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Why does ECS binpack strategy on memory always scale up a new EC2 instance despite available resources?

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I'm using ECS with an Auto Scaling group as a capacity provider to deploy tasks on EC2 (t3.micro, 2 vCPU, 1 GB memory). I've set the task placement strategy to binpack based on memory. However, I've noticed that ECS always scales up a new EC2 instance to deploy a new task, even if an existing instance has enough CPU and memory available. Consequently, there is only one task per EC2 instance. I expect that all tasks should be placed on a single EC2 instance if it has sufficient memory.

Here are some actions I've already checked:

  1. Port conflict: The network mode is set to awsvpc in the ECS task definition, so each task gets its own ENI, which prevents port conflicts.
  2. EC2 storage: Each EC2 instance has a storage size of 30GB (EBS GP3). My container is a nginx-based web app with 1 MB of static files, so the storage is more than sufficient for running multiple containers.

The following configurations might be related to this issue,

Capacity provider configurations

capacity provider: autoscaling group
base: 0
weight: 100
target capacity: 100%
managed instance draining: true
managed instance scaling: true
scale in protection: false

ECS service configurations

desired count: 1
placement strategy:
  type: binpack
  field: memory
scheduling strategy: REPLICA
service connect:
  enabled: true
  namespace: my_namespace
  services:
    - port name: web
      discovery name: hello
      client aliases:
        - port: 80
          dns name: hello
  

ECS task definition

network mode: awsvpc
CPU: 256
Memory: 128
container definitions:
  - name: web
    image: nginx
    port mappings:
      - name: web
        container port: 80
        protocol: tcp

Any insights or suggestions?

Additional information
I changed the instance type from t3.micro to t3.small (2 vCPUs, 2 GB memory) and deployed 4 ECS tasks. The ECS cluster autoscaled up 2 EC2 instances, placing 2 tasks on each instance.

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Hi,

I would assume it is perhaps due to the baseline constraints for T series instances which can burst upto 100% but normally operates at baseline. (Cannot corroborate this with AWS docs unfortunately)

You might want to check on a small non T type instance.

Update: I tried launching 8 tasks on c7g.medium instance. It launched 4 tasks on single node before scaling another node for the other 4. A slight improvement as compared to T.x instances but definitely not as expected for binpack. Maybe some more underlying factors determining binpack behaviour

--Syd

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  • I changed the instance type to c7i.large (2 vCPU, 4 GB memory). Unfortunately, the ECS cluster still scaled up 2 instances and placed 2 tasks on each instance. Thanks your advice.

  • I also cleanuped the container instance memory cache/buffer (showed by free -hb). It didn't solve the problem.

  • Can you also define the same limits at the container level (in addition to the task definition level). After setting the limits on the container lever to 0.125vCPU /0.125GB (Memory hard limit) I was able to launch 8 tasks on one c7g.medium instance even though there were two instances running. Also i tested with network mode default

  • Thank you for your assistance. I have set the resource limits for the container level to 0.125 vCPU and 0.125 GB (hard limit). However, it still placed two tasks on each EC2 instance (c7i.large). I need to enable Service Connect, so I have defined the network mode as awsvpc.

  • Can I ask for your configuration details about capacity provider, autoscaling group, ECS service, ECS task definition?

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