Linux上shmmax参数的设置及含义

王朝system·作者佚名 2008-05-19

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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

0x00000000 917505

oracle

640

33554432

0x00000000 950274

oracle

640

33554432

0x00000000 983043

oracle

640

33554432

0x00000000 1015812

oracle

640

33554432

0x00000000 1048581

oracle

640

33554432

0x00000000 1081350

oracle

640

33554432

0x00000000 1114119

oracle

640

33554432

0x00000000 1146888

oracle

640

33554432

0x00000000 1179657

oracle

640

33554432

0x00000000 1212426

oracle

640

33554432

0x00000000 1245195

oracle

640

33554432

0x00000000 1277964

oracle

640

33554432

0x00000000 1310733

oracle

640

33554432

0x00000000 1343502

oracle

640

33554432

0x00000000 1376271

oracle

640

33554432

0x00000000 1409040

oracle

640

33554432

0x00000000 1441809

oracle

640

33554432

0x00000000 1474578

oracle

640

33554432

0x00000000 1507347

oracle

640

33554432

0x00000000 1540116

oracle

640

33554432

0x00000000 1572885

oracle

640

33554432

0x00000000 1605654

oracle

640

33554432

0x00000000 1638423

oracle

640

33554432

0x00000000 1671192

oracle

640

33554432

0x00000000 1703961

oracle

640

33554432

0x7a9c9900 1736730

oracle

640

4194304

------ 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

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

------ 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