Determining the task size for Amazon ECS

One of the most important choices when you deploy containers on Amazon ECS is your container and task sizes. Your container and task sizes are essential for scaling and capacity planning.

Amazon ECS uses two resource metrics for capacity: CPU and memory. Amazon ECS measures CPU in units of 1/1024 of a full vCPU (where 1024 units equals 1 whole vCPU). Amazon ECS measures memory in megabytes.

In your task definition, you can declare resource reservations and limits.

When you declare a reservation, you declare the minimum amount of resources that a task requires. Your task receives at least the amount of resources you request. Your application might be able to use more CPU or memory than the reservation you declare. However, this is subject to any limits you also declared.

Using more than the reservation amount is known as bursting. Bursting means your application uses more resources than you reserved but stays within your declared limits. Amazon ECS guarantees reservations. For example, if you use Amazon EC2 instances to provide capacity, Amazon ECS doesn't place a task on an instance where it can't fulfill the reservation.

A limit is the maximum amount of CPU units or memory that your container or task can use. If your container tries to use more CPU than this limit, Amazon ECS throttles it. If your container tries to use more memory than this limit, Amazon ECS stops your container.

Choosing these values can be challenging. The values that work best for your application depend greatly on your application's resource requirements.

Load testing your application is the key to successful resource requirement planning. Load testing helps you better understand your application's requirements.

Stateless applications

For stateless applications that scale horizontally, such as an application behind a load balancer, we recommend that you first determine how much memory your application consumes when it serves requests.

To do this, you can use traditional tools such as ps or top. You can also use monitoring solutions such as CloudWatch Container Insights.

When you determine a CPU reservation, consider how you want to scale your application to meet your business requirements.

You can use smaller CPU reservations, such as 256 CPU units (or 1/4 vCPU), to scale out in a fine-grained way that minimizes cost. But they might not scale fast enough to meet significant spikes in demand.

You can use larger CPU reservations to scale in and out more quickly. This helps you match demand spikes more quickly. However, larger CPU reservations cost more.

Other applications

For applications that don't scale horizontally, such as singleton workers or database servers, available capacity and cost are your most important considerations.

Choose the amount of memory and CPU based on what load testing shows you need to serve traffic and meet your service-level objective. Amazon ECS ensures that your application is placed on a host that has adequate capacity.