Selecting Spot 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. One of the best practices for successful adoption of Spot instances is to implement Spot instance diversification as part of your configuration. Spot instance diversification helps to acquire 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 and size (for example, m5.large), availability zone (AZ), in the same region

We can diversify Spot instances by selecting a mix of instances types and families from different pools that meet the same vCPU’s and memory criteria. In the case of ECS we can check what’s the ratio of vCPU and Memory used by our task resources. For example, look at the ECS task resource reservation in the file ec2-task.json:

"cpu": "480", "memory": "1920"

This means the ratio for vCPU:Memory in our ECS task that would run in the cluster is 1:4. Ideally, we should select instance types with similar vCPU:Memory ratio, in order to have good utilization of the resources in the EC2 instances. There are over 270 different instance types available on EC2 which can make selecting appropriate instance types difficult. amazon-ec2-instance-selector helps you select compatible instance types for your application to run on. We can pass the command line options for resource criteria like vCPUs, memory, network performance, and much more and then return the available, matching instance types.

To learn more about EC2 instance types, click here. As for ec2-instance-selector, it is an open source tool that makes calls to DescribeInstanceTypes APIs, on the specific region and filters instances based on the criteria selected in the command line. We encourage you to test what are the options available with ec2-instance-selector and run a few commands with it to familiarize yourself with the tool.For example, try running the same commands as you did before with the extra parameter --output table-wide.

In our case, for the 1:4 vCPU to memory ratio, the smallest instance type which would satisfy these criteria from the latest generation of x86_64 EC2 instance types is m5.large. We will use amazon-ec2-instance-selector to help us select the relevant instance types and families with an enough number of vCPUs and Memory. Let’s first install amazon-ec2-instance-selector :

curl -Lo ec2-instance-selector`uname | tr '[:upper:]' '[:lower:]'`-amd64 && chmod +x ec2-instance-selector
sudo mv ec2-instance-selector /usr/local/bin/
ec2-instance-selector --version

Now that you have ec2-instance-selector installed, you can run ec2-instance-selector --help to understand how you could use it for selecting Instances that match your workload requirements. For this workshop, we need to first get a group of instances that meet the following criteria:

  • 1:4 vCPU:RAM Ratio
  • Instances have 2 vCPUs
  • Instances don’t have a GPUs
  • Instances Architecture is: x86_64 (no ARM instances like A1 or m6g instances, for example)
  • Instances of current generation (4th gen onwards)
  • Instances that don’t meet the regular expression .*n.*|.*d.*, so effectively discard instances such as: m5n, m5dn, m5d.
ec2-instance-selector --vcpus-to-memory-ratio 1:4 --vcpus=2 --gpus 0 --current-generation -a x86_64 --deny-list '.*n.*|.*d.*'     

This should display a list like the one that follows (note results might differ depending on the region). We will use these instance types as part or EC2 Auto Scaling groups.


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.

While in this example we have restricted our selection to similar instances and pools of the same size (of memory and vCPUs), in production workloads the recommendation is to increase the instance selection by adding other sizes that respect the same vCPU to memory ratio.

As a summary of Spot Best practices selection for ECS Auto Scaling Groups and Capacity Providers : a) Use different AZ’s b) Diversify across multiple instance types-pools c) Diversify using multiple generation of similar hardware that keep the multiplier or ratio of cpu/mem close i.e: m4.large, m5.large, m4.xlarge, m5.xlarge. d) While diversifying in size, avoid very large spreads in instance sizes, and add contiguous 2 to 3 sizes i.e: large, xlarge, 2xlarge. or 2xlarge, 4xlarge, 8xlarge.

Exercise : How would you change the ec2-instance-selector command above to provide other instance sizes?

Click here to show the answer