Configuration of Resource Limits


Several ways exist to restrict resources on a linux system from a global level down to user/groups or even shells and the processes started by them.


Sysctl is used to modify kernel parameters at runtime. E.g. to set the maximum number of files

 $ sysctl -w fs.file-max=100000

To permanently set them append to the main configuration file and reload the settings

 $ echo fs.file-max=100000 >> /etc/sysctl.conf
 $ sysctl -p

More infos can be found via man sysctl


Allows to restrict resources an a global, group or user level. E.g:

  $ cat /etc/security/limits.d/90-nproc.conf 
  # Default limit for number of user's processes to prevent
  # accidental fork bombs.
  # See rhbz #432903 for reasoning.
  *          soft    nproc     1024

From man limits.conf:

Also, please note that all limit settings are set per login. They are not global, nor are they permanent; existing only for the duration of the session.

The limits per login are applied via the pam stack. See man pam and man pam_limits for more details. As those limits are bound to sessions they don't affect most daemons started by our supported init systems or init utils. Most state that they are ignored by design, see upstart, systemd and start-stop-daemon


From man bash

ulimit [-HSTabcdefilmnpqrstuvx [limit]] Provides control over the resources available to the shell and to processes started by it, on systems that allow such control.

This is what we use in our System V compatible init scripts to increase resources for the open-xchange process across multiple distros. Currently only the maximum number of processes and the maximum number of open file descriptors available to a single user are increased via ulimit. The values are specified in /opt/open-xchange/


Control Groups

Control groups should only affect the OX middleware if you create/manage them yourself of if you are using a modern distribution that already uses systemd as init.

Citing from the kernel cgroup [documentation]:

1-2. What is cgroup?

cgroup is a mechanism to organize processes hierarchically and distribute system resources along the hierarchy in a controlled and configurable manner.

cgroup is largely composed of two parts - the core and controllers. cgroup core is primarily responsible for hierarchically organizing processes. A cgroup controller is usually responsible for distributing a specific type of system resource along the hierarchy although there are utility controllers which serve purposes other than resource distribution.

cgroups form a tree structure and every process in the system belongs to one and only one cgroup. All threads of a process belong to the same cgroup. On creation, all processes are put in the cgroup that the parent process belongs to at the time. A process can be migrated to another cgroup. Migration of a process doesn't affect already existing descendant processes.

Following certain structural constraints, controllers may be enabled or disabled selectively on a cgroup. All controller behaviors are hierarchical - if a controller is enabled on a cgroup, it affects all processes which belong to the cgroups consisting the inclusive sub-hierarchy of the cgroup. When a controller is enabled on a nested cgroup, it always restricts the resource distribution further. The restrictions set closer to the root in the hierarchy can not be overridden from further away.

So processes are organized into a tree structure of control groups and controllers are responsible for the distribution of resources. So what kind of controllers exist?

5. Controllers

5-1. CPU

The "cpu" controllers regulates distribution of CPU cycles. This controller implements weight and absolute bandwidth limit models for normal scheduling policy and absolute bandwidth allocation model for realtime scheduling policy.

5-2. Memory

The "memory" controller regulates distribution of memory. ... While not completely water-tight, all major memory usages by a given cgroup are tracked so that the total memory consumption can be accounted and controlled to a reasonable extent.

5-3. IO

The "io" controller regulates the distribution of IO resources. This controller implements both weight based and absolute bandwidth or IOPS limit distribution; however, weight based distribution is available only if cfq-iosched is in use and neither scheme is available for blk-mq devices.

The open-xchange service is simply put into the default system.slice without applying further limits.

 singlenode$ systemd-cgls --no-pager
 ├─1 /sbin/init
 │ ├─avahi-daemon.service
 │ │ ├─501 avahi-daemon: running [singlenode]
 │ │ └─514 avahi-daemon: chroot helper
 │ ├─console-kit-daemon.service
 │ │ └─16164 /usr/sbin/console-kit-daemon --no-daemon
 │ ├─dbus.service
 │ │ └─508 /usr/bin/dbus-daemon --system --address=systemd: --nofork --nopidfile --systemd-activation
 │ ├─munin-node.service
 │ │ └─4290 /usr/bin/perl -wT /usr/sbin/munin-node
 │ ├─open-xchange.service
 │ │ └─6037 /usr/bin/java -Dnetworkaddress.cache.ttl=3600 -Dnetworkaddress.cache.negative.ttl=10 ...

