mirror of https://github.com/k3s-io/k3s
85 lines
4.2 KiB
Markdown
85 lines
4.2 KiB
Markdown
<!-- BEGIN MUNGE: UNVERSIONED_WARNING -->
|
|
|
|
<!-- BEGIN STRIP_FOR_RELEASE -->
|
|
|
|
<img src="http://kubernetes.io/img/warning.png" alt="WARNING"
|
|
width="25" height="25">
|
|
<img src="http://kubernetes.io/img/warning.png" alt="WARNING"
|
|
width="25" height="25">
|
|
<img src="http://kubernetes.io/img/warning.png" alt="WARNING"
|
|
width="25" height="25">
|
|
<img src="http://kubernetes.io/img/warning.png" alt="WARNING"
|
|
width="25" height="25">
|
|
<img src="http://kubernetes.io/img/warning.png" alt="WARNING"
|
|
width="25" height="25">
|
|
|
|
<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.
|
|
|
|
<strong>
|
|
The latest 1.0.x release of this document can be found
|
|
[here](http://releases.k8s.io/release-1.0/docs/devel/scheduler.md).
|
|
|
|
Documentation for other releases can be found at
|
|
[releases.k8s.io](http://releases.k8s.io).
|
|
</strong>
|
|
--
|
|
|
|
<!-- END STRIP_FOR_RELEASE -->
|
|
|
|
<!-- END MUNGE: UNVERSIONED_WARNING -->
|
|
|
|
# The Kubernetes Scheduler
|
|
|
|
The Kubernetes scheduler runs as a process alongside the other master
|
|
components such as the API server. Its interface to the API server is to watch
|
|
for Pods with an empty PodSpec.NodeName, and for each Pod, it posts a Binding
|
|
indicating where the Pod should be scheduled.
|
|
|
|
## The scheduling process
|
|
|
|
The scheduler tries to find a node for each Pod, one at a time, as it notices
|
|
these Pods via watch. There are three steps. First it applies a set of "predicates" that filter out
|
|
inappropriate nodes. For example, if the PodSpec specifies resource limits, then the scheduler
|
|
will filter out nodes that don't have at least that much resources available (computed
|
|
as the capacity of the node minus the sum of the resource limits of the containers that
|
|
are already running on the node). Second, it applies a set of "priority functions"
|
|
that rank the nodes that weren't filtered out by the predicate check. For example,
|
|
it tries to spread Pods across nodes while at the same time favoring the least-loaded
|
|
nodes (where "load" here is sum of the resource limits of the containers running on the node,
|
|
divided by the node's capacity).
|
|
Finally, the node with the highest priority is chosen
|
|
(or, if there are multiple such nodes, then one of them is chosen at random). The code
|
|
for this main scheduling loop is in the function `Schedule()` in
|
|
[plugin/pkg/scheduler/generic_scheduler.go](../../plugin/pkg/scheduler/generic_scheduler.go)
|
|
|
|
## Scheduler extensibility
|
|
|
|
The scheduler is extensible: the cluster administrator can choose which of the pre-defined
|
|
scheduling policies to apply, and can add new ones. The built-in predicates and priorities are
|
|
defined in [plugin/pkg/scheduler/algorithm/predicates/predicates.go](../../plugin/pkg/scheduler/algorithm/predicates/predicates.go) and
|
|
[plugin/pkg/scheduler/algorithm/priorities/priorities.go](../../plugin/pkg/scheduler/algorithm/priorities/priorities.go), respectively.
|
|
The policies that are applied when scheduling can be chosen in one of two ways. Normally,
|
|
the policies used are selected by the functions `defaultPredicates()` and `defaultPriorities()` in
|
|
[plugin/pkg/scheduler/algorithmprovider/defaults/defaults.go](../../plugin/pkg/scheduler/algorithmprovider/defaults/defaults.go).
|
|
However, the choice of policies
|
|
can be overridden by passing the command-line flag `--policy-config-file` to the scheduler, pointing to a JSON
|
|
file specifying which scheduling policies to use. See
|
|
[examples/scheduler-policy-config.json](../../examples/scheduler-policy-config.json) for an example
|
|
config file. (Note that the config file format is versioned; the API is defined in
|
|
[plugin/pkg/scheduler/api](../../plugin/pkg/scheduler/api/)).
|
|
Thus to add a new scheduling policy, you should modify predicates.go or priorities.go,
|
|
and either register the policy in `defaultPredicates()` or `defaultPriorities()`, or use a policy config file.
|
|
|
|
## Exploring the code
|
|
|
|
If you want to get a global picture of how the scheduler works, you can start in
|
|
[plugin/cmd/kube-scheduler/app/server.go](../../plugin/cmd/kube-scheduler/app/server.go)
|
|
|
|
|
|
<!-- BEGIN MUNGE: GENERATED_ANALYTICS -->
|
|
[]()
|
|
<!-- END MUNGE: GENERATED_ANALYTICS -->
|