Task definition CPU reservation on AWS ECS EC2

Question

I am building my cluster on ECS while using EC2 instances. I am curious about specifying CPU reservation on my Task definitions. How does AWS manage my tasks inside of EC2 instances when i leave CPU reservation empty or write 0?

I have read this article: https://aws.amazon.com/blogs/containers/how-amazon-ecs-manages-cpu-and-memory-resources/

And here it says:

when you don’t specify any CPU units for a container, ECS intrinsically enforces two Linux CPU shares for the cgroup (which is the minimum allowed).

I am not really sure what this means and is it different for Tasks, because this is specifically stated for containers?

Answer 1

Cgroups are a feature of the Linux kernel that allow the distribution and hierarchy of services that run on your host.

This enables your containers to operate independently from each other (they will have access to a portion of the available CPU), whilst also providing the ability for higher priority tasks to gain access to the CPU if it required.

A CPU share defines how much of the overall CPU your container can have access to, as you add more containers this becomes a ratio of division between each container. Each of your containers in your case will get 2, if there are 4 containers this is a ratio of 0.25 of the available CPU per each one.

If you define in a task the limits you can cap the maximum of the resource on the host that can be used, of which the CPU shares will then be split in a ratio of. However, this will affect scheduling of new containers (if there is not enough resource available for the task and auto scaling is not enabled your chosen task cannot be scheduled).

There is some documentation here on cgroups, it is technical so if you have little experience of Linux it might be a little confusing.