Linux Container Primitives: cgroups, namespaces, and more! (LinuxFest Northwest 2019)

In this session, we’ll explore the different Linux primitives that are commonly used in implementing container runtimes. We’ll learn about the Linux primitives that underlie container runtimes like Docker, including cgroups, namespaces, and union filesystems. We’ll see how Docker uses these primitives, and how the OCI standard makes it possible to customize how your containers run. We’ll also discuss alternative container runtimes like CRI-O, rkt, and systemd-nspawn and what makes them different. This will be an interactive session with a live demo and open questions.

A recording of this presentation is available at https://www.youtube.com/watch?v=Ueu8TuW-IOg

Transcript

1. © 2019, Amazon Web Services, Inc. or its Affiliates. All rights
   reserved.
   Samuel Karp, Amazon Container Services – @samuelkarp
   April 27, 2019 – LinuxFest Nortwest
   Linux Container Primitives
   cgroups, namespaces, and more!
   

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2. © 2019, Amazon Web Services, Inc. or its Affiliates. All rights reserved.
   Agenda
   Container primitives overview
   Control groups (cgroups)
   Namespaces
   Union filesystems
   Runtimes
   

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3. © 2019, Amazon Web Services, Inc. or its Affiliates. All rights reserved.
   Linux container primitives
   

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4. © 2019, Amazon Web Services, Inc. or its Affiliates. All rights
   reserved.
   © 2019, Amazon Web Services, Inc. or its Affiliates. All rights reserved.
   Containers are an abstraction over
   several different Linux
   technologies
   

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5. © 2019, Amazon Web Services, Inc. or its Affiliates. All rights reserved.
   Linux Kernel
   Container runtime
   Container 1 Container 2 Container 3 Container 4 Container 5 Container 6
   Namespaces Control groups Union filesystem
   

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6. © 2019, Amazon Web Services, Inc. or its Affiliates. All rights reserved.
   Control groups
   

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7. © 2019, Amazon Web Services, Inc. or its Affiliates. All rights reserved.
   What do control groups (cgroups) do?
   • Organize all processes in the system
   • Limit or prioritize resource utilization
   • Account for resource usage and gather utilization data
   

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8. © 2019, Amazon Web Services, Inc. or its Affiliates. All rights reserved.
   Subsystems
   • Control groups is an abstract
   framework
   • Subsystems are concrete
   implementations
   • Different subsystems can
   organize processes separately
   • Most subsystems are resource
   controllers
   Examples of subsystems:
   • Memory
   • CPU time
   • Block I/O
   • Number of discrete processes
   (pids)
   • CPU & memory pinning
   • Freezer (used by docker pause)
   • Devices
   • Network priority
   

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9. © 2019, Amazon Web Services, Inc. or its Affiliates. All rights reserved.
   Hierarchical representation
   • Independent subsystem
   hierarchies
   • Every pid is represented
   exactly once in each
   subsystem
   • New processes inherit
   cgroups from their parents
   ├── blkio
   │ └── docker
   │ └── b211c37
   ├── cpu,cpuacct
   │ └── docker
   │ └── b211c37
   ├── cpuset
   │ └── docker
   │ └── b211c37
   ├── devices
   │ └── docker
   │ └── b211c37
   ├── freezer
   │ └── docker
   │ └── b211c37
   ├── hugetlb
   │ └── docker
   │ └── b211c37
   ├── memory
   │ └── docker
   │ └── b211c37
   

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10. © 2019, Amazon Web Services, Inc. or its Affiliates. All rights reserved.
    cgroup virtual filesystem
    • Typically mounted at
    /sys/fs/cgroup
    • tasks virtual file holds all
    pids in the cgroup
    • Other files have settings and
    utilization data
    ├── cgroup.clone_children
    ├── cgroup.procs
    ├── cgroup.sane_behavior
    ├── cpuacct.stat
    ├── cpuacct.usage
    ├── cpuacct.usage_all
    ├── cpuacct.usage_percpu
    ├── cpuacct.usage_percpu_sys
    ├── cpuacct.usage_percpu_user
    ├── cpuacct.usage_sys
    ├── cpuacct.usage_user
    ├── cpu.cfs_period_us
    ├── cpu.cfs_quota_us
    ├── cpu.rt_period_us
    ├── cpu.rt_runtime_us
    ├── cpu.shares
    ├── cpu.stat
    ├── notify_on_release
    ├── release_agent
    └── tasks
    

