Allocation strategies - Amazon EC2 Auto Scaling
Spot InstancesOn-Demand InstancesHow the allocation strategies work with weights
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Allocation strategies

When you use multiple instance types, you manage how Amazon EC2 Auto Scaling fulfills your On-Demand and Spot capacities from the possible instance types. To do this, you specify allocation strategies.

To review the best practices for a mixed instances group, see Setup overview.

Spot Instances

Amazon EC2 Auto Scaling provides the following allocation strategies for Spot Instances:

price-capacity-optimized (recommended)

The price and capacity optimized allocation strategy looks at both price and capacity to select the Spot Instance pools that are the least likely to be interrupted and have the lowest possible price.

We recommend this strategy when you're getting started. For more information, see Introducing the price-capacity-optimized allocation strategy for EC2 Spot Instances in the Amazon blog.

capacity-optimized

Amazon EC2 Auto Scaling requests your Spot Instance from the pool with optimal capacity for the number of instances that are launching.

With Spot Instances, the pricing changes slowly over time based on long-term trends in supply and demand. However, capacity fluctuates in real time. The capacity-optimized strategy automatically launches Spot Instances into the most available pools by looking at real-time capacity data and predicting which are the most available. This helps to minimize possible disruptions for workloads that might have a higher cost of interruption associated with restarting work and checkpointing. To give certain instance types a higher chance of launching first, use capacity-optimized-prioritized.

capacity-optimized-prioritized

You set the order of instance types for the launch template overrides from highest to lowest priority (from first to last in the list). Amazon EC2 Auto Scaling honors the instance type priorities on a best-effort basis but optimizes for capacity first. This is a good option for workloads where the possibility of disruption must be minimized, but the preference for certain instance types matters, too. If the On-Demand allocation strategy is set to prioritized, the same priority is applied when fulfilling On-Demand capacity.

lowest-price

Amazon EC2 Auto Scaling requests your Spot Instances using the lowest priced pools within an Availability Zone, across the N number of Spot pools that you specify for the Lowest priced pools setting. For example, if you specify four instance types and four Availability Zones, your Auto Scaling group can access up to 16 Spot pools. (Four in each Availability Zone.) If you specify two Spot pools (N=2) for the allocation strategy, your Auto Scaling group can draw on the two lowest priced pools per Availability Zone to fulfill your Spot capacity.

Because this strategy only considers instance price and not capacity availability, it might lead to high interruption rates.

Amazon EC2 Auto Scaling makes an effort to draw Spot Instances from the N number of pools that you specify. However, if a pool runs out of Spot capacity before fulfilling your desired capacity, Amazon EC2 Auto Scaling continues to fulfill your request by drawing from the next lowest priced pool. To meet your desired capacity, you might receive Spot Instances from more pools than your specified N number. Likewise, if most of the pools have no Spot capacity, you might receive your full desired capacity from fewer pools than your specified N number.

Note

If you configure your Spot Instances to launch with AMD SEV-SNP turned on, you are charged an additional hourly usage fee that is equivalent to 10% of the On-Demand hourly rate of the selected instance type. If the allocation strategy uses price as an input, Amazon EC2 Auto Scaling does not include this additional fee; only the Spot price is used.

On-Demand Instances

Amazon EC2 Auto Scaling provides the following allocation strategies that can be used for On-Demand Instances:

lowest-price

Amazon EC2 Auto Scaling automatically deploys the lowest priced instance type in each Availability Zone based on the current On-Demand price.

To meet your desired capacity, you might receive On-Demand Instances of more than one instance type in each Availability Zone. This depends on how much capacity you request.

prioritized

When fulfilling On-Demand capacity, Amazon EC2 Auto Scaling determines which instance type to use first based on the order of instance types in the list of launch template overrides. For example, let's say that you specify three launch template overrides in the following order: c5.large, c4.large, and c3.large. When your On-Demand Instances launch, the Auto Scaling group fulfills On-Demand capacity in the following order: c5.large, c4.large, and then c3.large.

Consider the following when managing the priority order of your On-Demand Instances:

  • You can pay for usage upfront to get significant discounts for On-Demand Instances by using Savings Plans or Reserved Instances. For more information, see the Amazon EC2 pricing page.

  • With Reserved Instances, your discounted rate of the regular On-Demand Instance pricing applies if Amazon EC2 Auto Scaling launches matching instance types. Therefore, if you have unused Reserved Instances for c4.large, you can set the instance type priority to give the highest priority for your Reserved Instances to a c4.large instance type. When a c4.large instance launches, you receive the Reserved Instance pricing.

  • With Savings Plans, your discounted rate of the regular On-Demand Instance pricing applies when using Amazon EC2 Instance Savings Plans or Compute Savings Plans. With Savings Plans, you have more flexibility when prioritizing your instance types. As long as you use instance types that are covered by your Savings Plan, you can set them in any priority order. You can also occasionally change the entire order of your instance types, while still receiving the Savings Plan discounted rate. For more information about Savings Plans, see the Savings Plans User Guide.

How the allocation strategies work with weights

When you specify the WeightedCapacity parameter in your overrides (or "DesiredCapacityType": "vcpu" or "DesiredCapacityType": "memory-mib" at the group level), the allocation strategies work exactly like they do for other Auto Scaling groups.

The only difference is that when you choose the lowest-price or price-capacity-optimized strategy, your instances come from the instance pools with the lowest price per unit in each Availability Zone. For more information, see Configure an Auto Scaling group to use instance weights.

For example, imagine you have an Auto Scaling group with several instance types that have varying amounts of vCPUs. You use lowest-price for your Spot and On-Demand allocation strategies. If you choose to assign weights based on the vCPU count of each instance type, Amazon EC2 Auto Scaling launches whichever instance types have the lowest price per your assigned weight values (for example, per vCPU) at the time of fulfillment. If it's a Spot Instance, then this means the lowest Spot price per vCPU. If it's an On-Demand Instance, then this means the lowest On-Demand price per vCPU.