Optimizing Amazon ECS task density using awsvpc network mode

This post is contributed by Tony Pujals | Senior Developer Advocate, AWS

AWS recently increased the number of elastic network interfaces available when you run tasks on Amazon ECS. Use the account setting called awsvpcTrunking. If you use the Amazon EC2 launch type and task networking (awsvpc network mode), you can now run more tasks on an instance—5 to 17 times as many—as you did before.

As more of you embrace microservices architectures, you deploy increasing numbers of smaller tasks. AWS now offers you the option of more efficient packing per instance, potentially resulting in smaller clusters and associated savings.

Overview

To manage your own cluster of EC2 instances, use the EC2 launch type. Use task networking to run ECS tasks using the same networking properties as if tasks were distinct EC2 instances.

Task networking offers several benefits. Every task launched with awsvpc network mode has its own attached network interface, a primary private IP address, and an internal DNS hostname. This simplifies container networking and gives you more control over how tasks communicate, both with each other and with other services within their virtual private clouds (VPCs).

Task networking also lets you take advantage of other EC2 networking features like VPC Flow Logs. This feature lets you monitor traffic to and from tasks. It also provides greater security control for containers, allowing you to use security groups and network monitoring tools at a more granular level within tasks. For more information, see Introducing Cloud Native Networking for Amazon ECS Containers.

However, if you run container tasks on EC2 instances with task networking, you can face a networking limit. This might surprise you, particularly when an instance has plenty of free CPU and memory. The limit reflects the number of network interfaces available to support awsvpc network mode per container instance.

Raise network interface density limits with trunking

The good news is that AWS raised network interface density limits by implementing a networking feature on ECS called “trunking.” This is a technique for multiplexing data over a shared communication link.

If you’re migrating to microservices using AWS App Mesh, you should optimize network interface density. App Mesh requires awsvpc networking to provide routing control and visibility over an ever-expanding array of running tasks. In this context, increased network interface density might save money.

By opting for network interface trunking, you should see a significant increase in capacity—from 5 to 17 times more than the previous limit. For more information on the new task limits per container instance, see Supported Amazon EC2 Instance Types.

Applications with tasks not hitting CPU or memory limits also benefit from this feature through the more cost-effective “bin packing” of container instances.

Trunking is an opt-in feature

AWS chose to make the trunking feature opt-in due to the following factors:

• Instance registration: While normal instance registration is straightforward with trunking, this feature increases the number of asynchronous instance registration steps that can potentially fail. Any such failures might add extra seconds to launch time.
• Available IP addresses: The “trunk” belongs to the same subnet in which the instance’s primary network interface originates. This effectively reduces the available IP addresses and potentially the ability to scale out on other EC2 instances sharing the same subnet. The trunk consumes an IP address. With a trunk attached, there are two assigned IP addresses per instance, one for the primary interface and one for the trunk.
• Differing customer preferences and infrastructure: If you have high CPU or memory workloads, you might not benefit from trunking. Or, you may not want awsvpc networking.

Consequently, AWS leaves it to you to decide if you want to use this feature. AWS might revisit this decision in the future, based on customer feedback. For now, your account roles or users must opt in to the awsvpcTrunking account setting to gain the benefits of increased task density per container instance.

Enable trunking

Enable the ECS elastic network interface trunking feature to increase the number of network interfaces that can be attached to supported EC2 container instance types. You must meet the following prerequisites before you can launch a container instance with the increased network interface limits:

• Your account must have the AWSServiceRoleForECS service-linked role for ECS.
• You must opt into the awsvpcTrunking  account setting.

Make sure that a service-linked role exists for ECS

A service-linked role is a unique type of IAM role linked to an AWS service (such as ECS). This role lets you delegate the permissions necessary to call other AWS services on your behalf. Because ECS is a service that manages resources on your behalf, you need this role to proceed.

In most cases, you won’t have to create a service-linked role. If you created or updated an ECS cluster, ECS likely created the service-linked role for you.

You can confirm that your service-linked role exists using the AWS CLI, as shown in the following code example:

$ aws iam get-role --role-name AWSServiceRoleForECS
{
    "Role": {
        "Path": "/aws-service-role/ecs.amazonaws.com/",
        "RoleName": "AWSServiceRoleForECS",
        "RoleId": "AROAJRUPKI7I2FGUZMJJY",
        "Arn": "arn:aws:iam::226767807331:role/aws-service-role/ecs.amazonaws.com/AWSServiceRoleForECS",
        "CreateDate": "2018-11-09T21:27:17Z",
        "AssumeRolePolicyDocument": {
            "Version": "2012-10-17",
            "Statement": [
                {
                    "Effect": "Allow",
                    "Principal": {
                        "Service": "ecs.amazonaws.com"
                    },
                    "Action": "sts:AssumeRole"
                }
            ]
        },
        "Description": "Role to enable Amazon ECS to manage your cluster.",
        "MaxSessionDuration": 3600
    }
}

If the service-linked role does not exist, create it manually with the following command:

aws iam create-service-linked-role --aws-service-name ecs.amazonaws.com

For more information, see Using Service-Linked Roles for Amazon ECS.

Opt in to the awsvpcTrunking account setting

Your account, IAM user, or role must opt in to the awsvpcTrunking account setting. Select this setting using the AWS CLI or the ECS console. You can opt in for an account by making awsvpcTrunking  its default setting. Or, you can enable this setting for the role associated with the instance profile with which the instance launches. For instructions, see Account Settings.

Other considerations

After completing the prerequisites described in the preceding sections, launch a new container instance with increased network interface limits using one of the supported EC2 instance types.

Keep the following in mind:

• It’s available with the latest variant of the ECS-optimized AMI.
• It only affects creation of new container instances after opting into awsvpcTrunking.
• It only affects tasks created with awsvpc network mode and EC2 launch type. Tasks created with the AWS Fargate launch type always have a dedicated network interface, no matter how many you launch.

For details, see ENI Trunking Considerations.

Summary

If you seek to optimize the usage of your EC2 container instances for clusters that you manage, enable the increased network interface density feature with awsvpcTrunking. By following the steps outlined in this post, you can launch tasks using significantly fewer EC2 instances. This is especially useful if you embrace a microservices architecture, with its increasing numbers of lighter tasks.

Hopefully, you found this post informative and the proposed solution intriguing. As always, AWS welcomes all feedback or comment.