Ashish pandey huawei osi_days2011_cgroups_understanding_better

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Cgroups: Understanding better!

OSI Days, November, 2011, Bangalore

Ashish Pandey, Liuyang, Roopesh Keeppattu, Sanil Kumar D.

HUAWEI TECHNOLOGIES CO., LTD. OSI Days Nov 2011

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Agenda• Setting the context…

- Background and Cgroups Definition - Significance of Cgroups• Cgroups: Deeper View - Features and Design - Cgroups Subsystems - Cgroups Implementation• Cgroups Use cases - Usage Illustration - Sample Use cases• Cgroups : Challenges and Future - Key Challenges - Future



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Background Clustering and Aggregation of Resources help

in Fairness in Large Systems Predictability in Performance depends on the

predictability in the availability of resource

Isolation and Aggregation of Resources for specific activities can improve the predictability of the performance for the specific activities. This also helps in Resource Tracking, Control and Management

Examples : Cloud, Virtualization, Containers

Cloud



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Cgroups Definition

Cgroups are like process container or process groups in Linux kernel

Provide a mechanism for aggregating / partitioning sets of tasks, and all their future children, into hierarchical groups with specialized behaviour

Associate a set of tasks with a set of parameters for one or more subsystems

‘Cgroups’ = ‘Control groups’

Cgroups: Process Container

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Cgroups helps for resource isolation and resource control

Cgroups help tuning the system to make best use of available hardware and network resources.



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Significance of Cgroups

Mainly 2 types of Subsystems supported:• Isolation and Special Controllers : cpuset, namespace,

freezer, device, checkpoint/restart• Resource Controllers : cpu(scheduler), memory, disk I/O,

network

As the systems are becoming more complex with dynamic resource needs, better resource management is inevitable to get optimized outout.

Resource planning is better than dynamic resource allocation

Cgroups with its tree-structured hierarchy and subsystems achieves resource planning for the native system

The significance of Cgroups is that it helps to implement the essential resource groups hierarchically in kernel to enable better resource management. Every task running in the system is in exactly one of the Cgroups in this hierarchy



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Features and Design

• Isolate the resources for specific similar task groups

• Separate Cpuset, Namespace, Device for groups

• Controls the resource for usage limits, freezing groups or check pointing and restarting

• Freezer, Checkpoint / Restart, Cpuacct, CPU, Memory, Disk I/O and Network I/O

• Resource Tracking to measure how much resources certain systems use for e.g. billing purposes

• State of the resource is tracked

Resource Isolation Resource Control Resource Tracking

task Userspace or kernel process

cgroup One or more tasks

subsystem Module to modify the

behaviour

of the tasks in a cgroup

hierarchy Several cgroups in a tree

Hierarchical based grouping of similar tasks in terms of resources is the design principle behind Cgroups



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Cgroups Subsystems

Modules which makes use of task grouping facilities provided by Cgroups to treat groups of tasks in particular way

Resource controllers allocating different levels of system resources to different Cgroups.



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Cgroups Subsystems DetailsSubsystem Type Description

cpuset Isolation Use cpuset to tie processes to system subsets of CPU

ns Isolation For showing private view of system to processes in cgroup; mainly used for OS level virtualization

cpu Control Shares CPU bandwidth between groups

cpuacct Accounting Groups tasks using cgroup and accounts the CPU usage

memory Control Limits memory and memory related parameters

devices Isolation Decides the access to devices by tasks in cgroup

freezer Control Freezes/unfreezes tasks in a group. Used for HPC clusters

net_cls Control Identification of packets originating from a specific cgroup and actions on the packet.

blk_io Control controls and monitors access to I/O on block devices by tasks in cgroup



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Cgroups Implementation

VFS based filesystem model implementation

Each task in the system has a reference-counted pointer to a css_set

A css_set contains a set of reference-counted pointers to cgroup_subsys_state objects, one for each cgroup subsystem registered in the system

A cgroup hierarchy filesystem can be mounted for browsing and manipulation from user space

You can list all the tasks (by pid) attached to any cgroup [cat /cgroup/tasks OR cat /cgroup/<group>/tasks]

Files: • kernel/cgroup.c• include/linux/cgroup.h• include/linux/cgroup_subsys.h• +various subsystems

implementations cgroups requires a few, simple hooks into

the rest of the kernel, these hooks are not in performance critical paths of the kernel.

• In init/main.c : To initialize C groups at boot time.

