ECS EC2 resource distribution

The behavior you're observing with your ECS cluster creating two m6a.2xlarge instances instead of one is likely due to several factors related to ECS task placement strategies and resource allocation.

First, it's important to note that ECS doesn't always pack tasks as tightly as possible onto instances by default. The default task placement strategy is "spread," which aims to distribute tasks across Availability Zones and instances for better fault tolerance.

Additionally, ECS reserves some resources on each instance for the ECS agent and other system processes. This means that not all of the instance's resources are available for task allocation.

For Task2, which has multiple containers, ECS treats this as a single task with combined resource requirements. The total resources for Task2 are: CPU: 256 + 2048 + 4096 = 6400 Memory: 512 + 2048 + 6144 = 8704 MB

Given these resource requirements, it's possible that ECS determined it couldn't fit both Task1 and Task2 on a single instance while maintaining sufficient reserved resources for the ECS agent and system processes.

To optimize resource utilization, you could consider:

1. Adjusting your task placement strategy to use "binpack" instead of "spread" if you want to maximize instance utilization.
2. Fine-tuning your task definitions to better match the instance size.
3. Using a different instance type that better aligns with your task requirements.
4. Investigating if there are any service-level constraints or scaling policies affecting instance allocation.

Remember that ECS aims to ensure reliable operation, sometimes at the cost of perfect resource utilization. If you need more precise control over resource allocation, you might want to consider using ECS with AWS Fargate, which provides more granular resource allocation without managing EC2 instances.
Sources
Does ECS have a limitation where only one task can run on the same EC2 instance? | AWS re:Post
ECS Service Registers EC2 Instance more than desired In my ECS cluster using EC2 launch type | AWS re:Post
Overview of Amazon EC2 and Amazon RDS Instance Families - Right Sizing: Provisioning Instances to Match Workloads