To check all the details use

singlenode:~ # systemctl show system.slice
Limits besides control groups

Besides control groups systemd allows you to apply other limits to the execution environment of your service. Here we can apply the limits that would normally be applied via limits.conf or ulimit. Systemd uses [setrlimit] for this. The options that we set by default are:

 * LimitNOFILE
 * LimitNPROC

You can check this by looking at the service file that is shipped by default service file

 singlenode:~ # cat /usr/lib/systemd/system/open-xchange.service 
 [Unit] ypbind.service sendmail.service cyrus.service  
 ExecStartPre=/opt/open-xchange/sbin/triggerupdatethemes -u
 ExecStop=/opt/open-xchange/sbin/shutdown -w
 ExecReload=/opt/open-xchange/sbin/triggerreloadconfiguration -d
Drop-in configs

Drop in configs allow administrator to easily override the default service unit files. So if you want to change the default limits or add additional limits have a look at

 singlenode:~ # cat /etc/systemd/system/open-xchange.service.d/limits.conf 
 # Override and add options in this file
 # See systemd.exec(5) for other limits

Open-Xchange middleware on specific distros

The support for the mentioned mechanism of resource control differ depending on the distribution and the init system in use.

Debian 7

System V style
OX Configurable Limits/Defaults
nofile, nproc

The mentioned limits can be configured via /opt/open-xchange/etc/ The limits are applied via ulimit in the service's init script. The open-xchange service is finally started via start-stop-daemon which doesn't doesn't consider /etc/security/limits.*

RHEL 6 / CentOS 6

Upstart, System V compatible
OX Configurable Limits/Defaults
nofile, nproc

The mentioned limits can be configured via /opt/open-xchange/etc/ The limits are applied via ulimit in the service's init script. Furthermore as the open-xchange service is finally started via su ... open-xchange on this distro a user session is opened via su/pam and the default CentOS pam config reads the /etc/security/limits.* configuration by loading the pam stack like:

-> /etc/pam.d/system-auth

If NPROC isn't configured for the open-xchange-server it's restricted to 1024 globally by default to prevent accidental fork bombs, see /etc/security/limits.d/90-nproc.conf which can result in severe problems modern multithreaded applications.

RHEL 7 / CentOS 7 / Debian 8 / SLE 12

OX Configurable Limits/Defaults
nofile, nproc

For systemd the default limits are configured directly in the service's unit file that is shipped by OX and located at /usr/lib/systemd/system/open-xchange.service. The drop-in config to override or extend the default unit file is located at /etc/systemd/system/open-xchange.service.d/limits.conf. Systemd.exec shows a whole lot of options that can be used by admins to adapt the default service to their specific needs.

Verify limits

System V

 singlenode:~ # read pid < /var/run/
 singlenode:~ # cat /proc/$pid/limits
 Limit                     Soft Limit           Hard Limit           Units
 Max cpu time              unlimited            unlimited            seconds
 Max file size             unlimited            unlimited            bytes
 Max data size             unlimited            unlimited            bytes
 Max stack size            8388608              unlimited            bytes
 Max core file size        0                    unlimited            bytes
 Max resident set          unlimited            unlimited            bytes
 Max processes             65536                65536                processes
 Max open files            65536                65536                files
 Max locked memory         65536                65536                bytes
 Max address space         unlimited            unlimited            bytes
 Max file locks            unlimited            unlimited            locks
 Max pending signals       24254                24254                signals
 Max msgqueue size         819200               819200               bytes
 Max nice priority         0                    0
 Max realtime priority     0                    0
 Max realtime timeout      unlimited            unlimited            us


 singlenode:~ # systemctl show open-xchange | grep Limit