Using AWS Spot instances

This page describes how to set up the system to take advantage of Amazon’s EC2 spot instances.

Nodemanager

Nodemanager should have configured cloud sizes that include the preemptible boolean parameter. For example, for every on-demand cloud node size, you could create a .spot variant, like this:

[Size m4.large]
cores = 2
scratch = 32000

[Size m4.large.spot]
cores = 2
instance_type = m4.large
preemptible = true
scratch = 32000

Slurm dispatcher

The crunch-dispatch-slurm service needs a matching instance type configuration on /etc/arvados/config.yml, following the previous example:

Clusters:
  uuid_prefix:
    InstanceTypes:
    - Name: m4.large
      VCPUs: 2
      RAM: 7782000000
      Scratch: 32000000000
      Price: 0.1
    - Name: m4.large.spot
      Preemptible: true
      VCPUs: 2
      RAM: 7782000000
      Scratch: 32000000000
      Price: 0.1

InstanceType names should match those defined on nodemanager’s config file because it’s crunch-dispatch-slurm’s job to select the instance type and communicate the decision to nodemanager via Slurm.

API Server

Container requests will need the preemptible scheduling parameter included, to make the dispatcher request a spot instance. The API Server configuration file includes an option that when active, will auto assign the preemptible parameter to any new child container request if it doesn’t have it already. To activate this feature, the following should be added to the application.yml file:

preemptible_instances: true

With this configuration active, child container requests should include the preemptible = false parameter at creation time to avoid being scheduled for spot instance usage.

AWS Permissions

When requesting spot instances, Amazon’s API may return an authorization error depending on how users and permissions are set on the account. If this is the case check nodemanager’s log for:

BaseHTTPError: AuthFailure.ServiceLinkedRoleCreationNotPermitted: The provided credentials do not have permission to create the service-linked role for EC2 Spot Instances.

The account needs to have a service linked role created. This can be done by logging into the AWS account, go to IAM Management → Roles and create the AWSServiceRoleForEC2Spot role by clicking on the Create button, selecting EC2 service and EC2 - Spot Instances use case.

Cost Tracking

Amazon’s Spot instances prices are declared at instance request time and defined by the maximum price that the user is willing to pay per hour. By default, this price is the same amount as the on-demand version of each instance type, and this setting is the one that nodemanager uses for now, as it doesn’t include any pricing data to the spot instance request.

The real price that a spot instance has at any point in time is discovered at the end of each usage hour, depending on instance demand. For this reason, AWS provides a data feed subscription to get hourly logs, as described on Amazon’s User Guide.

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