• In fork and exit calls : To attach and detach a task from cgroups

A new file system, of type "cgroup" should be mounted, to enable browsing and modifying the all the registered Cgroups subsystem



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Cgroups : How to write a new subsystem

Major modifications to be done: • init/Kconfig: kernel

configuration parameters (general setup)

• include/linux/cgroup_subsys.h: cgroup subsystem definition

• kernel/cgroup_example.c: cgroup subsystem implementation [Implement Kernel APIs as per cgroup_subsys]

• kernel/Makefile: Makefile of the core kernel components

• Add the appropriate hooks into the kernel

APIs to be implemented by a new subsystem

As Cgroups provide clean kernel APIs , new subsystem addition and management are simpler



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mount -t tmpfs none /sys/fs/cgroupmkdir /sys/fs/cgroup/memorymount -t cgroup none /sys/fs/cgroup/memory -o memory#Make the new group and move bash into itmkdir /sys/fs/cgroup/memory/0echo $$ > /sys/fs/cgroup/memory/0/tasks#Since now we're in the 0 cgroup, we can alter the memory limit:echo 4M > /sys/fs/cgroup/memory/0/memory.limit_in_bytescat /sys/fs/cgroup/memory/0/memory.limit_in_bytes4194304#We can check the usage:cat /sys/fs/cgroup/memory/0/memory.usage_in_bytes1216512

Usage Illustration

mount -t cgroup -ocpuset cpuset /sys/fs/cgroup/cpusetcd /sys/fs/cgroup/cpusetmkdir Charliecd Charlie/bin/echo 2-3 > cpuset.cpus/bin/echo 1 > cpuset.mems/bin/echo $$ > taskssh# The subshell 'sh' is now running in cpuset Charlie# The next line should display '/Charlie'cat /proc/self/cpuset

CPUset SubsystemMemory Subsystem



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mkdir /sys/fs/cgroup/freezermount -t cgroup -ofreezer freezer /sys/fs/cgroup/freezermkdir /sys/fs/cgroup/freezer/0echo $some_pid > /sys/fs/cgroup/freezer/0/tasks#to get status of the freezer subsystem : cat /sys/fs/cgroup/freezer/0/freezer.stateTHAWED#to freeze all tasks in the container :echo FROZEN > /sys/fs/cgroup/freezer/0/freezer.state#at /sys/fs/cgroup/freezer/0/freezer.state FROZEN#to unfreeze all tasks in the container :echo THAWED > /sys/fs/cgroup/freezer/0/freezer.statecat /sys/fs/cgroup/freezer/0/freezer.stateTHAWED

Usage Illustration

#An entry is added using devices.allow, and removed using devices.deny. For #instanceecho 'c 1:3 mr' > /sys/fs/cgroup/1/devices.allow#allows cgroup 1 to read and mknod the device usually known as dev/null.#Doingecho a > /sys/fs/cgroup/1/devices.deny#will remove the default 'a *:* rwm' entry. Doingecho a > /sys/fs/cgroup/1/devices.allow#will add the 'a *:* rwm' entry to the whitelist.

Device SubsystemFreezer Subsystem



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Sample Use casesWithout CPU cgroup subsystem(10 tasks in “multimedia” and 5 tasks in “browser”)

With CPU cgroup subsystem(10 tasks in “multimedia” and 5 tasks in “browser”)

Cgroups can provide better resource control and management



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Sample Use cases (contd.)

Users

Professor

CPU (0-3)

Disk (50%)

Memory(50%) N/W

W W W b ro w s in g (

2 0 % ) NFS(25%)

Student

CPU((4-7)

Disk(50%)

Memory(50%) N/W

W W W b ro w s in g (

1 0 % ) NFS(25%)

A large university server with various users - students, professors, system tasks etc. The resource planning for this server could be along the following lines with cgroups

Consider a batch job management system which requires start and stop sets of tasks in order to schedule the resources of a machine .This kind of program is often used on HPC clusters to schedule access to the cluster as a whole

Batch Processing System

Batch Process1

Cpu(0-4)

Disk(50%)

Memory(50%)

No Freeze

Batch Process2

Cpu(5-8)

Disk(40%)

Memory(30%)

Can be freezed



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Key Challenges

Throughput intensive system, Cgroups may add performance overhead

It's not currently possible to bind a new subsystem to an active cgroup hierarchy, or to unbind a subsystem from an active cgroup hierarchy

Though Cgroups acts like lighter virtualization, the same can be applied only to native system

Large multi-core systems, the linear performance scalability is needed to utilise the resource better



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Future…

Throughput optimization

Virtualization Possibilities and Lower Level Resource Abstraction

Tuning and Development for Large Multi-core systems

Multicore clustering support and Specialized Subsystems

and …shaping as an inevitable component in linux kernel for effective resource control and management…



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Ashish Pandey ([email protected]) Roopesh Keeppattu ([email protected])

Liuyang([email protected]) Sanil Kumar D. ([email protected])

mailto:[email protected]




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Ashish pandey huawei osi_days2011_cgroups_understanding_better