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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/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
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