Services or capabilities described in Amazon Web Services documentation might vary by Region. To see the differences applicable to the China Regions, see Getting Started with Amazon Web Services in China (PDF).

Configure an Auto Scaling group to use instance weights

When you use multiple instance types, you can specify how many units to associate with each instance type, and then specify the capacity of your group with the same unit of measurement. This capacity specification option is known as weights.

For example, let's say that you run a compute-intensive application that performs best with at least 8 vCPUs and 15 GiB of RAM. If you use c5.2xlarge as your base unit, any of the following EC2 instance types would meet your application needs.

By default, all instance types have equal weight regardless of size. In other words, whether Amazon EC2 Auto Scaling launches a large or small instance type, each instance counts the same toward the desired capacity of the Auto Scaling group.

With weights, however, you assign a number value that specifies how many units to associate with each instance type. For example, if the instances are of different sizes, a c5.2xlarge instance could have the weight of 2, and a c5.4xlarge (which is two times bigger) could have the weight of 4, and so on. Then, when Amazon EC2 Auto Scaling scales the group, these weights translate into the number of units that each instance counts toward your desired capacity.

The weights do not change which instance types Amazon EC2 Auto Scaling chooses to launch; instead, the allocation strategies do that. For more information, see Allocation strategies.

Considerations

This section discusses key considerations for effectively implementing weights.

Note the following when updating existing groups:

Instance weight behaviors

When you use instance weights, Amazon EC2 Auto Scaling behaves in the following way:

Configure an Auto Scaling group to use weights

You can configure an Auto Scaling group to use weights, as shown in the following Amazon CLI examples. For instructions on using the console, see Create a mixed instances group by manually choosing instance types.

To configure a new Auto Scaling group to use weights (Amazon CLI)

Use the create-auto-scaling-group command. For example, the following command creates a new Auto Scaling group and assigns weights by specifying the following:

aws autoscaling create-auto-scaling-group --cli-input-json file://~/config.json

The config.json file contains the following content.

{
    "AutoScalingGroupName": "my-asg",
    "MixedInstancesPolicy": {
        "LaunchTemplate": {
            "LaunchTemplateSpecification": {
                "LaunchTemplateName": "my-launch-template",
                "Version": "$Latest"
            },
            "Overrides": [
                {
                    "InstanceType": "m4.16xlarge",
                    "WeightedCapacity": "16"
                },
                {
                    "InstanceType": "m5.24xlarge",
                    "WeightedCapacity": "24"
                }
            ]
        },
        "InstancesDistribution": {
            "OnDemandPercentageAboveBaseCapacity": 0,
            "SpotAllocationStrategy": "capacity-optimized"
        }
    },
    "MinSize": 160,
    "MaxSize": 720,
    "DesiredCapacity": 480,
    "VPCZoneIdentifier": "subnet-5ea0c127,subnet-6194ea3b,subnet-c934b782",
    "Tags": []
}

To configure an existing Auto Scaling group to use weights (Amazon CLI)

Use the update-auto-scaling-group command. For example, the following command assigns weights to instance types in an existing Auto Scaling group by specifying the following:

aws autoscaling update-auto-scaling-group --cli-input-json file://~/config.json

The config.json file contains the following content.

{
    "AutoScalingGroupName": "my-existing-asg",
    "MixedInstancesPolicy": {
        "LaunchTemplate": {
            "Overrides": [
                {
                    "InstanceType": "c5.18xlarge",
                    "WeightedCapacity": "18"
                },
                {
                    "InstanceType": "c5.24xlarge",
                    "WeightedCapacity": "24"
                },
                {
                    "InstanceType": "c5.2xlarge",
                    "WeightedCapacity": "2"
                },
                {
                    "InstanceType": "c5.4xlarge",
                    "WeightedCapacity": "4"
                }
            ]
        }
    },
    "MinSize": 0,
    "MaxSize": 100,
    "DesiredCapacity": 100
}

To verify the weights using the command line

Use one of the following commands:

Spot price per unit hour example

The following table compares the hourly price for Spot Instances in different Availability Zones in US East (Northern Virginia) with the price for On-Demand Instances in the same Region. The prices shown are example pricing and not current pricing. These are your costs per instance hour.

With instance weights, you can evaluate your costs based on what you use per unit hour. You can determine the price per unit hour by dividing your price for an instance type by the number of units that it represents. For On-Demand Instances, the price per unit hour is the same when deploying one instance type as it is when deploying a different size of the same instance type. In contrast, however, the Spot price per unit hour varies by Spot pool.

The following example shows how the Spot price per unit hour calculation works with instance weights. For ease of calculation, let's say you want to launch Spot Instances only in us-east-1a. The per unit hour price is captured in the following table.