Very often ‘too many open files’ errors occur on high-load Linux servers. It means that a process has opened too many files (file descriptors) and cannot open new ones. On Linux, the “max open file limit” is set by default per process or user and the values are rather small.

In this article, we’ll look at how to check the current limits on the maximum number of open files in Linux, and how to change this setting globally for the entire server, for specific services, and for a user session. The article is applicable to most modern Linux distros (Debian, Ubuntu, CentOS, RHEL, Oracle Linux, Rocky, etc.)

‘Too Many Open Files’ Error and Open Files Limit in Linux

First of all, let’s see where the ‘too many open files’ errors appear. Most often it occurs on the servers with an installed Nginx /httpd web server or a database server running MySQL/MariaDB/PostgreSQL when reading a large number of log files. For example, when an Nginx exceeds the open files limit, you will see an error:

socket () failed (29: Too many open files) while connecting to upstream

Or:

HTTP: Accept error: accept tcp [::]:<port_number>: accept4: too many open files.

In Python apps:

OSError: [Errno 24] Too many open files.

Using this command, you can get the maximum number of file descriptors your system can open:

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

To find out how many files are currently open, run:

# cat /proc/sys/fs/file-nr

7122??? 123????? 92312720

• 7122 — total number of open files
• 123– number of open files that are not currently in use
• 92312720– maximum number of files allowed to be opened

In Linux, you can configure max open files limits at several levels:

• OS kernel
• Service
• User

To display the current limit on the number of open files in the Linux kernel, run:
# sysctl fs.file-max

fs.file-max = 92233720

Let’s display the open files limit for one process of the current user:

# ulimit -n

By default, the number of files for one process of this is limited to 1024.

Let’s display the maximum number for one user (max user processes):

# ulimit –u

5041

Multiplying 1024 * 5041 gives us 5161984 – this is the maximum number of open files by all user processes.

There are two types of limits on the number of open files: Hard?and?Soft. Soft limits are advisory. If the number of open files has exceeded the hard limit, the user won’t be able to open new files until the previously opened ones are closed.

To view the current limits, use the ulimit command with the -S (soft) or -H (hard) option and the -n (the maximum number of open file descriptors) option.

To display the soft limit, run this command:

# ulimit –Sn

To display the hard limit value:

# ulimit -Hn

How to Increase the Max Open Files Limit in Linux?

To allow all services to open a large number of files, you can change the limits in your Linux OS. To make new settings permanent and prevent their reset after a server or session restart, you must make changes to /etc/security/limits.conf. This file allows limiting the number of various system resources available to a user process. Use the following format:

fs.file-max = 500000

Check that the file /etc/pam.d/common-session (Debian/Ubuntu) or /etc/pam.d/login (CentOS/RedHat/Fedora) contains the line:

session required pam_limits.so

If not, add it to the end of the config file. This parameter allows applying open file limits after user authentication.

After making any changes, re-open the console, and check the max_open_files value:

# ulimit -n

50000

Increase the Maximum Number of Open File Descriptors per Service

You can increase the max open file descriptors for a specific service, rather than for the entire operating system. Let’s take apache as an example. Open the service settings using?systemctl:

# systemctl edit httpd.service

Add the limits you want, e.g.:

[Service]
LimitNOFILE=16000
LimitNOFILESoft=16000

After making the changes, update the service configuration, and restart it:

# systemctl daemon-reload
# systemctl restart httpd.service

To check if the values have changed, get the service PID:

# systemctl status httpd.service

For example, the service PID is 3724:

# cat /proc/3724/limits | grep "Max open files"

The value must be 16000.

Thus, you have changed Max open files value for a specific service.

How to Set Max Open Files for Nginx and Apache?

After you have increased the limit on the number of open files for a server, you also have to change the service configuration file. For example, specify/change the following directive value in the Nginx configuration file /etc/nginx/nginx.conf:

worker_rlimit_nofile 16000

The worker_rlimit_nofile directive sets the limit on the number of files open by a worker process ( RLIMIT_NOFILE ). Nginx needs file descriptors to return a static file from the cache for each client connection. The more users connect to your server and the more static files Nginx returns, the more file descriptors are used. The maximum number of handles is limited at the OS and/or service level. If the number of open files in Nginx is exceeded, a “socket() failed (24: Too many open files) while connecting to upstream error” will occur.

Then restart Nginx.

# nginx -t &&?service?nginx -s reload

For Apache, you need to create a directory:

# mkdir /lib/systemd/system/httpd.service.d/

Then create the limit_nofile.conf file:

# nano /lib/systemd/system/httpd.service.d/limit_nofile.conf

Add to it:

[Service]
LimitNOFILE=16000

Don’t forget to restart the httpd service.

Change the Open File Limit for the Current User Session

To change the maximum open file limits for your current user session, run this command:

# ulimit -n 3000

If you specify a value here greater than that specified in the hard limit, an error will appear:

-bash: ulimit: open files: cannot modify limit: Operation not permitted

After closing the session and opening a new one, the limits will return to the initial values specified in /etc/security/limits.conf.

In this article, we have learned how to solve the issue when the value of open file descriptors limit in Linux is too small, and looked at several options for changing these limits on the server.