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ulimit
Settings
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.
Note
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.
mongod
¶
- 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 whenjournal
istrue
. - 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.
mongos
¶
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 themongos
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 themongod
instances.Note
You cannot set
maxConns
to a value higher than 20000.
Review and Set Resource Limits¶
ulimit
¶
Note
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
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:
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.
Note
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¶
Note
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:
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:
Recommended Settings¶
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]-n
(open files):64000
-m
(memory size):unlimited
[1]-u
(processes/threads):64000
Always remember to restart your mongod
and
mongos
instances after changing the ulimit
settings to
make sure that the settings change takes effect.
[1] | (1, 2) If you limit virtual or resident memory size on a system running MongoDB the operating system will refuse to honor additional allocation requests. |