UNIX ulimit Settings

Most UNIX-like operating systems, including Linux and OS X, provide ways to limit and control the usage of system resources such as threads, files, and network connections on a per-process and per-user basis. These “ulimits” prevent single users from using too many system resources. Sometimes, these limits have low default values that can cause a number of issues in the course of normal MongoDB operation.


Red Hat Enterprise Linux and CentOS 6 place a max process limitation of 1024 which overrides ulimit settings. 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. See /etc/security/limits.d/90-nproc.conf file as an example.

Resource Utilization

mongod and mongos each use threads and file descriptors to track connections and manage internal operations. This section outlines the general resource utilization patterns for MongoDB. Use these figures in combination with the actual information about your deployment and its use to determine ideal ulimit settings.

Generally, all mongod and mongos instances:

  • track each incoming connection with a file descriptor and a thread.
  • track each internal thread or pthread as a system process.


  • 1 file descriptor for each data file in use by the mongod instance.
  • 1 file descriptor for each journal file used by the mongod instance when journal is true.
  • In replica sets, each mongod maintains a connection to all other members of the set.

mongod uses background threads for a number of internal processes, including TTL collections, replication, and replica set health checks, which may require a small number of additional resources.


In addition to the threads and file descriptors for client connections, mongos must maintain connects to all config servers and all shards, which includes all members of all replica sets.

For mongos, consider the following behaviors:

  • mongos instances maintain a connection pool to each shard so that the mongos can reuse connections and quickly fulfill requests without needing to create new connections.

  • You can limit the number of incoming connections using the maxConns run-time option.

    By restricting the number of incoming connections you can prevent a cascade effect where the mongos creates too many connections on the mongod instances.


    You cannot set maxConns to a value higher than 20000.

Review and Set Resource Limits



Both the “hard” and the “soft” ulimit 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.

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

$ ulimit -a
-t: cpu time (seconds)         unlimited
-f: file size (blocks)         unlimited
-d: data seg size (kbytes)     unlimited
-s: stack size (kbytes)        8192
-c: core file size (blocks)    0
-m: resident set size (kbytes) unlimited
-u: processes                  192276
-n: file descriptors           21000
-l: locked-in-memory size (kb) 40000
-v: address space (kb)         unlimited
-x: file locks                 unlimited
-i: pending signals            192276
-q: bytes in POSIX msg queues  819200
-e: max nice                   30
-r: max rt priority            65
-N 15:                         unlimited

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.

You can change ulimit settings by issuing a command in the following form:

ulimit -n <value>

For many distributions of Linux you can change values by substituting the -n option for any possible value in the output of ulimit -a. On OS X, use the launchctl limit command. See your operating system documentation for the precise procedure for changing system limits on running systems.


After changing the ulimit settings, you must restart the process to take advantage of the modified settings. 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 system a system restart. Check your distribution and operating system documentation for more information.

/proc File System


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/<pid>/limits, where <pid> 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:


     for process in $@; do
          process_pids=`ps -C $process -o pid --no-headers | cut -d " " -f 2`

          if [ -z $@ ]; then
             echo "[no $process running]"
             for pid in $process_pids; do
                   echo "[$process #$pid -- limits]"
                   cat /proc/$pid/limits



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:

return-limits mongod
return-limits mongos
return-limits mongod mongos