1. file-max的含義

man proc，可得到file-max的描述：

/proc/sys/fs/file-max
              This  file defines a system-wide limit on the number of open files for all processes.  (See
              also setrlimit(2),  which  can  be  used  by  a  process  to  set  the  per-process  limit,
              RLIMIT_NOFILE,  on  the  number  of  files it may open.)  If you get lots of error messages
              about running out of file handles, try increasing this value:

即file-max是設定

也就是說，這項引數是系統級別的。

echo  6553560 > /proc/sys/fs/file-max

或修改 /etc/sysctl.conf, 加入

fs.file-max = 6553560 重啟生效

[System-wide File Descriptors (FD) Limits

The number of concurrently open file descriptors throughout the system can be changed via /etc/sysctl.conf file under Linux operating systems.]

2. ulimit的

Provides control over the resources available to the shell and to processes started by it, on systems that allow  such control.

即設定當前shell以及由它啟動的程序的資源限制。

顯然，對伺服器來說，file-max, ulimit都需要設定，否則就可能出現檔案描述符用盡的問題，為了讓機器在重啟之後仍然有效，強烈建立作以下配置，以確保file-max, ulimit的值正確無誤：

1. 修改/etc/sysctl.conf, 加入

fs.file-max = 6553560

2.系統預設的ulimit對檔案開啟數量的限制是1024，修改/etc/security/limits.conf並加入以下配置，永久生效

* soft nofile 65535 
* hard nofile 65535

3.nr_open是單個程序可分配的最大檔案數，所以在使用ulimit或limits.conf來設定時，如果要超過預設的1048576值時需要先增大nr_open值（sysctl -w fs.nr_open=100000000或者直接寫入sysctl.conf檔案）
nr_open:
This denotes the maximum number of file-handles a process can
allocate. Default value is 1024*1024 (1048576) which should be
enough for most machines. Actual limit depends on RLIMIT_NOFILE
resource limit.

修改完之後，重啟即可生效

[User Level FD Limits

The above procedure sets system-wide file descriptors (FD) limits. However, you can limit httpd (or any other users) user to specific limits by editing /etc/security/limits.conf file]

3,nofile的最大值就是nr_open的值，預設是1024*1024

/etc/security/limits.conf nofile absolute maximum
Apparently unlimited != unlimited in the Linux kernel for maximum number of open files. After some extensive digging, I finally found the actual maximum to the nofile setting in /etc/security/limits.conf. Yes I was searching in the context of Oracle (Imagine that) for a maximum number of procs / files. The Linux kernel has a hard upper limit of 1024*1024 (1048576 – a magical number I won’t soon forget).


Consider the following:


[[email protected] ~]# grep mrsmith /etc/security/limits.conf
mrsmith     soft    nofile      unlimited   
mrsmith     hard    nofile      unlimited
Trying to su to mrsmith suddenly casts you aside:


[[email protected] ~]# su - mrsmith
could not open session
Displaying the open file setting for mrsmith shows something odd:


[[email protected] ~]# ulimit -au mrsmith
...
open files                      (-n) 1024
Why would our number of open files be 1024? This apparently is the default setting for users without a custom nofile set based on the following bugzilla.


Then what is our actual upper bound for nofile in /etc/security/limits.conf? This isn’t well documented anywhere I’ve seen, but based on some research it appears to be 1024*1024. Setting nofile to 1048577 produces the same “could not open session” error as mentioned previously. 1048576 however seems to work just fine. Although you can set your nofile ulimit to this, the effects of such are unknown and I would highly recommend testing any such setting.