Linux Container Primitives and Runtimes (re:Invent 2018, CON407)

In this session, we'll explore the different Linux primitives that are commonly used in implementing container runtimes. Starting with Docker containers and moving down through the stack, we'll cover the underlying Linux primitives like cgroups, namespaces, and union filesystems, as well as how OCI runtimes like runc use them. 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.

Transcript

2. © 2018, Amazon Web Services, Inc. or its affiliates. All rights reserved. Linux Container Primitives and Runtimes Samuel Karp Senior Software Development Engineer Amazon Web Services – Container Services C O N 4 0 7
3. © 2018, Amazon Web Services, Inc. or its affiliates. All rights reserved. Agenda Container primitives overview Control groups (cgroups) Namespaces Union filesystems Runtimes
4. © 2018, Amazon Web Services, Inc. or its affiliates. All rights reserved. Related breakouts Monday, November 26 CON355 Best Practices for Container Image Management 10:45 a.m. – 11:45 a.m. | Venetian, Level 3, Murano 3302 Tuesday, November 27 CON410-R Deep Dive into Container Networking 9:15 a.m. – 10:15 a.m. | MGM, Level 3, South Concourse 301 Wednesday, November 28 CON410-R1 Deep Dive into Container Networking 6:15 p.m. – 7:15 p.m. | Aria West, Level 3, Starvine 2
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6. Containers are an abstraction over several different Linux technologies
7. © 2018, 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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9. © 2018, 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
10. © 2018, 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
11. © 2018, 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
12. © 2018, 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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14. © 2018, 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
15. © 2018, Amazon Web Services, Inc. or its affiliates. All rights reserved. Further reading • Linux: Documentation/cgroup-v1
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17. © 2018, Amazon Web Services, Inc. or its affiliates. All rights reserved. What do namespaces do? • Isolation mechanism for resources • Changes to resources within namespace are invisible outside the namespace*
18. © 2018, 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
19. © 2018, 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]
20. © 2018, 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 Elastic Container Service (Amazon ECS) tasks with the awsvpc networking mode
21. © 2018, 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/overl ay2/l/Q5EBZ7CIJYELLG2MBKZIRRFWW6:/var/lib/d ocker/overlay2/l/PKATP76T57BQZ5D44JXYFIB26E ,upperdir=/var/lib/docker/overlay2/88816f95 10a9ff38b31eaaceccbef6ffc9cc3c06bcc451f9684 850db5ee1b152/diff,workdir=/var/lib/docker/ overlay2/88816f9510a9ff38b31eaaceccbef6ffc9 cc3c06bcc451f9684850db5ee1b152/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,p tmxmode=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)
22. © 2018, 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/$$/ns/* cgroup:[4026531835] ipc:[4026531839] mnt:[4026531840] net:[4026531993] pid:[4026531836] user:[4026531837] uts:[4026531838]
23. © 2018, Amazon Web Services, Inc. or its affiliates. All rights reserved. Creating namespaces • clone(2) and unshare(2) • CLONE_NEW* flags to specify which namespaces • clone(2) is for new processes to create new namespaces • unshare(2) is for existing processes to create new namespaces
24. © 2018, Amazon Web Services, Inc. or its affiliates. All rights reserved. Persisting namespaces • The kernel automatically garbage-collects namespaces by reference-counting • New namespace remains open as long as • a process runs or • a mount is open • Bind-mount a file in /proc/$$/ns to another place on the filesystem $ mount \ --bind /proc/$$/ns/net \ /var/run/netns/con407
25. © 2018, Amazon Web Services, Inc. or its affiliates. All rights reserved. Entering namespaces • Open a file from /proc/$$/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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27. © 2018, 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
28. © 2018, 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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30. © 2018, 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
31. © 2018, Amazon Web Services, Inc. or its affiliates. All rights reserved. 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 Top layer (read-write) Intermediate layer (read- only) Base layer (read-only)
32. © 2018, 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
33. © 2018, 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”
34. © 2018, 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
35. © 2018, 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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37. © 2018, 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
38. © 2018, Amazon Web Services, Inc. or its affiliates. All rights reserved. Further reading • Linux: Documentation/filesystems/overlay.txt
39. © 2018, Amazon Web Services, Inc. or its affiliates. All rights reserved.
40. © 2018, 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
41. © 2018, 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"}, ⋮ ], ⋮ }
42. © 2018, 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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45. Thank you! © 2018, Amazon Web Services, Inc. or its affiliates. All rights reserved. Samuel Karp @samuelkarp