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Scheduling Framework

The scheduling framework is a new pluggable architecture for Kubernetes Scheduler that makes scheduler customizations easy. It adds a new set of “plugin” APIs to the existing scheduler. Plugins are compiled into the scheduler. The APIs allow most scheduling features to be implemented as plugins, while keeping the scheduling “core” simple and maintainable. Refer to the design proposal of the scheduling framework for more technical information on the design of the framework.

• Scheduling Cycle & Binding Cycle
• Extension points
• Plugin API

Framework workflow

The Scheduling Framework defines a few extension points. Scheduler plugins register to be invoked at one or more extension points. Some of these plugins can change the scheduling decisions and some are informational only.

Each attempt to schedule one Pod is split into two phases, the scheduling cycle and the binding cycle.

Scheduling Cycle & Binding Cycle

The scheduling cycle selects a node for the Pod, and the binding cycle applies that decision to the cluster. Together, a scheduling cycle and binding cycle are referred to as a “scheduling context”.

Scheduling cycles are run serially, while binding cycles may run concurrently.

A scheduling or binding cycle can be aborted if the Pod is determined to be unschedulable or if there is an internal error. The Pod will be returned to the queue and retried.

Extension points

The following picture shows the scheduling context of a Pod and the extension points that the scheduling framework exposes. In this picture “Filter” is equivalent to “Predicate” and “Scoring” is equivalent to “Priority function”.

One plugin may register at multiple extension points to perform more complex or stateful tasks.

Queue sort

These plugins are used to sort Pods in the scheduling queue. A queue sort plugin essentially will provide a “less(Pod1, Pod2)” function. Only one queue sort plugin may be enabled at a time.

Pre-filter

These plugins are used to pre-process info about the Pod, or to check certain conditions that the cluster or the Pod must meet. If a pre-filter plugin returns an error, the scheduling cycle is aborted.

Filter

These plugins are used to filter out nodes that cannot run the Pod. For each node, the scheduler will call filter plugins in their configured order. If any filter plugin marks the node as infeasible, the remaining plugins will not be called for that node. Nodes may be evaluated concurrently.

Post-filter

This is an informational extension point. Plugins will be called with a list of nodes that passed the filtering phase. A plugin may use this data to update internal state or to generate logs/metrics.

Note: Plugins wishing to perform “pre-scoring” work should use the post-filter extension point.

Scoring

These plugins are used to rank nodes that have passed the filtering phase. The scheduler will call each scoring plugin for each node. There will be a well defined range of integers representing the minimum and maximum scores. After the normalize scoring phase, the scheduler will combine node scores from all plugins according to the configured plugin weights.

Normalize scoring

These plugins are used to modify scores before the scheduler computes a final ranking of Nodes. A plugin that registers for this extension point will be called with the scoring results from the same plugin. This is called once per plugin per scheduling cycle.

For example, suppose a plugin BlinkingLightScorer ranks Nodes based on how many blinking lights they have.

func ScoreNode(_ *v1.pod, n *v1.Node) (int, error) {
   return getBlinkingLightCount(n)
}

However, the maximum count of blinking lights may be small compared to NodeScoreMax. To fix this, BlinkingLightScorer should also register for this extension point.

func NormalizeScores(scores map[string]int) {
   highest := 0
   for _, score := range scores {
      highest = max(highest, score)
   }
   for node, score := range scores {
      scores[node] = score*NodeScoreMax/highest
   }
}

If any normalize-scoring plugin returns an error, the scheduling cycle is aborted.

Note: Plugins wishing to perform “pre-reserve” work should use the normalize-scoring extension point.

Reserve

This is an informational extension point. Plugins which maintain runtime state (aka “stateful plugins”) should use this extension point to be notified by the scheduler when resources on a node are being reserved for a given Pod. This happens before the scheduler actually binds the Pod to the Node, and it exists to prevent race conditions while the scheduler waits for the bind to succeed.

