修改Linux下最大open files 数

Redhat 帮助文档[http://kbase.redhat.com/faq/FAQ_80_1540.shtm]
Issue:
How do I set the maximum number of files allowed to be open on a system
Resolution:
The current setting for maximum number of open files can be viewed with the command:

    ulimit -n

This number indicates the maximum number of files normal users (i.e. non-root) can have open in a single session. Note that for the root user, ulimit -n will sometimes output 1024 even after following the procedure to increase the maximum number of open files. This won't effect root's ability to open large numbers of files, as only normal users are bound by this value.

To increase the maximum number of open files beyond the default of 1024, two changes to the system may be necessary. In these examples, we will increase the maximum number of open files to the arbitrary value of 2048. All changes need to be made by the root user and users will need to log out and log back in before the changes will take effect.

   1. Configure the system to accept the desired value for maximum number of open files Check the value in /proc/sys/fs/file-max to see if it is larger than the value needed for the maximum number of open files:

      # cat /proc/sys/fs/file-max

      If the value isn't large enough, echo an appropriate number into the variable and add the change to /etc/sysctl.conf to make it persistent across reboots. If the number is already larger than the value you wish to use, skip to step 2.

      # echo 2048 > /proc/sys/fs/file-max

      and edit /etc/sysctl.conf to include the line:

      fs.file-max = 2048

   2. Set the value for maximum number of open files In the file /etc/security/limits.conf, below the commented line that reads

      #<domain>      <type>  <item>         <value>

      add this line:

      * - nofile 2048

      This line sets the default number of open file descriptors for every user on the system to 2048. Note that the "nofile" item has two possible limit values under the <type> header: hard and soft. Both types of limits must be set before the change in the maximum number of open files will take effect. By using the "-" character, both hard and soft limits are set simultaneously.

      The hard limit represents the maximum value a soft limit may have and the soft limit represents the limit being actively enforced on the system at that time. Hard limits can be lowered by normal users, but not raised and soft limits cannot be set higher than hard limits. Only root may raise hard limits.

When increasing file limit descriptors, you may want to simply double the value. For example, if you need to increase the default value of 1024, increase the value to 2048 first. If you need to increase it again, try 4096, etc.


==================================================================
1。ulimit -a 查看系统目前资源限制的设定。
[root@test security]# ulimit -a
core file size        (blocks, -c) 0
data seg size         (kbytes, -d) unlimited
file size             (blocks, -f) unlimited
max locked memory     (kbytes, -l) unlimited
max memory size       (kbytes, -m) unlimited
open files                    (-n) 1024
pipe size          (512 bytes, -p) 8
stack size            (kbytes, -s) 8192
cpu time             (seconds, -t) unlimited
max user processes            (-u) 7168
virtual memory        (kbytes, -v) unlimited
[root@test security]#
通过以上命令，我们可以看到open files 的最大数为1024
那么我们可以通过一下命令修改该参数的最大值
2. ulimit -n 4096
[root@test security]# ulimit -n 4096
[root@test security]# ulimit -a
core file size        (blocks, -c) 0
data seg size         (kbytes, -d) unlimited
file size             (blocks, -f) unlimited
max locked memory     (kbytes, -l) unlimited
max memory size       (kbytes, -m) unlimited
open files                    (-n) 4096
pipe size          (512 bytes, -p) 8
stack size            (kbytes, -s) 8192
cpu time             (seconds, -t) unlimited
max user processes            (-u) 7168
virtual memory        (kbytes, -v) unlimited

这样我们就修改了系统在同一时间打开文件资源的最大数，基本解决以上问题。

另外我们可以通过lsof  -p [进程的 pid]来查看系统当前打开的文件资源，可以了解不同时期系统的文件资源的使用情况，可根据情况进行系统资源的配置。
===================================================================
从这里学到的linux的命令
查看进程所占的文件描述符  lsof -p 390 | wc -l
查看系统的各种资源限制或者更改这个限制    ulimit -a