Best practices to optimize your Amazon EC2 Spot Instances usage

This blog post is written by Pranaya Anshu, EC2 PMM, and Sid Ambatipudi, EC2 Compute GTM Specialist.

Amazon EC2 Spot Instances are a powerful tool that thousands of customers use to optimize their compute costs. The National Football League (NFL) is an example of customer using Spot Instances, leveraging 4000 EC2 Spot Instances across more than 20 instance types to build its season schedule. By using Spot Instances, it saves 2 million dollars every season! Virtually any organization – small or big – can benefit from using Spot Instances by following best practices.

Overview of Spot Instances

Spot Instances let you take advantage of unused EC2 capacity in the AWS cloud and are available at up to a 90% discount compared to On-Demand prices. Through Spot Instances, you can take advantage of the massive operating scale of AWS and run hyperscale workloads at a significant cost saving. In exchange for these discounts, AWS has the option to reclaim Spot Instances when EC2 requires the capacity. AWS provides a two-minute notification before reclaiming Spot Instances, allowing workloads running on those instances to be gracefully shut down.

In this blog post, we explore four best practices that can help you optimize your Spot Instances usage and minimize the impact of Spot Instances interruptions: diversifying your instances, considering attribute-based instance type selection, leveraging Spot placement scores, and using the price-capacity-optimized allocation strategy. By applying these best practices, you’ll be able to leverage Spot Instances for appropriate workloads and ultimately reduce your compute costs. Note for the purposes of this blog, we will focus on the integration of Spot Instances with Amazon EC2 Auto Scaling groups.

Pre-requisites

Spot Instances can be used for various stateless, fault-tolerant, or flexible applications such as big data, containerized workloads, CI/CD, web servers, high-performance computing (HPC), and AI/ML workloads. However, as previously mentioned, AWS can interrupt Spot Instances with a two-minute notification, so it is best not to use Spot Instances for workloads that cannot handle individual instance interruption — that is, workloads that are inflexible, stateful, fault-intolerant, or tightly coupled.

Best practices

1. Diversify your instances

The fundamental best practice when using Spot Instances is to be flexible. A Spot capacity pool is a set of unused EC2 instances of the same instance type (for example, m6i.large) within the same AWS Region and Availability Zone (for example, us-east-1a). When you request Spot Instances, you are requesting instances from a specific Spot capacity pool. Since Spot Instances are spare EC2 capacity, you want to base your selection (request) on as many spare pools of capacity as possible in order to increase your likelihood of getting Spot Instances. You should diversify across instance sizes, generations, instance types, and Availability Zones to maximize your savings with Spot Instances. For example, if you are currently using c5a.large in us-east-1a, consider including c6a instances (newer generation of instances), c5a.xl (larger size), or us-east-1b (different Availability Zone) to increase your overall flexibility. Instance diversification is beneficial not only for selecting Spot Instances, but also for scaling, resilience, and cost optimization.

To get hands-on experience with Spot Instances and to practice instance diversification, check out Amazon EC2 Spot Instances workshops. And once you’ve diversified your instances, you can leverage AWS Fault Injection Simulator (AWS FIS) to test your applications’ resilience to Spot Instance interruptions to ensure that they can maintain target capacity while still benefiting from the cost savings offered by Spot Instances. To learn more about stress testing your applications, check out the Back to Basics: Chaos Engineering with AWS Fault Injection Simulator video and AWS FIS documentation.

2. Consider attribute-based instance type selection

We have established that flexibility is key when it comes to getting the most out of Spot Instances. Similarly, we have said that in order to access your desired Spot Instances capacity, you should select multiple instance types. While building and maintaining instance type configurations in a flexible way may seem daunting or time-consuming, it doesn’t have to be if you use attribute-based instance type selection. With attribute-based instance type selection, you can specify instance attributes — for example, CPU, memory, and storage — and EC2 Auto Scaling will automatically identify and launch instances that meet your defined attributes. This removes the manual-lift of configuring and updating instance types. Moreover, this selection method enables you to automatically use newly released instance types as they become available so that you can continuously have access to an increasingly broad range of Spot Instance capacity. Attribute-based instance type selection is ideal for workloads and frameworks that are instance agnostic, such as HPC and big data workloads, and can help to reduce the work involved with selecting specific instance types to meet specific requirements.

For more information on how to configure attribute-based instance selection for your EC2 Auto Scaling group, refer to Create an Auto Scaling Group Using Attribute-Based Instance Type Selection documentation. To learn more about attribute-based instance type selection, read the Attribute-Based Instance Type Selection for EC2 Auto Scaling and EC2 Fleet news blog or check out the Using Attribute-Based Instance Type Selection and Mixed Instance Groups section of the Launching Spot Instances workshop.

