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<h2>PLEASE NOTE: This document applies to the HEAD of the source tree</h2>
If you are using a released version of Kubernetes, you should
refer to the docs that go with that version.
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<strong>
The latest release of this document can be found
[here](http://releases.k8s.io/release-1.3/docs/proposals/kubelet-systemd.md).
Documentation for other releases can be found at
[releases.k8s.io](http://releases.k8s.io).
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# Kubelet and systemd interaction
**Author**: Derek Carr (@derekwaynecarr)
**Status**: Proposed
## Motivation
Many Linux distributions have either adopted, or plan to adopt `systemd` as their init system.
This document describes how the node should be configured, and a set of enhancements that should
be made to the `kubelet` to better integrate with these distributions independent of container
runtime.
## Scope of proposal
This proposal does not account for running the `kubelet` in a container.
## Background on systemd
To help understand this proposal, we first provide a brief summary of `systemd` behavior.
### systemd units
`systemd` manages a hierarchy of `slice`, `scope`, and `service` units.
* `service` - application on the server that is launched by `systemd`; how it should start/stop;
when it should be started; under what circumstances it should be restarted; and any resource
controls that should be applied to it.
* `scope` - a process or group of processes which are not launched by `systemd` (i.e. fork), like
a service, resource controls may be applied
* `slice` - organizes a hierarchy in which `scope` and `service` units are placed. a `slice` may
contain `slice`, `scope`, or `service` units; processes are attached to `service` and `scope`
units only, not to `slices`. The hierarchy is intended to be unified, meaning a process may
only belong to a single leaf node.
### cgroup hierarchy: split versus unified hierarchies
Classical `cgroup` hierarchies were split per resource group controller, and a process could
exist in different parts of the hierarchy.
For example, a process `p1` could exist in each of the following at the same time:
* `/sys/fs/cgroup/cpu/important/`
* `/sys/fs/cgroup/memory/unimportant/`
* `/sys/fs/cgroup/cpuacct/unimportant/`
In addition, controllers for one resource group could depend on another in ways that were not
always obvious.
For example, the `cpu` controller depends on the `cpuacct` controller yet they were treated
separately.
Many found it confusing for a single process to belong to different nodes in the `cgroup` hierarchy
across controllers.
The Kernel direction for `cgroup` support is to move toward a unified `cgroup` hierarchy, where the
per-controller hierarchies are eliminated in favor of hierarchies like the following:
* `/sys/fs/cgroup/important/`
* `/sys/fs/cgroup/unimportant/`
In a unified hierarchy, a process may only belong to a single node in the `cgroup` tree.
### cgroupfs single writer
The Kernel direction for `cgroup` management is to promote a single-writer model rather than
allowing multiple processes to independently write to parts of the file-system.
In distributions that run `systemd` as their init system, the cgroup tree is managed by `systemd`
by default since it implicitly interacts with the cgroup tree when starting units. Manual changes
made by other cgroup managers to the cgroup tree are not guaranteed to be preserved unless `systemd`
is made aware. `systemd` can be told to ignore sections of the cgroup tree by configuring the unit
to have the `Delegate=` option.
See: http://www.freedesktop.org/software/systemd/man/systemd.resource-control.html#Delegate=
### cgroup management with systemd and container runtimes
A `slice` corresponds to an inner-node in the `cgroup` file-system hierarchy.
For example, the `system.slice` is represented as follows:
`/sys/fs/cgroup/<controller>/system.slice`
A `slice` is nested in the hierarchy by its naming convention.
For example, the `system-foo.slice` is represented as follows:
`/sys/fs/cgroup/<controller>/system.slice/system-foo.slice/`
A `service` or `scope` corresponds to leaf nodes in the `cgroup` file-system hierarchy managed by
`systemd`. Services and scopes can have child nodes managed outside of `systemd` if they have been
delegated with the `Delegate=` option.
