image_locality.go

/*
Copyright 2016 The Kubernetes Authors.

Licensed under the Apache License, Version 2.0 (the "License");
you may not use this file except in compliance with the License.
You may obtain a copy of the License at

    http://www.apache.org/licenses/LICENSE-2.0

Unless required by applicable law or agreed to in writing, software
distributed under the License is distributed on an "AS IS" BASIS,
WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
See the License for the specific language governing permissions and
limitations under the License.
*/

package priorities

import (
	"fmt"
	"strings"

	"k8s.io/api/core/v1"
	schedulerapi "k8s.io/kubernetes/pkg/scheduler/api"
	schedulercache "k8s.io/kubernetes/pkg/scheduler/cache"
	"k8s.io/kubernetes/pkg/util/parsers"
)

// The two thresholds are used as bounds for the image score range. They correspond to a reasonable size range for
// container images compressed and stored in registries; 90%ile of images on dockerhub drops into this range.
const (
	mb           int64 = 1024 * 1024
	minThreshold int64 = 23 * mb
	maxThreshold int64 = 1000 * mb
)

// ImageLocalityPriorityMap is a priority function that favors nodes that already have requested pod container's images.
// It will detect whether the requested images are present on a node, and then calculate a score ranging from 0 to 10
// based on the total size of those images.
// - If none of the images are present, this node will be given the lowest priority.
// - If some of the images are present on a node, the larger their sizes' sum, the higher the node's priority.
func ImageLocalityPriorityMap(pod *v1.Pod, meta interface{}, nodeInfo *schedulercache.NodeInfo) (schedulerapi.HostPriority, error) {
	node := nodeInfo.Node()
	if node == nil {
		return schedulerapi.HostPriority{}, fmt.Errorf("node not found")
	}

	var score int
	if priorityMeta, ok := meta.(*priorityMetadata); ok {
		score = calculatePriority(sumImageScores(nodeInfo, pod.Spec.Containers, priorityMeta.totalNumNodes))
	} else {
		// if we are not able to parse priority meta data, skip this priority
		score = 0
	}

	return schedulerapi.HostPriority{
		Host:  node.Name,
		Score: score,
	}, nil
}

// calculatePriority returns the priority of a node. Given the sumScores of requested images on the node, the node's
// priority is obtained by scaling the maximum priority value with a ratio proportional to the sumScores.
func calculatePriority(sumScores int64) int {
	if sumScores < minThreshold {
		sumScores = minThreshold
	} else if sumScores > maxThreshold {
		sumScores = maxThreshold
	}

	return int(int64(schedulerapi.MaxPriority) * (sumScores - minThreshold) / (maxThreshold - minThreshold))
}

// sumImageScores returns the sum of image scores of all the containers that are already on the node.
// Each image receives a raw score of its size, scaled by scaledImageScore. The raw scores are later used to calculate
// the final score. Note that the init containers are not considered for it's rare for users to deploy huge init containers.
func sumImageScores(nodeInfo *schedulercache.NodeInfo, containers []v1.Container, totalNumNodes int) int64 {
	var sum int64
	imageStates := nodeInfo.ImageStates()

	for _, container := range containers {
		if state, ok := imageStates[normalizedImageName(container.Image)]; ok {
			sum += scaledImageScore(state, totalNumNodes)
		}
	}

	return sum
}

// scaledImageScore returns an adaptively scaled score for the given state of an image.
// The size of the image is used as the base score, scaled by a factor which considers how much nodes the image has "spread" to.
// This heuristic aims to mitigate the undesirable "node heating problem", i.e., pods get assigned to the same or
// a few nodes due to image locality.
func scaledImageScore(imageState *schedulercache.ImageStateSummary, totalNumNodes int) int64 {
	spread := float64(imageState.NumNodes) / float64(totalNumNodes)
	return int64(float64(imageState.Size) * spread)
}

// normalizedImageName returns the CRI compliant name for a given image.
// TODO: cover the corner cases of missed matches, e.g,
// 1. Using Docker as runtime and docker.io/library/test:tag in pod spec, but only test:tag will present in node status
// 2. Using the implicit registry, i.e., test:tag or library/test:tag in pod spec but only docker.io/library/test:tag
// in node status; note that if users consistently use one registry format, this should not happen.
func normalizedImageName(name string) string {
	if strings.LastIndex(name, ":") <= strings.LastIndex(name, "/") {
		name = name + ":" + parsers.DefaultImageTag
	}
	return name
}