Spot instances

# cluster.yaml

node_groups:
  - name: node-group-1

    # whether to use spot instances for this node group (default: false)
    spot: false

    spot_config:
      # additional instance types with identical or better specs than the primary cluster instance type (defaults to only the primary instance type)
      instance_distribution: # [similar_instance_type_1, similar_instance_type_2]

      # minimum number of on demand instances (default: 0)
      on_demand_base_capacity: 0

      # percentage of on demand instances to use after the on demand base capacity has been met [0, 100] (default: 50)
      # note: setting this to 0 may hinder cluster scale up when spot instances are not available
      on_demand_percentage_above_base_capacity: 0

      # max price for spot instances (default: the on-demand price of the primary instance type)
      max_price: # <float>

      # number of spot instance pools across which to allocate spot instances [1, 20] (default: number of instances in instance distribution)
      instance_pools: 3

Spot instances are not guaranteed to be available. The chances of getting spot instances can be improved by providing instance_distribution, a list of alternative instance types to the primary instance_type you specified. If left blank, Cortex will only include the primary instance type in the instance_distribution. When using instance_distribution, use the instance type with the fewest compute resources as your primary instance_type. Note that the default value for max_price is the on-demand price of the primary instance type, but you may wish to set this to the on-demand price of the most expensive instance type in your instance_distribution.

Spot instances can be mixed with on-demand instances by configuring on_demand_base_capacity and on_demand_percentage_above_base_capacity. on_demand_base_capacity enforces the minimum number of nodes that will be fulfilled by on-demand instances as your cluster is scaling up. on_demand_percentage_above_base_capacity defines the percentage of instances that will be on-demand after the base capacity has been fulfilled (the rest being spot instances). instance_pools is the number of pools per availability zone to allocate your instances from. See here for more details.

Even if multiple instances are specified in your instance_distribution on-demand instances are mixed, there is still a possibility of running into scale up issues when attempting to spin up spot instances. Spot instance requests may not be fulfilled for several reasons. Spot instance pricing fluctuates, therefore the max_price may be lower than the current spot pricing rate. Another possibility could be that the availability zones of the cluster ran out of spot instances. The addition of another on-demand node group to node_groups with a lower priority (by having a higher index in the node_groups list) can mitigate the impact of unfulfilled spot requests by enabling the cluster to spin up on-demand instances if spot instance requests are not fulfilled within 5 minutes.

There is a spot instance limit associated with your AWS account for each instance family in each region. You can check your current limit and request an increase here (set the region in the upper right corner to your desired region, type "spot" in the search bar, and click on the quota that matches your instance type). Note that the quota values indicate the number of vCPUs available, not the number of instances; different instances have a different numbers of vCPUs, which can be seen here.

Example spot configuration

Only spot instances

node_groups:
  - name: node-group-1
    spot: true

3 on-demand base capacity with 0% on-demand above base capacity

node_groups:
  - name: node-group-1
    min_instances: 0
    max_instances: 5
    spot: true
    spot_config:
      on_demand_base_capacity: 3
      on_demand_percentage_above_base_capacity: 0

# instance 1-3: on-demand
# instance 4-5: spot

0 on-demand base capacity with 50% on-demand above base capacity

node_groups:
  - name: node-group-2
    min_instances: 0
    max_instances: 4
    spot: true
    spot_config:
      on_demand_base_capacity: 0
      on_demand_percentage_above_base_capacity: 50

# instance 1: on-demand
# instance 2: spot
# instance 3: on-demand
# instance 4: spot