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11. © 2019, Amazon Web Services, Inc. or its Affiliates. All rights
    reserved.
    © 2019, Amazon Web Services, Inc. or its Affiliates. All rights reserved.
    Demo
    

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12. © 2019, Amazon Web Services, Inc. or its Affiliates. All rights reserved.
    What can you use cgroups for?
    • cgroups can be used
    independently of containers
    • cgroups control resource limits
    for processes
    • Monitor processes and
    organize them
    • Be careful not to break any
    assumptions your container
    runtime or orchestrator might
    have
    

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13. © 2019, Amazon Web Services, Inc. or its Affiliates. All rights reserved.
    Further reading
    • Linux: Documentation/cgroup-v1
    

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14. © 2019, Amazon Web Services, Inc. or its Affiliates. All rights reserved.
    Namespaces
    

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15. © 2019, Amazon Web Services, Inc. or its Affiliates. All rights reserved.
    What do namespaces do?
    • Isolation mechanism for resources
    • Changes to resources within namespace are (generally)
    invisible outside the namespace
    

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16. © 2019, Amazon Web Services, Inc. or its Affiliates. All rights reserved.
    What namespaces are available?
    • Network
    • Filesystem (mounts)
    • Processes (pid)
    • Inter-process communication (ipc)
    • Hostname and domain name (uts)
    • User and group IDs
    • cgroup
    

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17. © 2019, Amazon Web Services, Inc. or its Affiliates. All rights reserved.
    Namespace sharing
    Process A Process B Process C Process D
    pid:[2]
    pid:[1] pid:[3]
    net:[4] net:[5] net:[6]
    mount:[7] mount:[8]
    

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18. © 2019, Amazon Web Services, Inc. or its Affiliates. All rights reserved.
    Network namespace
    • Frequently used in containers
    • docker run uses a separate network namespace per
    container
    • Multiple containers can share a network namespace
    • Kubernetes pods
    • Amazon ECS tasks with the awsvpc networking mode
    

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19. © 2019, Amazon Web Services, Inc. or its Affiliates. All rights reserved.
    Mount namespace
    • Used for giving containers
    their own filesystem
    • Container image is mounted
    as the root filesystem
    • More about filesystems to
    come!
    bash-4.2# mount
    overlay on / type overlay
    (rw,relatime,lowerdir=/var/lib/docker/overlay2
    /l/Q5EBZ7CIJYELLG2MBKZIRRFWW6:/var/lib/docker/
    overlay2/l/PKATP76T57BQZ5D44JXYFIB26E,upperdir
    =/var/lib/docker/overlay2/88816f9510a9ff38b31e
    aaceccbef6ffc9cc3c06bcc451f9684850db5ee1b152/d
    iff,workdir=/var/lib/docker/overlay2/88816f951
    0a9ff38b31eaaceccbef6ffc9cc3c06bcc451f9684850d
    b5ee1b152/work)
    proc on /proc type proc
    (rw,nosuid,nodev,noexec,relatime)
    tmpfs on /dev type tmpfs
    (rw,nosuid,size=65536k,mode=755)
    devpts on /dev/pts type devpts
    (rw,nosuid,noexec,relatime,gid=5,mode=620,ptmx
    mode=666)
    sysfs on /sys type sysfs
    (ro,nosuid,nodev,noexec,relatime)
    tmpfs on /sys/fs/cgroup type tmpfs
    (ro,nosuid,nodev,noexec,relatime,mode=755)
    

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20. © 2019, Amazon Web Services, Inc. or its Affiliates. All rights reserved.
    procfs virtual filesystem
    • Namespaces are visible in
    /proc
    • Files are symbolic links to
    the namespace
    • The link contains the
    namespace type and inode
    number to identify the
    namespace
    $ readlink /proc/$$/ns/*
    cgroup:[4026531835]
    ipc:[4026531839]
    mnt:[4026531840]
    net:[4026531993]
    pid:[4026531836]
    user:[4026531837]
    uts:[4026531838]
    

