Vertical Pod Autoscaler (VPA)

Guide to Kubernetes Autoscaling
calendar October 11, 2021
Chapter 1 Vertical Pod Autoscaler (VPA)

Vertical Pod Autoscaler (VPA) is a Kubernetes (K8s) resource that helps compute the right size for resource requests associated with application pods (Deployments). This article will explore VPA’s features, provide instructions for using VPA, explain its limitations, and point to an alternative resource optimization approach in conclusion.

The Pod Resource Allocation Dilemma

Consider the sample Kubernetes resource usage dashboard presented in the screenshot below. We can see CPU and memory resources are over-allocated. In other words, the actual usage of those resources is too low compared to the allocated resources. If you are not as familiar with pod resource allocation, you can learn more about it by reading our article about Kubernetes resource limits and requests.

Dashboard showing extremely low utilization
Dashboard showing extremely low utilization

You may encounter this scenario frequently while managing and administering K8s clusters. As a cluster admin, how can you allocate resources more efficiently?

This problem occurs because the Kubernetes scheduler does not re-evaluate the pod’s resource needs after a pod is scheduled with a given set of requests. As a result, over-allocated resources are not freed or scaled-down. Conversely, if a pod didn’t request sufficient resources, the scheduler won’t increase them to meet the higher demand.

To summarize:

  • If you over-allocate resources: you add unnecessary workers, waste unused resources, and increase your monthly bill.
  • If you under-allocate resources: resources will get used up quickly, application performance will suffer, and the kubelet may start killing pods until resource utilization drops.

The Vertical Pod Autoscaler or VPA functionality provided in the Kubernetes platform aims to address this problem.

Kubernetes Vertical Pod Autoscaler (VPA)

The first reaction to VPA might be, “since legacy stateful applications require expensive refactoring to scale horizontally, let’s instead use a single pod and vertically scale the pod.” Unfortunately, stateful scaling is not yet a supported VPA use case.

How Does Kubernetes Vertical Pod Autoscaler Work?

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For simple Deployments or ReplicaSets (that are not stateful), VPA suggests values of requested pod resources and provides those values as recommendations for future Deployments. Alternatively, we can configure VPA to auto-apply those values to our Deployment objects.

How Does Horizontal Pod Autoscaler (HPA) Relate to VPA?

HPA scales the application pods horizontally by running more copies of the same pod (assuming that the hosted application supports horizontal scaling via replication). VPA is still relevant when horizontally scaling as it will compute and recommend the correct values for the requested resources of the pods managed by HPA.

For example, you can use a customized metric such as the number of incoming session requests by end-users to a service load balancer to scale horizontally via HPA. Meanwhile, CPU and memory resource usage metrics can be used separately to adjust each pod’s allocated resources via VPA.

How VPA works

The VPA controller observes the resource usage of an application. Then, using that usage information as a baseline, VPA recommends a lower bound, an upper bound, and target values for resource requests for those application pods.

In simple terms, we can summarize the VPA workflow as:

observe resource usage → recommend resources requests → update resources

How the K8s VPA Recommender works
The VPA Recommender

Depending on how you configure VPA, it can either:

  • Apply the recommendations directly by updating/recreating the pods (updateMode = auto).
  • Store the recommended values for reference (updateMode = off).
  • Apply the recommended values to newly created pods only (updateMode = initial).

Keep in mind that updateMode = auto is ok to use in testing or staging environments but not in production. The reason is that the pod restarts when VPA applies the change, which causes a workload disruption.

We should set updateMode = off in production, feed the recommendations to a capacity monitoring dashboard such as Grafana, and apply the recommendations in the next deployment cycle.

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How to use VPA

Here is a sample Kubernetes Deployment that uses VPA for resource recommendations.

First, create the Deployment resource using the following YAML manifest shown below. Note that there are no CPU or memory requests. The pods in the Deployment belong to the VerticalPodAutoscaler (shown in the next paragraph) as they are designated with the kind, Deployment and name, nginx-deployment.

apiVersion: apps/v1
  kind: Deployment
    name: nginx-deployment
      app: nginx
    replicas: 2
        app: nginx
          app: nginx
        - name: nginx
          image: nginx:1.7.8
          - containerPort: 80

Then, create the VPA resources using the following manifest:

  kind: VerticalPodAutoscaler
    name: nginx-deployment-vpa
      apiVersion: "apps/v1"
      kind:       Deployment
      name:       nginx-deployment
      updateMode: "Off"

Note that the update mode is set to off. This will just get the recommendations, but not auto-apply them. Once the configuration is applied, get the VPA recommendations by using the kubectl describe vpa nginx-deployment-vpa command.

