Linux上shmmax参数的设置及含义

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/shmmax33554432

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 3102oracle 3102 1 0 09:27 ? 00:00:26 ora_dbw0_hsmktroot 3447 3397 0 10:39 pts/4 00:00:00 grep 3102[root@neirong root]# pmap 3102ora_dbw0_hsmkt[3102]08048000 (37308 KB) r-xp (68:06 1525072) /opt/oracle/product/9.2.0/bin/oracle0a4b7000 (8804 KB) rw-p (68:06 1525072) /opt/oracle/product/9.2.0/bin/oracle0ad50000 (380 KB) rw-p (00:00 0) 50000000 (4096 KB) rw-s (00:04 884736) /SYSV0000000051000000 (32768 KB) rw-s (00:04 917505) /SYSV0000000053000000 (32768 KB) rw-s (00:04 950274) /SYSV0000000055000000 (32768 KB) rw-s (00:04 983043) /SYSV0000000057000000 (32768 KB) rw-s (00:04 1015812) /SYSV0000000059000000 (32768 KB) rw-s (00:04 1048581) /SYSV000000005b000000 (32768 KB) rw-s (00:04 1081350) /SYSV000000005d000000 (32768 KB) rw-s (00:04 1114119) /SYSV000000005f000000 (32768 KB) rw-s (00:04 1146888) /SYSV0000000061000000 (32768 KB) rw-s (00:04 1179657) /SYSV0000000063000000 (32768 KB) rw-s (00:04 1212426) /SYSV0000000065000000 (32768 KB) rw-s (00:04 1245195) /SYSV0000000067000000 (32768 KB) rw-s (00:04 1277964) /SYSV0000000069000000 (32768 KB) rw-s (00:04 1310733) /SYSV000000006b000000 (32768 KB) rw-s (00:04 1343502) /SYSV000000006d000000 (32768 KB) rw-s (00:04 1376271) /SYSV000000006f000000 (32768 KB) rw-s (00:04 1409040) /SYSV0000000071000000 (32768 KB) rw-s (00:04 1441809) /SYSV0000000073000000 (32768 KB) rw-s (00:04 1474578) /SYSV0000000075000000 (32768 KB) rw-s (00:04 1507347) /SYSV0000000077000000 (32768 KB) rw-s (00:04 1540116) /SYSV0000000079000000 (32768 KB) rw-s (00:04 1572885) /SYSV000000007b000000 (32768 KB) rw-s (00:04 1605654) /SYSV000000007d000000 (32768 KB) rw-s (00:04 1638423) /SYSV000000007f000000 (32768 KB) rw-s (00:04 1671192) /SYSV0000000081000000 (32768 KB) rw-s (00:04 1703961) /SYSV0000000083000000 (4 KB) r--s (00:04 1736730) /SYSV7a9c990083001000 (644 KB) rw-s (00:04 1736730) /SYSV7a9c9900830a2000 (4 KB) r--s (00:04 1736730) /SYSV7a9c9900830a3000 (3444 KB) rw-s (00:04 1736730) /SYSV7a9c9900b6ec2000 (44 KB) r-xp (68:03 32811) /lib/libnss_files-2.3.2.sob6ecd000 (4 KB) rw-p (68:03 32811) /lib/libnss_files-2.3.2.sob6ece000 (512 KB) rw-p (68:03 40360) /dev/zerob6f4e000 (1140 KB) rw-p (00:00 0) b706b000 (1224 KB) r-xp (68:03 114692) /lib/tls/libc-2.3.2.sob719d000 (12 KB) rw-p (68:03 114692) /lib/tls/libc-2.3.2.sob71a0000 (12 KB) rw-p (00:00 0) b71a3000 (72 KB) r-xp (68:03 32795) /lib/libnsl-2.3.2.sob71b5000 (4 KB) rw-p (68:03 32795) /lib/libnsl-2.3.2.sob71b6000 (8 KB) rw-p (00:00 0) b71b8000 (52 KB) r-xp (68:03 114696) /lib/tls/libpthread-0.60.sob71c5000 (4 KB) rw-p (68:03 114696) /lib/tls/libpthread-0.60.sob71c6000 (8 KB) rw-p (00:00 0) b71c8000 (132 KB) r-xp (68:03 114694) /lib/tls/libm-2.3.2.sob71e9000 (4 KB) rw-p (68:03 114694) /lib/tls/libm-2.3.2.sob71ea000 (8 KB) r-xp (68:03 32791) /lib/libdl-2.3.2.sob71ec000 (4 KB) rw-p (68:03 32791) /lib/libdl-2.3.2.sob71ed000 (2940 KB) r-xp (68:06 491677) /opt/oracle/product/9.2.0/lib/libjox9.sob74cc000 (1088 KB) rw-p (68:06 491677) /opt/oracle/product/9.2.0/lib/libjox9.sob75dc000 (12 KB) rw-p (00:00 0) b75df000 (4 KB) r-xp (68:06 491685) /opt/oracle/product/9.2.0/lib/libskgxn9.sob75e0000 (8 KB) rw-p (68:06 491685) /opt/oracle/product/9.2.0/lib/libskgxn9.sob75e2000 (4 KB) r-xp (68:06 491567) /opt/oracle/product/9.2.0/lib/libskgxp9.sob75e3000 (4 KB) ---p (68:06 491567) /opt/oracle/product/9.2.0/lib/libskgxp9.sob75e4000 (4 KB) rw-p (68:06 491567) /opt/oracle/product/9.2.0/lib/libskgxp9.sob75e5000 (4 KB) r-xp (68:06 491688) /opt/oracle/product/9.2.0/lib/libodmd9.sob75e6000 (4 KB) rw-p (68:06 491688) /opt/oracle/product/9.2.0/lib/libodmd9.sob75e7000 (4 KB) r-xp (68:03 101245) /etc/libcwait.sob75e8000 (4 KB) rw-p (68:03 101245) /etc/libcwait.sob75ea000 (4 KB) rw-p (00:00 0) b75eb000 (84 KB) r-xp (68:03 32778) /lib/ld-2.3.2.sob7600000 (4 KB) rw-p (68:03 32778) /lib/ld-2.3.2.sobfff8000 (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/shmmax1073741824

