Linux上shmmax參數(shù)的設(shè)置及含義

浸心閣 2015-09-18

展開全文

shmmax內(nèi)核參數(shù)定義單個(gè)共享內(nèi)存段的最大值，如果該參數(shù)設(shè)置小于Oracle SGA設(shè)置，那么SGA就會(huì)被分配多個(gè)共享內(nèi)存段。這在繁忙的系統(tǒng)中可能成為性能負(fù)擔(dān)，帶來系統(tǒng)問題。

Linux上該參數(shù)的缺省值通常為32M。

[root@neirong root]# more /proc/sys/kernel/shmmax
33554432

可以通過ipcs命令查看此設(shè)置下共享內(nèi)存的分配，我們可以看到Oracle分配了多個(gè)共享內(nèi)存段以滿足SGA設(shè)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我們可以看到每個(gè)共享內(nèi)存段的地址空間

[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

為了避免多個(gè)共享內(nèi)存段，我們可以修改shmmax內(nèi)核參數(shù)，使SGA存在于一個(gè)共享內(nèi)存段中。
通過修改/proc/sys/kernel/shmmax參數(shù)可以達(dá)到此目的。

[root@neirong root]# echo 1073741824 > /proc/sys/kernel/shmmax
[root@neirong root]# more /proc/sys/kernel/shmmax
1073741824

這里設(shè)為1G。
對(duì)于shmmax文件的修改，系統(tǒng)重新啟動(dòng)后會(huì)復(fù)位?？梢酝ㄟ^修改 /etc/sysctl.conf 使更改永久化。
在該文件內(nèi)添加以下一行
這個(gè)更改在系統(tǒng)重新啟動(dòng)后生效
kernel.shmmax = 1073741824
重起數(shù)據(jù)庫使更改生效:

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

此時(shí)進(jìn)程的pmap映射顯示為：

[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

實(shí)際上，如果沒有修改shmmax參數(shù)，Oracle在啟動(dòng)過程中就會(huì)報(bào)出以下錯(cuò)誤:

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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