You can use the ulimit command at the system prompt to check system limits, as in the following example:

ulimit refers to the per-user limitations for various resources. Therefore, if your mongod instance executes as a user that is also running multiple processes, or multiple mongod processes, you might see contention for these resources. Also, be aware that the processes value (i.e. -u) refers to the combined number of distinct processes and sub-process threads.

On Linux, you can change ulimit settings by issuing a command in the following form:

There are both "hard" and the "soft" ulimit s that affect MongoDB's performance. The "hard" ulimit refers to the maximum number of processes that a user can have active at any time. This is the ceiling: no non-root process can increase the "hard" ulimit. In contrast, the "soft" ulimit is the limit that is actually enforced for a session or process, but any process can increase it up to "hard" ulimit maximum.

A low "soft" ulimit can cause can't create new thread, closing connection errors if the number of connections grows too high. For this reason, it is extremely important to set both ulimit values to the recommended values.

ulimit will modify both "hard" and "soft" values unless the -H or -S modifiers are specified when modifying limit values.

For many distributions of Linux you can change values by substituting the -n option for any possible value in the output of ulimit -a.

After changing the ulimit settings, you must restart the process to take advantage of the modified settings. On Linux, you can use the /proc file system to see the current limitations on a running process.

Depending on your system's configuration, and default settings, any change to system limits made using ulimit may revert following a system restart. Check your distribution and operating system documentation for more information.

You should typically start mongod using systemctl, which uses the ulimit settings:

If you do not start mongod using systemctl, systemd overrides some of the ulimit settings. For example, if you start mongod as shown in the following command, then a user slice (such as user-1000.slice) systemd setting is used:

A systemd user slice limits the resources for the user's processes.

For macOS systems that have installed MongoDB Community using the brew installation method, the recommended open files value is automatically set when you start MongoDB through brew services.

For macOS systems running MongoDB Enterprise or using the TGZ installation method, use the ulimit command to set the recommended values. See your operating system documentation for the precise procedure for changing system limits on running systems.

Red Hat Enterprise Linux and CentOS 6 and 7 enforce a separate max process limitation, nproc, which overrides ulimit settings. This value is defined in the following configuration file, depending on version:

To configure an nproc value for these versions, create a file named /etc/security/limits.d/99-mongodb-nproc.conf with new soft nproc and hard nproc values to increase the process limit. For recommended values, see Recommended ulimit Settings.

With RHEL / CentOS 8, separate nproc values are no longer necessary. The ulimit command is sufficient to configure the required max process values on RHEL / CentOS 8.

Every deployment may have unique requirements and settings; however, the following thresholds and settings are particularly important for mongod and mongos deployments:

-f (file size): unlimited

-t (cpu time): unlimited

-v (virtual memory): unlimited [1]

-l (locked-in-memory size): unlimited

-n (open files): 64000

-m (memory size): unlimited [1] [2]

-u (processes/threads): 64000

Always remember to restart your mongod and mongos instances after changing the ulimit settings to ensure that the changes take effect.

Incoming connections to a mongod or mongos instance require two file descriptors.

For the macOS platform, the recommended process limit is 2500, which is the maximum configurable value for this platform.

For Linux distributions that use Upstart, you can specify limits within service scripts if you start mongod and/or mongos instances as Upstart services. You can do this by using limit stanzas.

Specify the Recommended ulimit Settings, as in the following example:

Each limit stanza sets the "soft" limit to the first value specified and the "hard" limit to the second.

After changing limit stanzas, ensure that the changes take effect by restarting the application services, using the following form:

If you start a mongod and/or mongos instance as a systemd service, you can specify limits within the [Service] section of its service file. The service file has a location like /etc/systemd/system/.service.

You can set limits by using resource limit directives.

Specify the Recommended ulimit Settings, as in the following example:

Each systemd limit directive sets both the "hard" and "soft" limits to the value specified.

After changing limit stanzas, ensure that the changes take effect by restarting the application services, using the following form:

If you installed MongoDB via a package manager such as yum or apt, the service file installed as part of your installation already contains these ulimit values.

This section applies only to Linux operating systems.

The /proc file-system stores the per-process limits in the file system object located at /proc//limits, where is the process's PID or process identifier. You can use the following bash function to return the content of the limits object for a process or processes with a given name:

You can copy and paste this function into a current shell session or load it as part of a script. Call the function with one the following invocations: