Managing container swap space

Amazon ECS enables you to control the usage of swap memory space on your Linux container instances at the container level. Using a per-container swap configuration, each container within a task definition can have swap enabled or disabled, and for those that have it enabled, the maximum amount of swap space used can be limited. For example, latency-critical containers can have swap disabled, whereas containers with high transient memory demands can have swap turned on to reduce the chances of out-of-memory errors when the container is under load.

The swap configuration for a container is managed by the following container definition parameters:

maxSwap

The total amount of swap memory (in MiB) a container can use. This parameter will be translated to the --memory-swap option to docker run where the value would be the sum of the container memory plus the maxSwap value.

If a maxSwap value of 0 is specified, the container will not use swap. Accepted values are 0 or any positive integer. If the maxSwap parameter is omitted, the container will use the swap configuration for the container instance it is running on. A maxSwap value must be set for the swappiness parameter to be used.

swappiness

This allows you to tune a container's memory swappiness behavior. A swappiness value of 0 will cause swapping to not happen unless absolutely necessary. A swappiness value of 100 will cause pages to be swapped very aggressively. Accepted values are whole numbers between 0 and 100. If the swappiness parameter is not specified, a default value of 60 is used. If a value is not specified for maxSwap then this parameter is ignored. This parameter maps to the --memory-swappiness option to docker run.

The following is an example showing the JSON syntax:

"containerDefinitions": [{
        ...
        "linuxParameters": {
            "maxSwap": integer,
            "swappiness": integer
        },
        ...
}]

Container swap considerations

Consider the following when you use a per-container swap configuration.

• Swap space must be enabled and allocated on the container instance for the containers to use.

  Note

  The Amazon ECS-optimized AMIs do not have swap enabled by default. You must enable swap on the instance to use this feature. For more information, see Instance Store Swap Volumes in the Amazon EC2 User Guide for Linux Instances or How do I allocate memory to work as swap space in an Amazon EC2 instance by using a swap file?.

• The swap space container definition parameters are only supported for task definitions using the EC2 launch type.

• This feature is only supported for Linux containers.

• If the maxSwap and swappiness container definition parameters are omitted from a task definition, each container will have a default swappiness value of 60 and the total swap usage will be limited to two times the memory reservation of the container.