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:
引用:[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
引用:[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：
引用:[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