3. Leverage Spot placement scores

Now that we’ve stressed the importance of flexibility when it comes to Spot Instances and covered the best way to select instances, let’s dive into how to find preferred times and locations to launch Spot Instances. Because Spot Instances are unused EC2 capacity, Spot Instances capacity fluctuates. Correspondingly, it is possible that you won’t always get the exact capacity at a specific time that you need through Spot Instances. Spot placement scores are a feature of Spot Instances that indicates how likely it is that you will be able to get the Spot capacity that you require in a specific Region or Availability Zone. Your Spot placement score can help you reduce Spot Instance interruptions, acquire greater capacity, and identify optimal configurations to run workloads on Spot Instances. However, it is important to note that Spot placement scores serve only as point-in-time recommendations (scores can vary depending on current capacity) and do not provide any guarantees in terms of available capacity or risk of interruption.  To learn more about how Spot placement scores work and to get started with them, see the Identifying Optimal Locations for Flexible Workloads With Spot Placement Score blog and Spot placement scores documentation.

As a near real-time tool, Spot placement scores are often integrated into deployment automation. However, because of its logging and graphic capabilities, you may find it to be a valuable resource even before you launch a workload in the cloud. If you are looking to understand historical Spot placement scores for your workload, you should check out the Spot placement score tracker, a tool that automates the capture of Spot placement scores and stores Spot placement score metrics in Amazon CloudWatch. The tracker is available through AWS Labs, a GitHub repository hosting tools. Learn more about the tracker through the Optimizing Amazon EC2 Spot Instances with Spot Placement Scores blog.

When considering ideal times to launch Spot Instances and exploring different options via Spot placement scores, be sure to consider running Spot Instances at off-peak hours – or hours when there is less demand for EC2 Instances. As you may assume, there is less unused capacity – Spot Instances – available during typical business hours than after business hours. So, in order to leverage as much Spot capacity as you can, explore the possibility of running your workload at hours when there is reduced demand for EC2 instances and thus greater availability of Spot Instances. Similarly, consider running your Spot Instances in “off-peak Regions” – or Regions that are not experiencing business hours at that certain time.

On a related note, to maximize your usage of Spot Instances, you should consider using previous generation of instances if they meet your workload needs. This is because, as with off-peak vs peak hours, there is typically greater capacity available for previous generation instances than current generation instances, as most people tend to use current generation instances for their compute needs.

4. Use the price-capacity-optimized allocation strategy

Once you’ve selected a diversified and flexible set of instances, you should select your allocation strategy. When launching instances, your Auto Scaling group uses the allocation strategy that you specify to pick the specific Spot pools from all your possible pools. Spot offers four allocation strategies: price-capacity-optimized, capacity-optimized, capacity-optimized-prioritized, and lowest-price. Each of these allocation strategies select Spot Instances in pools based on price, capacity, a prioritized list of instances, or a combination of these factors.

The price-capacity-optimized strategy launched in November 2022. This strategy makes Spot Instance allocation decisions based on the most capacity at the lowest price. It essentially enables Auto Scaling groups to identify the Spot pools with the highest capacity availability for the number of instances that are launching. In other words, if you select this allocation strategy, we will find the Spot capacity pools that we believe have the lowest chance of interruption in the near term. Your Auto Scaling groups then request Spot Instances from the lowest priced of these pools.

We recommend you leverage the price-capacity-optimized allocation strategy for the majority of your workloads that run on Spot Instances. To see how the price-capacity-optimized allocation strategy selects Spot Instances in comparison with lowest-price and capacity-optimized allocation strategies, read the Introducing the Price-Capacity-Optimized Allocation Strategy for EC2 Spot Instances blog post.

Clean-up

If you’ve explored the different Spot Instances workshops we recommended throughout this blog post and spun up resources, please remember to delete resources that you are no longer using to avoid incurring future costs.

Conclusion

Spot Instances can be leveraged to reduce costs across a wide-variety of use cases, including containers, big data, machine learning, HPC, and CI/CD workloads. In this blog, we discussed four Spot Instances best practices that can help you optimize your Spot Instance usage to maximize savings: diversifying your instances, considering attribute-based instance type selection, leveraging Spot placement scores, and using the price-capacity-optimized allocation strategy.

To learn more about Spot Instances, check out Spot Instances getting started resources. Or to learn of other ways of reducing costs and improving performance, including leveraging other flexible purchase models such as AWS Savings Plans, read the Increase Your Application Performance at Lower Costs eBook or watch the Seven Steps to Lower Costs While Improving Application Performance webinar.