AWS ECS Tasks: parallelly::availableCores()

[jeffkeller-einc]

Does anyone know how I can correctly detect the number of available CPUs when running code as an AWS ECS Task? I'm using a Managed Instances Capacity Provider (i.e. not Fargate).

When my task gets placed on an instance with more CPU than explicitly requested, parallelly::availableCores() returns the number of CPUs on the EC2 instance, not what has been allocated to the task.

For example, if I set my task and container vCPU limit to 1024 with placement constraints that guarantee an oversized instance:

{
  "expression": "attribute:ecs.instance-type==m5.2xlarge"
  "type": "memberOf"
}

Then parallelly::availableCores(which = "all") returns:

system /proc/self/status             nproc 
     8                 8                 8

If I launch the same task as a Kubernetes pod then I get the expected number of cores (1).

[HenrikBengtsson]

For example, if I set my task and container vCPU limit to 1024 ...

If AWS ECS Tasks has a standard way to querying CPU constraints, I'm happy to add support for it to the parallelly package. I don't have easy access to AWS ECS Tasks and I don't think I've every used it. Can you please share exactly how you make the above setting/request? That would help to figure out what's possible and not.

[jeffkeller-einc]

I'm not aware of a standard way to query CPU constraints. Most of my reading on the matter have referred to cgroups and CPU shares / quotas with varying levels of ambiguity (docs, docs).

There is quite a lot of infrastructure involved so I don't think I could realistically provide a working minimal example, but something like this using the AWS CLI would be close:

aws ecs run-task --cluster "my-cluster" \
  --capacity-provider-strategy "capacityProvider=my-managed-instances-provider" \
  --task-definition "my-task-definition"
  --placement-constraints "type=memberOf,expression='attribute:ecs.instance-type==m5.2xlarge'"

However, I would be happy to intercept an instance and run some commands to probe the situation if it would help.