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21. © 2019, Amazon Web Services, Inc. or its Affiliates. All rights reserved.
    Creating namespaces
    • clone(2) and
    unshare(2)
    • CLONE_NEW* flags to
    specify which namespaces
    • clone(2) is for new
    processes to create new
    namespaces
    • unshare(2) is for existing
    processes to create new
    namespaces
    

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22. © 2019, Amazon Web Services, Inc. or its Affiliates. All rights reserved.
    Persisting namespaces
    • The kernel automatically
    garbage-collects namespaces
    by reference-counting
    • New namespace remains open
    as long as
    • a process runs or
    • a mount is open
    • Bind-mount a file in
    /proc/$$/ns to another place
    on the filesystem
    $ mount \
    --bind /proc/$$/ns/net \
    /var/run/netns/lfnw
    

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23. © 2019, Amazon Web Services, Inc. or its Affiliates. All rights reserved.
    Entering namespaces
    • Open a file from
    /proc/$$/ns (or a bind-
    mount)
    • Pass to setns(2) to enter the
    existing namespace
    • Namespace remains open as
    long as the process is running,
    even if the original file goes
    away
    • nsenter(1) is a command for
    doing this interactively
    • ip-netns(8) works
    specifically for network
    namespaces
    

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24. © 2019, Amazon Web Services, Inc. or its Affiliates. All rights
    reserved.
    © 2019, Amazon Web Services, Inc. or its Affiliates. All rights reserved.
    Demo
    

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25. © 2019, Amazon Web Services, Inc. or its Affiliates. All rights reserved.
    How can you leverage this?
    • Use nsenter or ip netns to troubleshoot container
    networking
    • Monitor containers by entering the pid namespace
    • Access binaries in your containers with the mount
    namespace
    

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26. © 2019, Amazon Web Services, Inc. or its Affiliates. All rights reserved.
    Further reading
    • man 7 namespaces
    • man 7 pid_namespaces
    • man 7 user_namespaces
    

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27. © 2019, Amazon Web Services, Inc. or its Affiliates. All rights reserved.
    Images, layers, and union
    filesystems
    

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28. © 2019, Amazon Web Services, Inc. or its Affiliates. All rights reserved.
    Filesystem images
    • Images are representations of a filesystem
    • Images are popular for virtualization and container
    systems
    • Docker helped popularize the concept of layers
    • A union filesystem is one where two or more filesystems
    are joined together in a unified view
    

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29. © 2019, Amazon Web Services, Inc. or its Affiliates. All rights reserved.
    Top layer
    (read-write)
    Intermediate
    layer
    (read-only)
    Base layer
    (read-only)
    How Docker layers work
    • A copy-on-write view of your
    files
    • New files exist only in the top
    layer
    • When a file is modified, it is
    copied up to the top layer
    • Unmodified files exist in
    whatever layer they were
    added/modified
    • Deleted files are hidden, but still
    exist
    

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30. © 2019, Amazon Web Services, Inc. or its Affiliates. All rights reserved.
    Union filesystems
    • Popular in container runtimes (like Docker) to
    implement layers
    • Efficient use of storage when starting multiple
    containers with identical images
    • Efficient use of storage when making minor
    modifications to images
    

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31. © 2019, Amazon Web Services, Inc. or its Affiliates. All rights reserved.
    Overlay filesystem
    • Joins two directories (upper and lower) to form a union
    • Uses file name to describe the files
    • When writing to the overlay
    • lowerdir is not modified, all changes go to upperdir
    • Existing files are copied-up to the upperdir for modificiation
    • Whole file is copied, not just blocks
    • “Deleting” a file in the upperdir creates a whiteout
    • Files: character devices with 0/0 device number
    • Directories: xattr “trusted.overlay.opaque” set to “y”
    

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32. © 2019, Amazon Web Services, Inc. or its Affiliates. All rights reserved.
    Overlay filesystem (continued)
    • An upperdir can have multiple lowerdirs
    • Overlay filesystems can be created with mount(2)
    • You can examine the mounts with
    • mount(8)
    • /proc/mounts
    • /proc/$$/mountinfo
    

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33. © 2019, Amazon Web Services, Inc. or its Affiliates. All rights reserved.
    Docker’s overlay driver
    • Docker’s default layer storage uses the overlay
    filesystem
    • upperdir, lowerdir, and diff directories are in
    /var/lib/docker/overlay2
    

