# cgroup

cgroup是计算机资源管理常用的管理设施，尤其是在云计算环境使用广泛。这里我们从cgroup的使用和架构上进行一次学习。

## 基本使用

对大多数人来说，接触cgroup还是使用cgroup来控制应用程序的资源。所以这一节，我们先来看看

[使用cgroup控制进程cpu和内存](/kernel-exploring/00-index/01-control_cpu_mem_by_cgroup.md)

## cgroup文件系统

在上一节基本使用中，我们通过创建目录，向指定文件写入内容来使用cgroup。我想你也一定好奇，为什么这么操作就能达到这样的效果？

这一切都要归功于[cgroup文件系统](/kernel-exploring/00-index/02-cgroup_fs.md)

## cgroup树形结构

从用户使用的角度看过了cgroup后，终于是时候揭开cgroup真实的面纱了。

在开始之前，我们先来好好看看cgroup的定义。来个借花献佛，我搬运一下[内核文档](https://www.kernel.org/doc/html/latest/admin-guide/cgroup-v1/cgroups.html)

```
Control Groups provide a mechanism for aggregating/partitioning sets of
tasks, and all their future children, into hierarchical groups with
specialized behaviour.

Definitions:

A *cgroup* associates a set of tasks with a set of parameters for one
or more subsystems.

A *subsystem* is a module that makes use of the task grouping
facilities provided by cgroups to treat groups of tasks in
particular ways. A subsystem is typically a "resource controller" that
schedules a resource or applies per-cgroup limits, but it may be
anything that wants to act on a group of processes, e.g. a
virtualization subsystem.

A *hierarchy* is a set of cgroups arranged in a tree, such that
every task in the system is in exactly one of the cgroups in the
hierarchy, and a set of subsystems; each subsystem has system-specific
state attached to each cgroup in the hierarchy.  Each hierarchy has
an instance of the cgroup virtual filesystem associated with it.
```

从上面的描述中可以看到cgroup有两个重要的概念：

* 层次结构
* 子系统

层次结构是对系统中进程的切分，而子系统则赋予了cgroup在不同层面对进程的切变控制的方法。

每个子系统对系统的控制有着不同的方式，所以在本章我们主要讲述[cgroup的树形结构](/kernel-exploring/00-index/03-hierarchy.md)。

虽然会涉及到一些子系统的内容，但其其中具体的精妙细节将留给各个子系统描述。

## cgroup和进程的关联

接下来我们需要了解的是cgroup和进程的关系。

从上面的定义中，我们看到cgroup联结了计算机系统中的两部分内容：

* 进程
* 系统资源

在上一节中，我们看到了cgroup的subsystem将系统的资源按照层次结构做了切分。（当然具体怎么切的需要看对应的subsystem，cgroup本身只提供了一种层次化结构框架。）那接下来就要看看内核是如何把进程和cgroup联系起来的。

[cgroup和进程的关联](/kernel-exploring/00-index/04-cgroup_and_process.md)

## cgroup数据统计

最后一个有意思的部分就是cgroup上如果做数据同步。当整个cgroup树变得庞大后，如何有效将子cgroup的信息同步收集到父cgroup，来保证cgroup能有效控制资源，这是一个非常有意思的问题。

[cgroup数据统计](/kernel-exploring/00-index/05-statistics.md)


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