Tuning file descriptor limits on Linux

Tuning file descriptor limits on Linux

Linux limits the number of file descriptors that any one process may open; the default limits are 1024 per process. These limits can prevent optimum performance of both benchmarking clients (such as httperf and apachebench) and of the web servers themselves (Apache is not affected, since it uses a process per connection, but single process web servers such as Zeus use a file descriptor per connection, and so can easily fall foul of the default limit).

The open file limit is one of the limits that can be tuned with the ulimit command. The command ulimit -aS displays the current limit, and ulimit -aH displays the hard limit (above which the limit cannot be increased without tuning kernel parameters in /proc).

The following is an example of the output of ulimit -aH. You can see that the current shell (and its children) is restricted to 1024 open file descriptors.

core file size (blocks) unlimited
data seg size (kbytes) unlimited
file size (blocks) unlimited
max locked memory (kbytes) unlimited
max memory size (kbytes) unlimited
open files 1024
pipe size (512 bytes) 8
stack size (kbytes) unlimited
cpu time (seconds) unlimited
max user processes 4094
virtual memory (kbytes) unlimited

Increasing the file descriptor limit

The file descriptor limit can be increased using the following procedure:

1. Edit /etc/security/limits.conf and add the lines:

* soft nofile 1024
* hard nofile 65535

2. Edit /etc/pam.d/login, adding the line:

session required /lib/security/pam_limits.so

3. The system file descriptor limit is set in /proc/sys/fs/file-max. The following command will increase the limit to 65535:

echo 65535 > /proc/sys/fs/file-max

4. You should then be able to increase the file descriptor limits using:

ulimit -n unlimited

The above command will set the limits to the hard limit specified in /etc/security/limits.conf.


or---
Increase the limit from 64 (default) to 2048 by issuing the command:

ulimit -n 2048

Also, some applications like the SCO JDK require that the virtual memory resource limit be set to unlimited.

ulimit -v unlimited


Note that you may need to log out and back in again before the changes take effect.