The recommended resource requests will look like this:

  - containerName: nginx
      cpu: 40m
      memory: 3100k
      cpu: 60m
      memory: 3500k
      cpu: 831m
      memory: 8000k

You can set the UpdateMode to auto in the above example to enable auto-update the resource requests (assuming that it’s not being used in a production environment). This will cause the pods to be recreated with new values for resource requests.

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Using VPA with HPA

As mentioned earlier in this article, you can use both VPA and HPA in a complementary fashion. When used together, VPA calculates and adjusts the right amount of CPU and memory to allocate, while HPA scales the pod horizontally using a custom metric.

Below is an example diagram showing HPA based on Istio metrics. The Istio telemetry service collects stats like HTTP request rate, response status codes, and duration from the Envoy sidecars that can drive the horizontal pod autoscaling. VPA will be driven only by CPU and Memory usage metrics.

The diagram below shows the Kubernetes and Prometheus (monitoring tool) components that come together to enable the use case described above.

HPA and VPA rely on the Kubernetes metrics server, metrics aggregator, Istio Telemetry, and the Prometheus custom metrics adapter
HPA and VPA rely on the Kubernetes metrics server, metrics aggregator, Istio Telemetry, and the Prometheus custom metrics adapter

VPA Limitations

While VPA is a helpful tool for recommending and applying resource allocations, it has several limitations. Below are ten important points to keep in mind when working with VPA.

  1. VPA is not aware of Kubernetes cluster infrastructure variables such as node size in terms of memory and CPU. Therefore, it doesn't know whether a recommended pod size will fit your node. This means that the resource requests recommendation may be too large to fit any node, and therefore pods may go to a pending state because the resource request can’t be met. Some cloud providers such as GKE provide a cluster autoscaler to spin up more worker nodes addressing pod pending issues, but if the Kubernetes environment has no cluster autoscaler feature, then pods will remain pending, causing downtime.
  2. VPA does not support StatefulSets yet. The problem is scaling pods in StatefulSet is not simple. Neither starting nor restarting can be done the way it’s done for a Deployment or ReplicaSet. Instead, the pods in StatefulSet are managed in a well-defined order. For example, a Postgres DB StatefulSet will first deploy the master pod and then deploy the slave or replication pods. The master pod can’t be simply replaced with just any other pod.
  3. In Kubernetes, the pod spec is immutable. This means that the pod spec can't be updated in place. To update or change the pod resource request, VPA needs to evict the pod and re-create it. This will disrupt your workload. As a result, running VPA in auto mode isn’t a viable option for many use cases. Instead, it is used for recommendations that can be applied manually during a maintenance window.
  4. VPA won't work with HPA using the same CPU and memory metrics because it would cause a race condition. Suppose HPA and VPA both use CPU and memory metrics for scaling decisions. HPA will try to scale out (horizontally) based on CPU and memory, while at the same time, VPA will try to scale the pods up (vertically). Therefore if you need to use both HPA and VPA together, you must configure HPA to use a custom metric such as web requests.
  5. VPA is not yet ready for JVM-based workloads. This shortcoming is due to its limited visibility into memory usage for Java virtual machine workloads,
  6. The performance of VPA is untested on large-scale clusters. Therefore, performance issues may occur when using VPA at scale. This is another reason why it’s not recommended to use VPA within large production environments.
  7. VPA doesn’t consider network and I/O. This is an important issue since ignoring I/O throughout (for writing to disk), and network bandwidth usage can cause application slow-downs and outages.
  8. VPA uses limited historical data. VPA requires eight days of historical data storage before it’s initiated. The limited use of only eight days of data would miss monthly, quarterly, annual, and seasonal fluctuations that could cause bottlenecks during peak usage.
  9. VPA requires configuration for each cluster. If you manage a dozen or more clusters, you would have to manage separate configurations for each cluster. More sophisticated optimization tools provide a governance workflow for approving and unifying configurations across multiple clusters.
  10. VPA policies lack flexibility. VPA uses a resource policy to control resource computations, and an update policy to control how to apply changes to Pods. The policy functionality is however limited. For example, the resource policy sets a higher and a lower value calculated based on historical CPU and memory measurements aggregated into percentiles (e.g., 95 percentile) and you can’t choose a more sophisticated machine learning algorithm to predict usage.

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VPA assists in adding or removing CPU and memory resources but its inherent limitations make it too risky to use in a production environment. Optimizing the Kubernetes resources assigned to essential applications requires a holistic optimization of containers, pods, namespaces, and nodes. Machine learning should power such an analysis to avoid human error and scale to support large estates. Another critical factor to consider is the need to consistently apply the same optimization technology to other resources, whether hosted in a public or private cloud. A cross-platform optimization solution reduces the need for organizational training, improves accuracy based on well-established policies, and provides unified reporting. To learn more, read about our holistic approach to optimizing the entire Kubernetes stack.

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