cgroups

cgroups (abbreviated from control groups) is a Linux kernel feature that limits, accounts for, and isolates the resource usage (CPU, memory, disk I/O, network, etc.) of a collection of processes.

Engineers at Google (primarily Paul Menage and Rohit Seth) started the work on this feature in 2006 under the name "process containers".^[1] In late 2007, the nomenclature changed to "control groups" to avoid confusion caused by multiple meanings of the term "container" in the Linux kernel context, and the control groups functionality was merged into the Linux kernel mainline in kernel version 2.6.24, which was released in January 2008.^[2] Since then, developers have added many new features and controllers, such as support for kernfs,^[3] firewalling,^[4] and unified hierarchy.^[5]

Versions

There are two versions of cgroups.

Cgroups was originally written by Paul Menage et al. and mainlined into the Linux kernel in 2007. Afterwards this is called cgroups version 1.^[6]

Then development and maintenance of cgroups was taken over by Tejun Heo. Tejun Heo redesigned and rewrote cgroups. This rewrite is now called version 2, the documentation of cgroups-v2 first appeared in Linux kernel 4.5 released on March 14, 2016.^[7]

Unlike v1, cgroup v2 has only a single process hierarchy and discriminates between processes, not threads.

Features

One of the design goals of cgroups is to provide a unified interface to many different use cases, from controlling single processes (by using nice, for example) to whole operating system-level virtualization (as provided by OpenVZ, Linux-VServer or LXC, for example). Cgroups provides:

• Resource limiting – groups can be set to not exceed a configured memory limit, which also includes the file system cache^[8][9]
• Prioritization – some groups may get a larger share of CPU utilization^[10] or disk I/O throughput^[11]
• Accounting – measures a group's resource usage, which may be used, for example, for billing purposes^[12]
• Control – freezing groups of processes, their checkpointing and restarting^[12]

Use

As an example of indirect usage, systemd assumes exclusive access to the cgroups facility

A control group (abbreviated as cgroup) is a collection of processes that are bound by the same criteria and associated with a set of parameters or limits. These groups can be hierarchical, meaning that each group inherits limits from its parent group. The kernel provides access to multiple controllers (also called subsystems) through the cgroup interface;^[2] for example, the "memory" controller limits memory use, "cpuacct" accounts CPU usage, etc.

Control groups can be used in multiple ways:

• By accessing the cgroup virtual file system manually.
• By creating and managing groups on the fly using tools like cgcreate, cgexec, and cgclassify (from libcgroup).
• Through the "rules engine daemon" that can automatically move processes of certain users, groups, or commands to cgroups as specified in its configuration.
• Indirectly through other software that uses cgroups, such as Docker, Linux Containers (LXC) virtualization,^[13] libvirt, systemd, Open Grid Scheduler/Grid Engine,^[14] and Google's lmctfy.

The Linux kernel documentation contains full technical details of the setup and use of control groups.^[15]

Redesign

Redesign of cgroups started in 2013,^[16] with additional changes brought by versions 3.15 and 3.16 of the Linux kernel.^[17][18][19]

Namespace isolation

While not technically part of the cgroups work, a related feature of the Linux kernel is namespace isolation, where groups of processes are separated such that they cannot "see" resources in other groups. For example, a PID namespace provides a separate enumeration of process identifiers within each namespace. Also available are mount, UTS, network and SysV IPC namespaces.

• The PID namespace provides isolation for the allocation of process identifiers (PIDs), lists of processes and their details. While the new namespace is isolated from other siblings, processes in its "parent" namespace still see all processes in child namespaces—albeit with different PID numbers.^[20]
• Network namespace isolates the network interface controllers (physical or virtual), iptables firewall rules, routing tables etc. Network namespaces can be connected with each other using the "veth" virtual Ethernet device.^[21]
• "UTS" namespace allows changing the hostname.
• Mount namespace allows creating a different file system layout, or making certain mount points read-only.^[22]
• IPC namespace isolates the System V inter-process communication between namespaces.
• User namespace isolates the user IDs between namespaces.^[23]

Namespaces are created with the "unshare" command or syscall, or as new flags in a "clone" syscall.^[24]

The "ns" subsystem was added early in cgroups development to integrate namespaces and control groups. If the "ns" cgroup was mounted, each namespace would also create a new group in the cgroup hierarchy. This was an experiment that was later judged to be a poor fit for the cgroups API, and removed from the kernel.

Linux namespaces were inspired by the more general namespace functionality used heavily throughout Plan 9 from Bell Labs.^[25]

Unified hierarchy

Whenever designing software, a software engineer seeks solutions which overall best address exigencies regarding stability, security, performance, as well as maintainability, programmability (API) and usability (ABI). By their nature, these exigencies balance each other, e.g., a mighty API to user space, that doesn't offer too much functionality, but carelessly exposes some key inner working, might seriously compromise stability and security. That is especially true if that software is part of the Linux kernel.

Tejun Heo decided to alter cgroups to prevent these scenarios, designing and implementing a unified hierarchy with only one user space entity that has exclusive access to the facilities offered by cgroups.

Kernfs was introduced into the Linux kernel with version 3.14, the main author being Tejun Heo.^[26] One of the main motivators for a separate kernfs is the cgroups file system. Kernfs is basically created by splitting off some of the sysfs logic into an independent entity so that other kernel subsystems can more easily implement their own virtual file system with handling for device connect and disconnect, dynamic creation and removal as needed or unneeded, and other attributes. Redesign continued into version 3.15 of the Linux kernel.^[27]

Kernel memory control groups (kmemcg)

Kernel memory control groups (kmemcg) were merged into version 3.8 of the Linux kernel mainline.^[28][29][30] The kmemcg controller can limit the amount of memory that the kernel can utilize to manage its own internal processes.

Adoption

Various projects use cgroups as their basis, including CoreOS, Docker, Hadoop, Jelastic, Kubernetes,^[31] lmctfy (Let Me Contain That For You), LXC (LinuX Containers), systemd, Mesos and Mesosphere,^[31] HTCondor.

See also

• Operating system–level virtualization implementations
• Process group
• Tc (Linux) – a traffic control utility that has slight functional overlap with network-oriented cgroup settings

References

External links

• Linux kernel documentation on cgroups
• Linux kernel Namespaces and cgroups by Rami Rosen
• Namespaces and cgroups, the basis of Linux containers (including cgroups v2), slides of a talk by Rami Rosen, Netdev 1.1, Seville, Spain, 2016
• Understanding the new control groups API, LWN.net, by Rami Rosen, March 2016
• Large-scale cluster management at Google with Borg, April 2015, by Abhishek Verma, Luis Pedrosa, Madhukar Korupolu, David Oppenheimer, Eric Tune and John Wilkes
• Job Objects, similar feature on Windows