FreeBSD provides several methods for an administrator to limit the amount of system resources an individual may use. Disk quotas limit the amount of disk space available to users. Quotas are discussed in Section 17.11, “Disk Quotas”.
Limits to other resources, such as CPU
and memory, can be set using either a flat file or a command to
configure a resource limits database. The traditional method
defines login classes by editing
/etc/login.conf. While this method is
still supported, any changes require a multi-step process of
editing this file, rebuilding the resource database, making
necessary changes to /etc/master.passwd,
and rebuilding the password database. This can become time
consuming, depending upon the number of users to
configure.
rctl can be used to provide a more
fine-grained method for controlling resource limits. This
command supports more than user limits as it can also be used to
set resource constraints on processes and jails.
This section demonstrates both methods for controlling resources, beginning with the traditional method.
In the traditional method, login classes and the resource
limits to apply to a login class are defined in
/etc/login.conf. Each user account can
be assigned to a login class, where default
is the default login class. Each login class has a set of
login capabilities associated with it. A login capability is
a
name=valuename is a well-known
identifier and value is an
arbitrary string which is processed accordingly depending on
the name.
Whenever /etc/login.conf is edited,
the /etc/login.conf.db must be updated
by executing the following command:
#cap_mkdb /etc/login.conf
Resource limits differ from the default login capabilities in two ways. First, for every limit, there is a soft and hard limit. A soft limit may be adjusted by the user or application, but may not be set higher than the hard limit. The hard limit may be lowered by the user, but can only be raised by the superuser. Second, most resource limits apply per process to a specific user.
Table 13.1, “Login Class Resource Limits” lists the most commonly used resource limits. All of the available resource limits and capabilities are described in detail in login.conf(5).
| Resource Limit | Description |
|---|---|
| coredumpsize | The limit on the size of a core file generated by
a program is subordinate to other limits on disk
usage, such as filesize or disk
quotas. This limit is often used as a less severe
method of controlling disk space consumption. Since
users do not generate core files and often do not
delete them, this setting may save them from running
out of disk space should a large program
crash. |
| cputime | The maximum amount of CPU time
a user's process may consume. Offending processes
will be killed by the kernel. This is a limit on
CPU time
consumed, not the percentage of the
CPU as displayed in some of the
fields generated by top and
ps. |
| filesize | The maximum size of a file the user may own. Unlike disk quotas (Section 17.11, “Disk Quotas”), this limit is enforced on individual files, not the set of all files a user owns. |
| maxproc | The maximum number of foreground and background
processes a user can run. This limit may not be
larger than the system limit specified by
kern.maxproc. Setting this limit
too small may hinder a user's productivity as some
tasks, such as compiling a large program, start lots
of processes. |
| memorylocked | The maximum amount of memory a process may request to be locked into main memory using mlock(2). Some system-critical programs, such as amd(8), lock into main memory so that if the system begins to swap, they do not contribute to disk thrashing. |
| memoryuse | The maximum amount of memory a process may consume at any given time. It includes both core memory and swap usage. This is not a catch-all limit for restricting memory consumption, but is a good start. |
| openfiles | The maximum number of files a process may have
open. In FreeBSD, files are used to represent sockets
and IPC channels, so be careful not
to set this too low. The system-wide limit for this
is defined by
kern.maxfiles. |
| sbsize | The limit on the amount of network memory a user may consume. This can be generally used to limit network communications. |
| stacksize | The maximum size of a process stack. This alone is not sufficient to limit the amount of memory a program may use, so it should be used in conjunction with other limits. |
There are a few other things to remember when setting resource limits:
Processes started at system startup by
/etc/rc are assigned to the
daemon login class.
Although the default
/etc/login.conf is a good source of
reasonable values for most limits, they may not be
appropriate for every system. Setting a limit too high
may open the system up to abuse, while setting it too low
may put a strain on productivity.
Xorg takes a lot of resources and encourages users to run more programs simultaneously.
Many limits apply to individual processes, not the
user as a whole. For example, setting
openfiles to 50
means that each process the user runs may open up to
50 files. The total amount of files a
user may open is the value of openfiles
multiplied by the value of maxproc.
This also applies to memory consumption.
For further information on resource limits and login classes and capabilities in general, refer to cap_mkdb(1), getrlimit(2), and login.conf(5).
The kern.racct.enable tunable must be
set to a non-zero value. Custom kernels require specific
configuration:
options RACCT options RCTL
Once the system has rebooted into the new kernel,
rctl may be used to set rules for the
system.
Rule syntax is controlled through the use of a subject, subject-id, resource, and action, as seen in this example rule:
user:trhodes:maxproc:deny=10/user
In this rule, the subject is user, the
subject-id is trhodes, the resource,
maxproc, is the maximum number of
processes, and the action is deny, which
blocks any new processes from being created. This means that
the user, trhodes, will be constrained to
no greater than 10 processes. Other
possible actions include logging to the console, passing a
notification to devd(8), or sending a sigterm to the
process.
Some care must be taken when adding rules. Since this
user is constrained to 10 processes, this
example will prevent the user from performing other tasks
after logging in and executing a
screen session. Once a resource limit has
been hit, an error will be printed, as in this example:
%man test/usr/bin/man: Cannot fork: Resource temporarily unavailable eval: Cannot fork: Resource temporarily unavailable
As another example, a jail can be prevented from exceeding a memory limit. This rule could be written as:
#rctl -a jail:httpd:memoryuse:deny=2G/jail
Rules will persist across reboots if they have been added
to /etc/rctl.conf. The format is a rule,
without the preceding command. For example, the previous rule
could be added as:
# Block jail from using more than 2G memory: jail:httpd:memoryuse:deny=2G/jail
To remove a rule, use rctl to remove it
from the list:
#rctl -r user:trhodes:maxproc:deny=10/user
A method for removing all rules is documented in rctl(8). However, if removing all rules for a single user is required, this command may be issued:
#rctl -r user:trhodes
Many other resources exist which can be used to exert
additional control over various subjects.
See rctl(8) to learn about them.
All FreeBSD documents are available for download at https://download.freebsd.org/ftp/doc/
Questions that are not answered by the
documentation may be
sent to <freebsd-questions@FreeBSD.org>.
Send questions about this document to <freebsd-doc@FreeBSD.org>.