Tutorial: Use Spot Fleet with instance weighting

This tutorial uses a fictitious company called Example Corp to illustrate the process of requesting a Spot Fleet using instance weighting.

Objective

Example Corp, a pharmaceutical company, wants to leverage the computational power of Amazon EC2 for screening chemical compounds that might be used to fight cancer.

Planning

Example Corp first reviews Spot Best Practices. Next, Example Corp determines the following requirements for their Spot Fleet.

Instance types

Example Corp has a compute- and memory-intensive application that performs best with at least 60 GB of memory and eight virtual CPUs (vCPUs). They want to maximize these resources for the application at the lowest possible price. Example Corp decides that any of the following EC2 instance types would meet their needs:

Instance type	Memory (GiB)	vCPUs
r3.2xlarge	61	8
r3.4xlarge	122	16
r3.8xlarge	244	32

Target capacity in units

With instance weighting, target capacity can equal a number of instances (the default) or a combination of factors such as cores (vCPUs), memory (GiBs), and storage (GBs). By considering the base for their application (60 GB of RAM and eight vCPUs) as 1 unit, Example Corp decides that 20 times this amount would meet their needs. So the company sets the target capacity of their Spot Fleet request to 20.

Instance weights

After determining the target capacity, Example Corp calculates instance weights. To calculate the instance weight for each instance type, they determine the units of each instance type that are required to reach the target capacity as follows:

• r3.2xlarge (61.0 GB, 8 vCPUs) = 1 unit of 20

• r3.4xlarge (122.0 GB, 16 vCPUs) = 2 units of 20

• r3.8xlarge (244.0 GB, 32 vCPUs) = 4 units of 20

Therefore, Example Corp assigns instance weights of 1, 2, and 4 to the respective launch configurations in their Spot Fleet request.

Price per unit hour

Example Corp uses the On-Demand price per instance hour as a starting point for their price. They could also use recent Spot prices, or a combination of the two. To calculate the price per unit hour, they divide their starting price per instance hour by the weight. For example:

Instance type	On-Demand price	Instance weight	Price per unit hour
r3.2xLarge	$0.7	1	$0.7
r3.4xLarge	$1.4	2	$0.7
r3.8xLarge	$2.8	4	$0.7

Example Corp could use a global price per unit hour of $0.7 and be competitive for all three instance types. They could also use a global price per unit hour of $0.7 and a specific price per unit hour of $0.9 in the r3.8xlarge launch specification.

Verify permissions

Before creating a Spot Fleet request, Example Corp verifies that it has an IAM role with the required permissions. For more information, see Spot Fleet permissions.

Create the request

Example Corp creates a file, config.json, with the following configuration for its Spot Fleet request:

{
  "SpotPrice": "0.70",
  "TargetCapacity": 20,
  "IamFleetRole": "arn:aws:iam::123456789012:role/aws-ec2-spot-fleet-tagging-role",
  "LaunchSpecifications": [
    {
      "ImageId": "ami-1a2b3c4d",
      "InstanceType": "r3.2xlarge",
      "SubnetId": "subnet-482e4972",
      "WeightedCapacity": 1
    },
    {
      "ImageId": "ami-1a2b3c4d",
      "InstanceType": "r3.4xlarge",
      "SubnetId": "subnet-482e4972",
      "WeightedCapacity": 2
    },
    {
      "ImageId": "ami-1a2b3c4d",
      "InstanceType": "r3.8xlarge",
      "SubnetId": "subnet-482e4972",
      "SpotPrice": "0.90",
      "WeightedCapacity": 4
    }
  ]
}

Example Corp creates the Spot Fleet request using the request-spot-fleet command.

aws ec2 request-spot-fleet --spot-fleet-request-config file://config.json

For more information, see Spot Fleet request types.

Fulfillment

The allocation strategy determines which Spot capacity pools your Spot Instances come from.

With the lowestPrice strategy (which is the default strategy), the Spot Instances come from the pool with the lowest price per unit at the time of fulfillment. To provide 20 units of capacity, the Spot Fleet launches either 20 r3.2xlarge instances (20 divided by 1), 10 r3.4xlarge instances (20 divided by 2), or 5 r3.8xlarge instances (20 divided by 4).

If Example Corp used the diversified strategy, the Spot Instances would come from all three pools. The Spot Fleet would launch 6 r3.2xlarge instances (which provide 6 units), 3 r3.4xlarge instances (which provide 6 units), and 2 r3.8xlarge instances (which provide 8 units), for a total of 20 units.