This is the last step in a scheduling cycle. Once a Pod is in the reserved state, it will either trigger Unreserve plugins (on failure) or Post-bind plugins (on success) at the end of the binding cycle.

Note: This concept used to be referred to as “assume”.

Permit

These plugins are used to prevent or delay the binding of a Pod. A permit plugin can do one of three things.

1. approve
   Once all permit plugins approve a Pod, it is sent for binding.

2. deny
   If any permit plugin denies a Pod, it is returned to the scheduling queue. This will trigger Unreserve plugins.

3. wait (with a timeout)
   If a permit plugin returns “wait”, then the Pod is kept in the permit phase until a plugin approves it. If a timeout occurs, wait becomes deny and the Pod is returned to the scheduling queue, triggering Unreserve plugins.

Approving a Pod binding

While any plugin can access the list of “waiting” Pods from the cache and approve them (see FrameworkHandle) we expect only the permit plugins to approve binding of reserved Pods that are in “waiting” state. Once a Pod is approved, it is sent to the pre-bind phase.

Pre-bind

These plugins are used to perform any work required before a Pod is bound. For example, a pre-bind plugin may provision a network volume and mount it on the target node before allowing the Pod to run there.

If any pre-bind plugin returns an error, the Pod is rejected and returned to the scheduling queue.

Bind

These plugins are used to bind a Pod to a Node. Bind plugins will not be called until all pre-bind plugins have completed. Each bind plugin is called in the configured order. A bind plugin may choose whether or not to handle the given Pod. If a bind plugin chooses to handle a Pod, the remaining bind plugins are skipped.

Post-bind

This is an informational extension point. Post-bind plugins are called after a Pod is successfully bound. This is the end of a binding cycle, and can be used to clean up associated resources.

Unreserve

This is an informational extension point. If a Pod was reserved and then rejected in a later phase, then unreserve plugins will be notified. Unreserve plugins should clean up state associated with the reserved Pod.

Plugins that use this extension point usually should also use Reserve.

Plugin API

There are two steps to the plugin API. First, plugins must register and get configured, then they use the extension point interfaces. Extension point interfaces have the following form.

type Plugin interface {
   Name() string
}

type QueueSortPlugin interface {
   Plugin
   Less(*v1.pod, *v1.pod) bool
}

type PreFilterPlugin interface {
   Plugin
   PreFilter(PluginContext, *v1.pod) error
}

// ...

Plugin Configuration

Plugins can be enabled in the scheduler configuration. Also, default plugins can be disabled in the configuration. In 1.15, there are no default plugins for the scheduling framework.

The scheduler configuration can include configuration for plugins as well. Such configurations are passed to the plugins at the time the scheduler initializes them. The configuration is an arbitrary value. The receiving plugin should decode and process the configuration.

The following example shows a scheduler configuration that enables some plugins at reserve and preBind extension points and disables a plugin. It also provides a configuration to plugin foo.

apiVersion: kubescheduler.config.k8s.io/v1alpha1
kind: KubeSchedulerConfiguration

...

plugins:
  reserve:
    enabled:
    - name: foo
    - name: bar
    disabled:
    - name: baz
  preBind:
    enabled:
    - name: foo
    disabled:
    - name: baz

pluginConfig:
- name: foo
  args: >
    Arbitrary set of args to plugin foo

When an extension point is omitted from the configuration default plugins for that extension points are used. When an extension point exists and enabled is provided, the enabled plugins are called in addition to default plugins. Default plugins are called first and then the additional enabled plugins are called in the same order specified in the configuration. If a different order of calling default plugins is desired, default plugins must be disabled and enabled in the desired order.

Assuming there is a default plugin called foo at reserve and we are adding plugin bar that we want to be invoked before foo, we should disable foo and enable bar and foo in order. The following example shows the configuration that achieves this:

apiVersion: kubescheduler.config.k8s.io/v1alpha1
kind: KubeSchedulerConfiguration

...

plugins:
  reserve:
    enabled:
    - name: bar
    - name: foo
    disabled:
    - name: foo

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