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Allocation strategies for Spot Instances

Your launch configuration determines all the possible Spot capacity pools (instance types and Availability Zones) from which EC2 Fleet can launch Spot Instances. However, when launching instances, EC2 Fleet uses the allocation strategy that you specify to pick the specific pools from all your possible pools.

Allocation strategies

You can specify one of the following allocation strategies for Spot Instances:

Choose the appropriate allocation strategy

You can optimize your fleet for your use case by choosing the appropriate Spot allocation strategy. For On-Demand Instance target capacity, EC2 Fleet always selects the least expensive instance type based on the public On-Demand price, while following the allocation strategy—either price-capacity-optimized, capacity-optimized, diversified, or lowest-price—for Spot Instances.

Balance lowest price and capacity availability

To balance the trade-offs between the lowest priced Spot capacity pools and the Spot capacity pools with the highest capacity availability, we recommend that you use the price-capacity-optimized allocation strategy. This strategy makes decisions about which pools to request Spot Instances from based on both the price of the pools and the capacity availability of Spot Instances in those pools. This means that we will request Spot Instances from the pools that we believe have the lowest chance of interruption in the near term, while still taking price into consideration.

If your fleet runs resilient and stateless workloads, including containerized applications, microservices, web applications, data and analytics jobs, and batch processing, then use the price-capacity-optimized allocation strategy for optimal cost savings and capacity availability.

If your fleet runs workloads that might have a higher cost of interruption associated with restarting work, then you should implement checkpointing so that applications can restart from that point if they're interrupted. By using checkpointing, you make the price-capacity-optimized allocation strategy a good fit for these workloads because it allocates capacity from the lowest priced pools that also offer a low Spot Instance interruption rate.

For an example configuration that uses the price-capacity-optimized allocation strategy, see Example 11: Launch Spot Instances in a price-capacity-optimized fleet.

When workloads have a high cost of interruption

You can optionally use the capacity-optimized strategy if you run workloads that either use similarly priced instance types, or where the cost of interruption is so significant that any cost saving is inadequate in comparison to a marginal increase in interruptions. This strategy allocates capacity from the most available Spot capacity pools that offer the possibility of fewer interruptions, which can lower the overall cost of your workload. For an example configuration that uses the capacity-optimized allocation strategy, see Example 9: Launch Spot Instances in a capacity-optimized fleet.

When the possibility of interruptions must be minimized but the preference for certain instance types matters, you can express your pool priorities by using the capacity-optimized-prioritized allocation strategy and then setting the order of instance types to use from highest to lowest priority. For an example configuration, see Example 10: Launch Spot Instances in a capacity-optimized fleet with priorities.

Note that using priorities is supported only if your fleet uses a launch template. Also note that when you set priorities for capacity-optimized-prioritized, the same priorities are also applied to your On-Demand Instances if the On-Demand AllocationStrategy is set to prioritized.

When your workload is time flexible and capacity availability is not a factor

If your fleet is small or runs for a short time, you can use price-capacity-optimized to maximize cost savings while still considering capacity availability.

If your workload is time flexible and capacity availability is not a factor, you can optionally use the lowest-price allocation strategy to maximize cost savings. Note, however, that because the lowest-price allocation strategy only considers instance price and not capacity availability, it might lead to high Spot Instance interruption rates.

When your fleet is large or runs for a long time

If your fleet is large or runs for a long time, you can improve the availability of your fleet by distributing the Spot Instances across multiple pools using the diversified strategy. For example, if your EC2 Fleet specifies 10 pools and a target capacity of 100 instances, the fleet launches 10 Spot Instances in each pool. If the Spot price for one pool exceeds your maximum price for this pool, only 10% of your fleet is affected. Using this strategy also makes your fleet less sensitive to increases in the Spot price in any one pool over time. With the diversified strategy, the EC2 Fleet does not launch Spot Instances into any pools with a Spot price that is equal to or higher than the On-Demand price.

To create an inexpensive and diversified fleet, use the lowest-price strategy in combination with InstancePoolsToUseCount. For example, if your target capacity is 10 Spot Instances, and you specify 2 Spot capacity pools (for InstancePoolsToUseCount), EC2 Fleet will draw on the two lowest priced pools to fulfill your Spot capacity.

You can use a low or high number of Spot capacity pools across which to allocate your Spot Instances. For example, if you run batch processing, we recommend specifying a low number of Spot capacity pools (for example, InstancePoolsToUseCount=2) to ensure that your queue always has compute capacity while maximizing savings. If you run a web service, we recommend specifying a high number of Spot capacity pools (for example, InstancePoolsToUseCount=10) to minimize the impact if a Spot capacity pool becomes temporarily unavailable.

Note that EC2 Fleet attempts to draw Spot Instances from the number of pools that you specify on a best effort basis. If a pool runs out of Spot capacity before fulfilling your target capacity, EC2 Fleet will continue to fulfill your request by drawing from the next lowest priced pool. To ensure that your target capacity is met, you might receive Spot Instances from more than the number of pools that you specified. Similarly, if most of the pools have no Spot capacity, you might receive your full target capacity from fewer than the number of pools that you specified.

Maintain target capacity

After Spot Instances are terminated due to a change in the Spot price or available capacity of a Spot capacity pool, an EC2 Fleet of type maintain launches replacement Spot Instances. The allocation strategy determines the pools from which the replacement instances are launched, as follows: