Selecting Instance Types

Amazon EC2 Spot Instances offer spare compute capacity available in the AWS Cloud at steep discounts compared to On-Demand prices. EC2 can interrupt Spot Instances with two minutes of notification when EC2 needs the capacity back. You can use Spot Instances for various fault-tolerant and flexible applications. Some examples are analytics, containerized workloads, high-performance computing (HPC), stateless web servers, rendering, CI/CD, and other test and development workloads.

One of the best practices to successfully adopt Spot instances is to implement Spot instance diversification as part of your configuration. Spot instance diversification helps to procure capacity from multiple Spot Instance pools, both for scaling up and for replacing spot instances that may receive a spot instance termination notification. A Spot instance pool is a set of unused EC2 instances with the same instance type (for example, m5.large), operating system, Availability Zone.

Cluster Autoscaler And Spot Instance Diversification

Cluster Autoscaler is a tool that automatically adjusts the size of the Kubernetes cluster when there are pods that fail to run in the cluster due to insufficient resources (Scale Out) or there are nodes in the cluster that have been underutilized for a period of time (Scale In).

When using Spot instances with Cluster Autoscaler there are a few things that should be considered. For example Cluster Autoscaler makes the assumption that all nodes within a nodegroup will have the same number of vCPUs and Memory.

When applying Spot Diversification best practices to EKS and K8s clusters, using Cluster Autoscaler to dynamically scale capacity, we must implement diversification in a way that adheres to Cluster Autoscaler expected operational mode. In this workshop we will assume that our cluster nodegroups should be provisioned with instance types that adhere to a 1vCPU:4GB RAM ratio.

We can diversify Spot instance pools using two strategies:

  • By creating multiple nodegroups, each of different sizes. For example a nodegroup of size 4VCPU’s and 16GB Ram, and another nodegroup of 8vCPU’s and 32GB Ram.

  • By Implementing instance diversification within the nodegroups, by selecting a mix of instances types and families from different Spot instance pools that meet the same vCPU’s and memory criteria.

Our goal in this workshop, is to create at least 2 diversified groups of instances that adhere the 1vCPU:4GB RAM ratio. We can use Spot Instance Advisor page to find the relevant instances types and families with sufficient number of vCPUs and RAM, and use this to also select instance types with low interruption rates.

The frequency of Spot Instance interruptions reflected in Spot Instance Advisor may change over time. Savings compared to On-Demand are calculated over the last 30 days. The above just provides a real world example from a specific time and will probably be different when you are performing this workshop. Note also that not all the instances are available in all the regions.

Selecting Instance Type with 4vCPU and 16GB

In this case with Spot Instance Advisor we can create a 4vCPUs_16GB nodegroup with the following diversified instances: m5.xlarge, m5d.xlarge, m4.xlarge, m5a.xlarge, t2.xlarge, t3.xlarge, t3a.xlarge

Your workload may have other constraints that you should consider when selecting instances types. For example. t2 and t3 instance types are burstable instances and might not be appropriate for CPU bound workloads that require CPU execution determinism. Instances such as m5a are AMD Instances, if your workload is sensitive to numerical differences (i.e: financial risk calculations, industrial simulations) mixing these instance types might not be appropriate.

Challenge

Find out another group that adheres to a 1vCPU:4GB ratio, this time using instances with 8vCPU’s and 32GB of RAM.

Expand this for an example on the list of instances