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]$ exitexitSQL> startupORACLE instance started.Total System Global Area 839980852 bytesFixed Size 452404 bytesVariable Size 201326592 bytesDatabase Buffers 637534208 bytesRedo Buffers 667648 bytesDatabase 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 0xfcf02e10360448 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 4178ora_lgwr_hsmkt[4178]08048000 (37308 KB) r-xp (68:06 1525072) /opt/oracle/product/9.2.0/bin/oracle0a4b7000 (8804 KB) rw-p (68:06 1525072) /opt/oracle/product/9.2.0/bin/oracle0ad50000 (3320 KB) rw-p (00:00 0) 50000000 (835584 KB) rw-s (00:04 1835008) /SYSV7a9c990083000000 (4 KB) r--s (00:04 1835008) /SYSV7a9c990083001000 (644 KB) rw-s (00:04 1835008) /SYSV7a9c9900830a2000 (4 KB) r--s (00:04 1835008) /SYSV7a9c9900830a3000 (3444 KB) rw-s (00:04 1835008) /SYSV7a9c9900b6bb7000 (4112 KB) rw-p (00:00 0) b6fbb000 (44 KB) r-xp (68:03 32811) /lib/libnss_files-2.3.2.sob6fc6000 (4 KB) rw-p (68:03 32811) /lib/libnss_files-2.3.2.sob6fc7000 (512 KB) rw-p (68:03 40360) /dev/zerob7047000 (144 KB) rw-p (00:00 0) b706b000 (1224 KB) r-xp (68:03 114692) /lib/tls/libc-2.3.2.sob719d000 (12 KB) rw-p (68:03 114692) /lib/tls/libc-2.3.2.sob71a0000 (12 KB) rw-p (00:00 0) b71a3000 (72 KB) r-xp (68:03 32795) /lib/libnsl-2.3.2.sob71b5000 (4 KB) rw-p (68:03 32795) /lib/libnsl-2.3.2.sob71b6000 (8 KB) rw-p (00:00 0) b71b8000 (52 KB) r-xp (68:03 114696) /lib/tls/libpthread-0.60.sob71c5000 (4 KB) rw-p (68:03 114696) /lib/tls/libpthread-0.60.sob71c6000 (8 KB) rw-p (00:00 0) b71c8000 (132 KB) r-xp (68:03 114694) /lib/tls/libm-2.3.2.sob71e9000 (4 KB) rw-p (68:03 114694) /lib/tls/libm-2.3.2.sob71ea000 (8 KB) r-xp (68:03 32791) /lib/libdl-2.3.2.sob71ec000 (4 KB) rw-p (68:03 32791) /lib/libdl-2.3.2.sob71ed000 (2940 KB) r-xp (68:06 491677) /opt/oracle/product/9.2.0/lib/libjox9.sob74cc000 (1088 KB) rw-p (68:06 491677) /opt/oracle/product/9.2.0/lib/libjox9.sob75dc000 (12 KB) rw-p (00:00 0) b75df000 (4 KB) r-xp (68:06 491685) /opt/oracle/product/9.2.0/lib/libskgxn9.sob75e0000 (8 KB) rw-p (68:06 491685) /opt/oracle/product/9.2.0/lib/libskgxn9.sob75e2000 (4 KB) r-xp (68:06 491567) /opt/oracle/product/9.2.0/lib/libskgxp9.sob75e3000 (4 KB) ---p (68:06 491567) /opt/oracle/product/9.2.0/lib/libskgxp9.sob75e4000 (4 KB) rw-p (68:06 491567) /opt/oracle/product/9.2.0/lib/libskgxp9.sob75e5000 (4 KB) r-xp (68:06 491688) /opt/oracle/product/9.2.0/lib/libodmd9.sob75e6000 (4 KB) rw-p (68:06 491688) /opt/oracle/product/9.2.0/lib/libodmd9.sob75e7000 (4 KB) r-xp (68:03 101245) /etc/libcwait.sob75e8000 (4 KB) rw-p (68:03 101245) /etc/libcwait.sob75ea000 (4 KB) rw-p (00:00 0) b75eb000 (84 KB) r-xp (68:03 32778) /lib/ld-2.3.2.sob7600000 (4 KB) rw-p (68:03 32778) /lib/ld-2.3.2.sobfffc000 (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 2005WARNING: EINVAL creating segment of size 0x0000000033400000fix shm parameters in /etc/system or equivalent