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34. © 2019, Amazon Web Services, Inc. or its Affiliates. All rights
    reserved.
    © 2019, Amazon Web Services, Inc. or its Affiliates. All rights reserved.
    Demo
    

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35. © 2019, Amazon Web Services, Inc. or its Affiliates. All rights reserved.
    How can you leverage this?
    • Locate files in your layers
    • Examine which files and
    layers contribute to your
    disk usage
    • Understand the impact of
    writable files in your
    containers and how to
    reduce
    # du -h . | sort -hr
    753M .
    211M ./e33f37/diff
    211M ./e33f37
    204M ./e33f37/diff/usr
    169M ./f87973/diff
    …
    # ls ./f87973
    diff link
    # ls ./e33f37
    diff link lower work
    Base layer!
    Intermediate layer!
    

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36. © 2019, Amazon Web Services, Inc. or its Affiliates. All rights reserved.
    Further reading
    • Linux:
    Documentation/filesystems/overlay.txt
    

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37. © 2019, Amazon Web Services, Inc. or its Affiliates. All rights reserved.
    Runtimes
    

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38. © 2019, Amazon Web Services, Inc. or its Affiliates. All rights reserved.
    What is a container runtime?
    • A software tool that configures Linux primitives to create and run containers
    on a host
    • Examples include:
    • Docker
    • containerd
    • CRI-O
    • rkt
    • systemd-nspawn
    • Open Containers Initiative (OCI) aims to standardize container runtimes,
    image format, and distribution
    • The OCI reference implementation (runc) powers Docker, containerd, and
    CRI O
    ‑
    

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39. © 2019, Amazon Web Services, Inc. or its Affiliates. All rights reserved.
    OCI runtime spec
    • Containers are “bundles”
    • Filesystem
    • JSON document
    • Filesystem can be a union
    • JSON document describes
    • cgroups
    • Namespaces
    • Additional mounts
    • Linux capabilities
    • Linux security modules
    • And more
    • Hooks can modify the bundle
    {
    "ociVersion": "1.0.1",
    ⋮
    "root": {
    "path": "/var/lib/docker/overlay2/03004c/merged"
    },
    ⋮
    "hooks": {
    "prestart": [{"path": "/proc/9306/exe"}]
    },
    "linux": {
    "resources": {
    "cpu": {"shares": 0},
    "pids": {“limit": 0},
    ⋮
    },
    "cgroupsPath": "/docker/bd5cebc8950c",
    "namespaces": [
    {"type": "mount"},
    {"type": "network"},
    ⋮
    ],
    ⋮
    }
    

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40. © 2019, Amazon Web Services, Inc. or its Affiliates. All rights reserved.
    OCI runtime hooks
    • Hooks run
    • Before a container starts
    • After a container starts
    • After a container stops
    • Hooks can modify the
    filesystem, modify the JSON
    file, or take other actions
    • Hooks run sequentially, in
    an order defined in the
    JSON file
    • Docker generates a bundle
    without hooks
    • Docker does let you specify
    your own runtime
    • Your runtime could inject
    hooks, then execute the real
    runtime
    • This is how Nvidia’s
    container runtime works
    

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41. © 2019, Amazon Web Services, Inc. or its Affiliates. All rights
    reserved.
    © 2019, Amazon Web Services, Inc. or its Affiliates. All rights reserved.
    Demo
    

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42. © 2019, Amazon Web Services, Inc. or its Affiliates. All rights
    reserved.
    © 2019, Amazon Web Services, Inc. or its Affiliates. All rights reserved.
    Questions?
    

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43. © 2019, Amazon Web Services, Inc. or its Affiliates. All rights reserved.
    A brief note before we finish —
    Session surveys provide valuable information to speakers
    Feedback that is very helpful:
    • Topics you were excited to learn about
    • Suggestions for improving understanding and clarity
    Feedback that is extremely unhelpful:
    • Comments unrelated to talk content (please refer to the LinuxFest
    Northwest Code of Conduct)
    The “hallway track” is always open!
    Feedback and questions welcome (skarp@amazon.com, @samuelkarp)
    For support, use the AWS Forums or contact AWS Support
    

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44. © 2019, Amazon Web Services, Inc. or its Affiliates. All rights reserved.
    Thank you!
    Samuel Karp
    @samuelkarp
    

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