 (8804 KB)     rw-p (68:06 1525072)  /opt/oracle/product/9.2.0/bin/oracle
0ad50000 (3320 KB)     rw-p (00:00 0)        
50000000 (835584 KB)   rw-s (00:04 1835008)  /SYSV7a9c9900
83000000 (4 KB)        r--s (00:04 1835008)  /SYSV7a9c9900
83001000 (644 KB)      rw-s (00:04 1835008)  /SYSV7a9c9900
830a2000 (4 KB)        r--s (00:04 1835008)  /SYSV7a9c9900
830a3000 (3444 KB)     rw-s (00:04 1835008)  /SYSV7a9c9900
b6bb7000 (4112 KB)     rw-p (00:00 0)        
b6fbb000 (44 KB)       r-xp (68:03 32811)    /lib/libnss_files-2.3.2.so
b6fc6000 (4 KB)        rw-p (68:03 32811)    /lib/libnss_files-2.3.2.so
b6fc7000 (512 KB)      rw-p (68:03 40360)    /dev/zero
b7047000 (144 KB)      rw-p (00:00 0)        
b706b000 (1224 KB)     r-xp (68:03 114692)   /lib/tls/libc-2.3.2.so
b719d000 (12 KB)       rw-p (68:03 114692)   /lib/tls/libc-2.3.2.so
b71a0000 (12 KB)       rw-p (00:00 0)        
b71a3000 (72 KB)       r-xp (68:03 32795)    /lib/libnsl-2.3.2.so
b71b5000 (4 KB)        rw-p (68:03 32795)    /lib/libnsl-2.3.2.so
b71b6000 (8 KB)        rw-p (00:00 0)        
b71b8000 (52 KB)       r-xp (68:03 114696)   /lib/tls/libpthread-0.60.so
b71c5000 (4 KB)        rw-p (68:03 114696)   /lib/tls/libpthread-0.60.so
b71c6000 (8 KB)        rw-p (00:00 0)        
b71c8000 (132 KB)      r-xp (68:03 114694)   /lib/tls/libm-2.3.2.so
b71e9000 (4 KB)        rw-p (68:03 114694)   /lib/tls/libm-2.3.2.so
b71ea000 (8 KB)        r-xp (68:03 32791)    /lib/libdl-2.3.2.so
b71ec000 (4 KB)        rw-p (68:03 32791)    /lib/libdl-2.3.2.so
b71ed000 (2940 KB)     r-xp (68:06 491677)   /opt/oracle/product/9.2.0/lib/libjox9.so
b74cc000 (1088 KB)     rw-p (68:06 491677)   /opt/oracle/product/9.2.0/lib/libjox9.so
b75dc000 (12 KB)       rw-p (00:00 0)        
b75df000 (4 KB)        r-xp (68:06 491685)   /opt/oracle/product/9.2.0/lib/libskgxn9.so
b75e0000 (8 KB)        rw-p (68:06 491685)   /opt/oracle/product/9.2.0/lib/libskgxn9.so
b75e2000 (4 KB)        r-xp (68:06 491567)   /opt/oracle/product/9.2.0/lib/libskgxp9.so
b75e3000 (4 KB)        ---p (68:06 491567)   /opt/oracle/product/9.2.0/lib/libskgxp9.so
b75e4000 (4 KB)        rw-p (68:06 491567)   /opt/oracle/product/9.2.0/lib/libskgxp9.so
b75e5000 (4 KB)        r-xp (68:06 491688)   /opt/oracle/product/9.2.0/lib/libodmd9.so
b75e6000 (4 KB)        rw-p (68:06 491688)   /opt/oracle/product/9.2.0/lib/libodmd9.so
b75e7000 (4 KB)        r-xp (68:03 101245)   /etc/libcwait.so
b75e8000 (4 KB)        rw-p (68:03 101245)   /etc/libcwait.so
b75ea000 (4 KB)        rw-p (00:00 0)        
b75eb000 (84 KB)       r-xp (68:03 32778)    /lib/ld-2.3.2.so
b7600000 (4 KB)        rw-p (68:03 32778)    /lib/ld-2.3.2.so
bfffc000 (16 KB)       rwxp (00:00 0)        
mapped: 899660 KB       writable/private: 18096 KB      shared: 839680 KB


实际上，如果没有修改shmmax参数，Oracle在启动过程中就会报出以下错误:
Starting ORACLE instance (normal)
Thu Nov 17 09:27:29 2005
WARNING: EINVAL creating segment of size 0x0000000033400000
fix shm parameters in /etc/system or equivalent



历史上的今天...
    >> 2014-11-17文章:
    >> 2011-11-17文章:
    >> 2009-11-17文章:
    >> 2008-11-17文章:
           什么是ABM、ABC、ABP - 学习
    >> 2006-11-17文章:
    >> 2004-11-17文章:
           Oracle X$ table 参考手册

By eygle on 2005-11-17 18:34 | Comments (7) | FAQ | Internal | System | 539 |

7 Comments

侯雪峰 | February 5, 2006 4:45 PM

[root@neirong root]# ps -ef|grep 3102
请问在以上命令中3102这个进程号是怎么得到的？？同理以下命令中4178进程号是怎样得到的？谢谢！！！！
[oracle@neirong bdump]$ pmap 4178

Zhu ji | February 2, 2007 12:22 AM

Hi Tiger,

Thank you so much for this doc, I use this as a reference to locate and solve a critical problem for In-House Database Server.

Zhu ji

zhu ji | February 2, 2007 12:26 AM

I am sorry, the last comments should start with

"Hi Eagle,"

:-)

eygle | February 3, 2007 1:57 PM

I am very glad to hear so..

panweiweb | August 7, 2007 3:44 PM

还得继续学习

ivan | August 8, 2007 1:50 PM

请教eygle，shmmax这个参数有没有制约因素？如果只是为了得到共享内存段，这个参数完全可以设置的很大？

luyingbb | May 15, 2010 11:26 AM

[root@neirong root]# ps -ef|grep 3102
请问在以上命令中3102这个进程号是怎么得到的？？同理以下命令中4178进程号是怎样得到的？谢谢！！！！
[oracle@neirong bdump]$ pmap 4178

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