Memory Not Decreasing for ECS With No Calls

Question

Hi- I have an ECS Cluster with two running instances.  There is a web server running on it to receive requests and provide responses.  A couple of days ago I sent many requests to do stress testing.  The CPU Utilization went up from around .1 % to around 4 % during stress testing, then went back down to normal.  The memory went up from around 8% to around 20%.  However the memory still has not gone down over the past few days even though we are not making calls to the service.  When I look at the memory chart it shows around 20% for the past 3 days.  Any idea why the memory would not decrease when we are not using or calling the service?

re:Post Agent · Answer

The behavior you're observing with your ECS cluster's memory usage is not uncommon and can be attributed to several factors:

1. Memory management in containerized environments: Containers, especially those running web servers or application servers, often allocate memory and may not release it immediately when it's no longer needed. This is partly due to how memory management works in many programming languages and frameworks, which may keep memory allocated for future use to improve performance.

2. Caching: Web servers and applications often use caching mechanisms to improve performance. These caches may retain data in memory even when the service is not actively receiving requests.

3. Memory allocation strategies: Some applications are designed to allocate memory upfront and keep it reserved, even during periods of low activity. This can help reduce the overhead of frequent memory allocation and deallocation.

4. Garbage collection: If your application is written in a language with garbage collection (like Java), the memory might not be immediately released back to the system even when it's no longer in use by the application.

5. CloudWatch metrics aggregation: It's worth noting that CloudWatch collects multiple data points every minute and aggregates them before sending to CloudWatch. This means that short-term fluctuations in memory usage might not be reflected in the overall metrics.

6. Soft limits vs. hard limits: If your task definition uses memoryReservation (soft limit) instead of memory (hard limit), the tasks could use more memory than specified as needed, which might explain why the usage doesn't decrease.

To address this, you could try the following:

1. Review your application code for memory leaks or inefficient memory usage.
2. Consider restarting your tasks or services to see if it releases the memory.
3. Monitor more granular, container-level metrics using CloudWatch Container Insights for a more detailed view of resource usage.
4. If the high memory usage is causing issues, you might need to adjust your task definitions or consider using a larger instance type.

Remember, a certain level of memory usage even during idle periods is normal and can actually be beneficial for performance when the service needs to handle requests quickly.  
**Sources**  
[Reported memory usage discrepancy between Elastic Beanstalk and EC2 | AWS re:Post](https://repost.aws/questions/QUB8jSFLIhQGSeCgOfKhyuZQ/reported-memory-usage-discrepancy-between-elastic-beanstalk-and-ec2)  
[Monitor high resource usage in ECS tasks and services | AWS re:Post](https://repost.aws/knowledge-center/ecs-task-monitor-high-usage)  
[Amazon ECS service utilization metrics - Amazon Elastic Container Service](https://docs.aws.amazon.com/AmazonECS/latest/developerguide/service_utilization.html)

Memory Not Decreasing for ECS With No Calls

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