Amazon ECS Task Definition - CPU units & Memory - set container to use 100% of the EC2 available Resources

Question

I'd like to have multiple different services running on an ECS cluster, each service should be running on a single EC2 instance. The EC2 instances type for all services are the same. And I would like those services to use all their hosting EC2 available resources.

I have the assumption that if i use only the soft memory parameter (without using the hard one ) in the Task Configuration, this will allow my container instance to use all the available memory on the EC2 instance hosting it and that i won't be limiting. Is that correct?

As for the EC2 type (t2.micro [vCPU=1, Memory=1Gib] for example) !! is it possible to simply put:

{  
   ...
   "memory": 1024,
   "cpu": 1024,
   ...
}

Since the EC2 should be already set up with a bunch of Container Service Requirements.

Answer 1

Is it correct that you're trying to have each ECS Instance handle only a single task per instance?

The short answer to your question is, no. Usually the amount of memory made available to your containers is a bit less than the amount of memory available on the machine itself. This is so that the operating system has enough memory to keep running. From my experience, a T2.Small, which has 2048 MB of memory will end up with 2004 MB available for containers.

When it comes to your task definition, there are two ways of specifying Memory. The memory setting is a hard limit. If the containers memory usage hits this amount, the container will be terminated. If on the other hand, you specify memoryReservation, that much memory will be reserved for the task, but it can use more, up to the total amount of the machine. Check out the Task Definition documentation for further details.

An important consideration here is that only one of memory and memoryReservation are required. If both are used, memoryReservation should be less than memory. If you are only going to specify one of these, I'd recommend memoryReservation, as it will allow your task to use up to the total memory on the machine. If both are used, the memoryReservation will be used in calculating the amount of memory consumed by a task.

When placing tasks on an instance, it looks at the amount of available memory, that is the registered amount of memory for the instance, minus any tasks already placed on it. If this number is less than the amount of memory required for a task, no task will be placed on it. If no instance has enough memory for the task, it will not be placed, and the error will be logged in the Services Events log.

So it's important to look at the amount of memory actually registered by your instance type, and then ensure your memory or memoryReservation are lower than the amount registered by your instances. Otherwise, your tasks will never be placed.

As for cpu, this value is not required, and if not specified, all tasks on an instance are allowed an equal portion of the CPU available on the system. If only one task is on the instance, it can use the entire CPU of the instance by default.