For example, if the `docker.service` is associated with the `system.slice`, it is
represented as follows:
`/sys/fs/cgroup/<controller>/system.slice/docker.service/`
To demonstrate the use of `scope` units using the `docker` container runtime, if a
user launches a container via `docker run -m 100M busybox`, a `scope` will be created
because the process was not launched by `systemd` itself. The `scope` is parented by
the `slice` associated with the launching daemon.
For example:
`/sys/fs/cgroup/<controller>/system.slice/docker-<container-id>.scope`
`systemd` defines a set of slices. By default, service and scope units are placed in
`system.slice`, virtual machines and containers registered with `systemd-machined` are
found in `machine.slice`, and user sessions handled by `systemd-logind` in `user.slice`.
## Node Configuration on systemd
### kubelet cgroup driver
The `kubelet` reads and writes to the `cgroup` tree during bootstrapping
of the node. In the future, it will write to the `cgroup` tree to satisfy other
purposes around quality of service, etc.
The `kubelet` must cooperate with `systemd` in order to ensure proper function of the
system. The bootstrapping requirements for a `systemd` system are different than one
without it.
The `kubelet` will accept a new flag to control how it interacts with the `cgroup` tree.
* `--cgroup-driver=` - cgroup driver used by the kubelet. `cgroupfs` or `systemd`.
By default, the `kubelet` should default `--cgroup-driver` to `systemd` on `systemd` distributions.
The `kubelet` should associate node bootstrapping semantics to the configured
`cgroup driver`.
### Node allocatable
The proposal makes no changes to the definition as presented here:
https://github.com/kubernetes/kubernetes/blob/master/docs/proposals/node-allocatable.md
The node will report a set of allocatable compute resources defined as follows:
`[Allocatable] = [Node Capacity] - [Kube-Reserved] - [System-Reserved]`
### Node capacity
The `kubelet` will continue to interface with `cAdvisor` to determine node capacity.
### System reserved
The node may set aside a set of designated resources for non-Kubernetes components.
The `kubelet` accepts the followings flags that support this feature:
* `--system-reserved=` - A set of `ResourceName`=`ResourceQuantity` pairs that
describe resources reserved for host daemons.
* `--system-container=` - Optional resource-only container in which to place all
non-kernel processes that are not already in a container. Empty for no container.
Rolling back the flag requires a reboot. (Default: "").
The current meaning of `system-container` is inadequate on `systemd` environments.
The `kubelet` should use the flag to know the location that has the processes that
are associated with `system-reserved`, but it should not modify the cgroups of
existing processes on the system during bootstrapping of the node. This is
because `systemd` is the `cgroup manager` on the host and it has not delegated
authority to the `kubelet` to change how it manages `units`.
The following describes the type of things that can happen if this does not change:
https://bugzilla.redhat.com/show_bug.cgi?id=1202859
As a result, the `kubelet` needs to distinguish placement of non-kernel processes
based on the cgroup driver, and only do its current behavior when not on `systemd`.
The flag should be modified as follows:
* `--system-container=` - Name of resource-only container that holds all
non-kernel processes whose resource consumption is accounted under
system-reserved. The default value is cgroup driver specific. systemd
defaults to system, cgroupfs defines no default. Rolling back the flag
requires a reboot.
The `kubelet` will error if the defined `--system-container` does not exist
on `systemd` environments. It will verify that the appropriate `cpu` and `memory`
controllers are enabled.
### Kubernetes reserved
The node may set aside a set of resources for Kubernetes components:
* `--kube-reserved=:` - A set of `ResourceName`=`ResourceQuantity` pairs that
describe resources reserved for host daemons.
The `kubelet` does not enforce `--kube-reserved` at this time, but the ability
to distinguish the static reservation from observed usage is important for node accounting.
This proposal asserts that `kubernetes.slice` is the default slice associated with
the `kubelet` and `kube-proxy` service units defined in the project. Keeping it
separate from `system.slice` allows for accounting to be distinguished separately.
