Use instance weighting to manage cost and performance of your EC2 Fleet or Spot Fleet

With instance weighting, you assign a weight to each instance type in your EC2 Fleet or Spot Fleet to represent their compute capacity and performance relative to each other. Based on the weights, the fleet can use any combination of the specified instance types, as long as it can fulfil the desired target capacity. This can help you manage the cost and performance of your fleet.

The weight represents the capacity units that an instance type contributes to the total target capacity.

Example: Use instance weighting for performance management

Suppose your fleet has two instance types, and you assign a different weight to each instance type to reflect how many you need of each to achieve the same performance, as follows:

By assigning these weights, you're saying that you'd need 4 m5.large instances to achieve the same performance as 1 m5.2xlarge.

To calculate how many instances of each instance type are needed for a given target capacity, use the following formula:

target capacity / weight = number of instances

If your target capacity is 8 units, the fleet could fulfill the target capacity with either m5.large or m5.2xlarge, or a mix of both, as follows:

Example: Use instance weighting for cost management

By default, the price that you specify is per instance hour. When you use the instance weighting feature, the price that you specify is per unit hour. You can calculate your price per unit hour by dividing your price for an instance type by the number of units that it represents. The fleet calculates the number of instances to launch by dividing the target capacity by the instance weight. If the result isn't an integer, the fleet rounds it up to the next integer, so that the size of your fleet is not below its target capacity. The fleet can select any pool that you specify in your launch specification, even if the capacity of the instances launched exceeds the requested target capacity.

The following table includes examples of calculations to determine the price per unit for a fleet with a target capacity of 10.

Use the fleet instance weighting as follows to provision the target capacity that you want in the pools with the lowest price per unit at the time of fulfillment:

Instance weighting example

Consider a fleet request with the following configuration:

The weights represent the number of units that instance type represents toward the target capacity. If the first launch specification provides the lowest price per unit (price for r3.2xlarge per instance hour divided by 6), the fleet would launch four of these instances (24 divided by 6).

If the second launch specification provides the lowest price per unit (price for c3.xlarge per instance hour divided by 5), the fleet would launch five of these instances (24 divided by 5, result rounded up).

Instance weighting and allocation strategy

Consider a fleet request with the following configuration:

The fleet would launch four instances (30 divided by 8, result rounded up). With the diversified strategy, the fleet launches one instance in each of the three pools, and the fourth instance in whichever of the three pools provides the lowest price per unit.