Change Open File Limits

A guide that shows how to change the open file limtits

VerneMQ can consume a large number of open file handles when thousands of clients are connected as every connection requires at least one file handle.

Most operating systems can change the open-files limit using the ulimit -n command. Example:

ulimit -n 65536

However, this only changes the limit for the current shell session. Changing the limit on a system-wide, permanent basis varies more between systems.

Linux

On most Linux distributions, the total limit for open files is controlled by sysctl.

sysctl fs.file-max
fs.file-max = 50384

As seen above, it is generally set high enough for VerneMQ. If you have other things running on the system, you might want to consult the sysctl manpage manpage for how to change that setting. However, what most needs to be changed is the per-user open files limit. This requires editing /etc/security/limits.conf, for which you'll need superuser access. If you installed VerneMQ from a binary package, add lines for the vernemq user like so, substituting your desired hard and soft limits:

vernemq soft nofile 4096
vernemq hard nofile 65536

On Ubuntu, if you’re always relying on the init scripts to start VerneMQ, you can create the file /etc/default/vernemq and specify a manual limit like so:

ulimit -n 65536

This file is automatically sourced from the init script, and the VerneMQ process started by it will properly inherit this setting. As init scripts are always run as the root user, there’s no need to specifically set limits in /etc/security/limits.conf if you’re solely relying on init scripts.

On CentOS/RedHat systems, make sure to set a proper limit for the user you’re usually logging in with to do any kind of work on the machine, including managing VerneMQ. On CentOS, sudo properly inherits the values from the executing user.

Systemd

Systemd allows you to set the open file limit. The LimitNOFILE parameter defines the maximum number of file descriptors that a service or system unit can open. In the past, "infinite" was often chosen, which actually means an OS/systemD dependent maximum number. However, in recent versions of systemd like RHEL 9, CentOS Stream 9, and others, the default value is set to around a billion, significantly higher than necessary and the defaults used in older distributions. It is advisable to set a reasonable default value for LimitNOFILE based on the specific use case. Please consult https://access.redhat.com/solutions/1479623 for more information (RHEL9).

Enable PAM-Based Limits for Debian & Ubuntu

It can be helpful to enable PAM user limits so that non-root users, such as the vernemq user, may specify a higher value for maximum open files. For example, follow these steps to enable PAM user limits and set the soft and hard values for all users of the system to allow for up to 65536 open files.

Edit /etc/pam.d/common-session and append the following line:

session    required   pam_limits.so

If /etc/pam.d/common-session-noninteractive exists, append the same line as above.

Save and close the file.

Edit /etc/security/limits.conf and append the following lines to the file:

*               soft     nofile          65536
*               hard     nofile          65536

1. Save and close the file.

2. (optional) If you will be accessing the VerneMQ nodes via secure shell (ssh), you should also edit /etc/ssh/sshd_config and uncomment the following line:

#UseLogin no

and set its value to yes as shown here:

UseLogin yes

1. Restart the machine so that the limits to take effect and verify

   that the new limits are set with the following command:

ulimit -a

Enable PAM-Based Limits for CentOS and Red Hat

1. Edit /etc/security/limits.conf and append the following lines to

   the file:

*               soft     nofile          65536
*               hard     nofile          65536

1. Save and close the file.

2. Restart the machine so that the limits to take effect and verify that the new limits are set with the following command:

ulimit -a

In the above examples, the open files limit is raised for all users of the system. If you prefer, the limit can be specified for the vernemq user only by substituting the two asterisks (*) in the examples with vernemq.

Solaris

In Solaris 8, there is a default limit of 1024 file descriptors per process. In Solaris 9, the default limit was raised to 65536. To increase the per-process limit on Solaris, add the following line to /etc/system:

set rlim_fd_max=65536

Reference:

Mac OS X

To check the current limits on your Mac OS X system, run:

launchctl limit maxfiles

The last two columns are the soft and hard limits, respectively.

To adjust the maximum open file limits in OS X 10.7 (Lion) or newer, edit /etc/launchd.conf and increase the limits for both values as appropriate.

For example, to set the soft limit to 16384 files, and the hard limit to 32768 files, perform the following steps:

1. Verify current limits:

   launchctl limit

   The response output should look something like this:

   cpu         unlimited      unlimited
   filesize    unlimited      unlimited
   data        unlimited      unlimited
   stack       8388608        67104768
   core        0              unlimited
   rss         unlimited      unlimited
   memlock     unlimited      unlimited
   maxproc     709            1064
   maxfiles    10240          10240

2. Edit (or create) /etc/launchd.conf and increase the limits. Add lines that look like the following (using values appropriate to your environment):

   limit maxfiles 16384 32768

3. Save the file, and restart the system for the new limits to take effect. After restarting, verify the new limits with the launchctl limit command:

   launchctl limit

   The response output should look something like this:

   cpu         unlimited      unlimited
   filesize    unlimited      unlimited
   data        unlimited      unlimited
   stack       8388608        67104768
   core        0              unlimited
   rss         unlimited      unlimited
   memlock     unlimited      unlimited
   maxproc     709            1064
   maxfiles    16384          32768

Attributions

This work, "Open File Limits", is a derivative of Open File Limits by Riak, used under Creative Commons Attribution 3.0 Unported License. "Open File Limits" is licensed under Creative Commons Attribution 3.0 Unported License by Erlio GmbH.