The `kubelet` will detect its `cgroup` to track `kube-reserved` observed usage on `systemd`.
If the `kubelet` detects that its a child of the `system-container` based on the observed
`cgroup` hierarchy, it will warn.
If the `kubelet` is launched directly from a terminal, it's most likely destination will
be in a `scope` that is a child of `user.slice` as follows:
`/sys/fs/cgroup/<controller>/user.slice/user-1000.slice/session-1.scope`
In this context, the parent `scope` is what will be used to facilitate local developer
debugging scenarios for tracking `kube-reserved` usage.
The `kubelet` has the following flag:
* `--resource-container="/kubelet":` Absolute name of the resource-only container to create
and run the Kubelet in (Default: /kubelet).
This flag will not be supported on `systemd` environments since the init system has already
spawned the process and placed it in the corresponding container associated with its unit.
### Kubernetes container runtime reserved
This proposal asserts that the reservation of compute resources for any associated
container runtime daemons is tracked by the operator under the `system-reserved` or
`kubernetes-reserved` values and any enforced limits are set by the
operator specific to the container runtime.
**Docker**
If the `kubelet` is configured with the `container-runtime` set to `docker`, the
`kubelet` will detect the `cgroup` associated with the `docker` daemon and use that
to do local node accounting. If an operator wants to impose runtime limits on the
`docker` daemon to control resource usage, the operator should set those explicitly in
the `service` unit that launches `docker`. The `kubelet` will not set any limits itself
at this time and will assume whatever budget was set aside for `docker` was included in
either `--kube-reserved` or `--system-reserved` reservations.
Many OS distributions package `docker` by default, and it will often belong to the
`system.slice` hierarchy, and therefore operators will need to budget it for there
by default unless they explicitly move it.
**rkt**
rkt has no client/server daemon, and therefore has no explicit requirements on container-runtime
reservation.
### kubelet cgroup enforcement
The `kubelet` does not enforce the `system-reserved` or `kube-reserved` values by default.
The `kubelet` should support an additional flag to turn on enforcement:
* `--system-reserved-enforce=false` - Optional flag that if true tells the `kubelet`
to enforce the `system-reserved` constraints defined (if any)
* `--kube-reserved-enforce=false` - Optional flag that if true tells the `kubelet`
to enforce the `kube-reserved` constraints defined (if any)
Usage of this flag requires that end-user containers are launched in a separate part
of cgroup hierarchy via `cgroup-root`.
If this flag is enabled, the `kubelet` will continually validate that the configured
resource constraints are applied on the associated `cgroup`.
### kubelet cgroup-root behavior under systemd
The `kubelet` supports a `cgroup-root` flag which is the optional root `cgroup` to use for pods.
This flag should be treated as a pass-through to the underlying configured container runtime.
If `--cgroup-enforce=true`, this flag warrants special consideration by the operator depending
on how the node was configured. For example, if the container runtime is `docker` and its using
the `systemd` cgroup driver, then `docker` will take the daemon wide default and launch containers
in the same slice associated with the `docker.service`. By default, this would mean `system.slice`
which could cause end-user pods to be launched in the same part of the cgroup hierarchy as system daemons.
In those environments, it is recommended that `cgroup-root` is configured to be a subtree of `machine.slice`.
### Proposed cgroup hierarchy
```
$ROOT
|
+- system.slice
| |
| +- sshd.service
| +- docker.service (optional)
| +- ...
|
+- kubernetes.slice
| |
| +- kubelet.service
| +- docker.service (optional)
|
+- machine.slice (container runtime specific)
| |
| +- docker-<container-id>.scope
|
+- user.slice
| +- ...
```
* `system.slice` corresponds to `--system-reserved`, and contains any services the
operator brought to the node as normal configuration.
* `kubernetes.slice` corresponds to the `--kube-reserved`, and contains kube specific
daemons.
* `machine.slice` should parent all end-user containers on the system and serve as the
root of the end-user cluster workloads run on the system.
* `user.slice` is not explicitly tracked by the `kubelet`, but it is possible that `ssh`
sessions to the node where the user launches actions directly. Any resource accounting
reserved for those actions should be part of `system-reserved`.
The container runtime daemon, `docker` in this outline, must be accounted for in either
`system.slice` or `kubernetes.slice`.
In the future, the depth of the container hierarchy is not recommended to be rooted
more than 2 layers below the root as it historically has caused issues with node performance
in other `cgroup` aware systems (https://bugzilla.redhat.com/show_bug.cgi?id=850718). It
is anticipated that the `kubelet` will parent containers based on quality of service
in the future. In that environment, those changes will be relative to the configured
`cgroup-root`.
### Linux Kernel Parameters
The `kubelet` will set the following:
* `sysctl -w vm.overcommit_memory=1`
* `sysctl -w vm.panic_on_oom=0`
* `sysctl -w kernel/panic=10`
* `sysctl -w kernel/panic_on_oops=1`
### OOM Score Adjustment
The `kubelet` at bootstrapping will set the `oom_score_adj` value for Kubernetes
daemons, and any dependent container-runtime daemons.
If `container-runtime` is set to `docker`, then set its `oom_score_adj=-900`
## Implementation concerns
### kubelet block-level architecture
```
+----------+ +----------+ +----------+
| | | | | Pod |
| Node <-------+ Container<----+ Lifecycle|
| Manager | | Manager | | Manager |
| +-------> | | |
+---+------+ +-----+----+ +----------+
| |
| |
| +-----------------+
| | |
| | |
+---v--v--+ +-----v----+
| cgroups | | container|
| library | | runtimes |
+---+-----+ +-----+----+
| |
| |
+---------+----------+
|
|
+-----------v-----------+
| Linux Kernel |
+-----------------------+
```
The `kubelet` should move to an architecture that resembles the above diagram:
* The `kubelet` should not interface directly with the `cgroup` file-system, but instead
should use a common `cgroups library` that has the proper abstraction in place to
work with either `cgroupfs` or `systemd`. The `kubelet` should just use `libcontainer`
abstractions to facilitate this requirement. The `libcontainer` abstractions as
currently defined only support an `Apply(pid)` pattern, and we need to separate that
abstraction to allow cgroup to be created and then later joined.
* The existing `ContainerManager` should separate node bootstrapping into a separate
`NodeManager` that is dependent on the configured `cgroup-driver`.
* The `kubelet` flags for cgroup paths will convert internally as part of cgroup library,
i.e. `/foo/bar` will just convert to `foo-bar.slice`
### kubelet accounting for end-user pods
This proposal re-enforces that it is inappropriate at this time to depend on `--cgroup-root` as the
primary mechanism to distinguish and account for end-user pod compute resource usage.
Instead, the `kubelet` can and should sum the usage of each running `pod` on the node to account for
end-user pod usage separate from system-reserved and kubernetes-reserved accounting via `cAdvisor`.
## Known issues
### Docker runtime support for --cgroup-parent
Docker versions <= 1.0.9 did not have proper support for `-cgroup-parent` flag on `systemd`. This
was fixed in this PR (https://github.com/docker/docker/pull/18612). As result, it's expected
that containers launched by the `docker` daemon may continue to go in the default `system.slice` and
appear to be counted under system-reserved node usage accounting.
If operators run with later versions of `docker`, they can avoid this issue via the use of `cgroup-root`
flag on the `kubelet`, but this proposal makes no requirement on operators to do that at this time, and
this can be revisited if/when the project adopts docker 1.10.
Some OS distributions will fix this bug in versions of docker <= 1.0.9, so operators should
be aware of how their version of `docker` was packaged when using this feature.
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