About Control Groups
Control groups, usually referred to as cgroups
, are an Oracle Linux kernel feature that enables processes (PIDs
)
to be organized into hierarchical groups for the purpose of resource allocation. For example,
if you have identified 3 sets of processes that need to be allocated CPU time in a ratio of
150:100:50, you can create 3 cgroups
, each with a CPU weight corresponding to
one of the 3 values in your ratio, and then assign the appropriate processes to each
cgroup
.
By default, systemd
creates a cgroup
for the following:
-
Each
systemd
service set up on the host.For example, a server might have control group
NetworkManager.service
to group processes owned by theNetworkManager
service, and control groupfirewalld.service
to group processes owned by thefirewalld
service, and so on. -
Each user (
UID
) on the host.
The cgroup
functionality is mounted as a virtual file system under
/sys/fs/cgroup
. Each cgroup has a corresponding folder within
/sys/fs/cgroup
file system. For example, the cgroups
created by systemd
for the services it manages can be seen by running the
command ls -l /sys/fs/cgroup/system.slice | grep ".service"
as shown in the
following sample code block:
ls -l /sys/fs/cgroup/system.slice | grep ".service"
...root root 0 Mar 22 10:47 atd.service
...root root 0 Mar 22 10:47 auditd.service
...root root 0 Mar 22 10:47 chronyd.service
...root root 0 Mar 22 10:47 crond.service
...root root 0 Mar 22 10:47 dbus-broker.service
...root root 0 Mar 22 10:47 dtprobed.service
...root root 0 Mar 22 10:47 firewalld.service
...root root 0 Mar 22 10:47 httpd.service
...
You can also create cgroups
of your own by creating your own folders under
the /sys/fs/cgroup
virtual file system and assigning process IDs
(PIDs
) to different cgroups
according to your system
needs. However, the recommended practice is to use systemd
to configure
cgroups
instead of creating the cgroups
manually under
/sys/fs/cgroup.
See Using systemd to Manage cgroups v2
for the recommended method of managing cgroups
through
systemd
.
Note:
Use systemd
to configure cgroups
.
Although the recommended method for configuring using systemd
to manage
cgroups, this topic also covers the manual creation of cgroup
folders in
the /sys/fs/cgroup
file system. However, this coverage is mainly to provide
background knowledge of the kernel cgroup
feature to which
systemd
provides access.
Oracle Linux provides two types of control groups:
- Control groups version 1 (
cgroups v1
) -
These groups provide a per-resource controller hierarchy. Each resource, such as CPU, memory, I/O, and so on, has its own control group hierarchy. A disadvantage of this group is the difficulty of establishing proper coordination of resource use among groups that might belong to different process hierarchies.
- Control groups version 2 (
cgroups v2)
-
These groups provide a single control group hierarchy against which all resource controllers are mounted. In this hierarchy, you can obtain better proper coordination of resource uses across different resource controllers. This version is an improvement over
cgroups v1
whose over flexibility prevented proper coordination of resource use among the system consumers.
Both versions are present in Oracle Linux. However, by default, the cgroups v1
functionality is enabled and
mounted on Oracle Linux 8 systems.
For more information about control groups of both versions, see the
cgroups(7)
and sysfs(5)
manual pages.