Increasing the number of available file descriptors on Centos and Fedora Linux Servers

While debugging an application a few weeks back, I noticed the following error in the application log:

Cannot open file : Too many open files

The application runs as an unprivileged user, and upon closer inspection I noticed that the maximum number of file descriptors available to the process was 1024:

$ ulimit -n

Increasing the maximum number of file descriptors is a two part process. First, you need to make sure the kernel’s file-max tunable is set sufficiently. This value controls the number of files that can be open system-side, and is controlled through the file-max proc setting:

$ cat /proc/sys/fs/file-max

$ echo 512000 > /proc/sys/fs/file-max

$ cat /proc/sys/fs/file-max

Once you know you have enough file descriptors available, you will need to add an entry for the user (or add * if this applies to all users) to the security limits file (/etc/security/limits.conf). This file is used to control resource limits for users and groups, and is processes when a user login session is initialized. To increase the number of file descriptors for all users to 8k, the following entry can be appended to the file:

*                -       nofile          8192

If you would prefer to set the value for a specific user, you can do so by replacing the asterisk with the

user1            -       nofile          8192

Once the file-max proc setting and security limits file are updated, ulimit should report the increase:

$ ulimit -n

If you want to test to see how many file descriptors are available to a given user, you can run the openfds program I wrote:

$ ./openfds
Error: Too many open files
Last file descriptor successfully opened: 8189

Openfds will print the string representation of the errno value that resulted from open returning -1, as well as the last file descriptor that was successfully opened. If you are curious why 8189 is reported, it’s because STDIN, STDOUT and STDERR take up 3 entries in the file descriptor table for the process.

1 thought on “Increasing the number of available file descriptors on Centos and Fedora Linux Servers”

  1. BTW: there is a difference between soft and hard limits. ulimit -Sn and -Hn. You can set both to the same limit, or different. (this is the – in the second column)

    Some applications (like Java) automatically up the soft limit to the hard limit.

    In some situations (like daemons started) or sshd with switched off pam, the file might not get used.


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