Clusters deployed with CfnCluster are elastic in several ways. The first is by simply setting the initial_queue_size and max_queue_size parameters of a cluster settings. The initial_queue_size sets minimum size value of the ComputeFleet Auto Scaling Group(ASG) and also the desired capacity value . The max_queue_size sets maximum size value of the ComputeFleet ASG. As part of the CfnCluster, two Amazon CloudWatch alarms are created. These alarms monitor a custom Amazon CloudWatch metric that is published by the MasterServer of each cluster, this is the second elastic nature of CfnCluster. This metric is called pending and is created per Stack and unique to each cluster. These Amazon CloudWatch alarms call ScaleUp policies associated with the ComputeFleet ASG. This is what handles the automatic addition of compute nodes when there is pending tasks in the cluster. It is actually capable to scaling the cluster with zero compute nodes until the alarms no longer trigger or the max_queue_size is reached.
Within AutoScaling, there is typically a Amazon CloudWatch alarm to remove instances when no longer needed. This alarm would operate on a aggregate metric such as CPU or network. When the aggregate metric fell below a certain level, it would make a call to a ScaleDown policy. The decision to remove which instance is complex and is not aware of individual instance utilization. For that reason, each one of the instances in the ComputeFleet ASG run a process called nodewatcher. The purpose of this process is to monitor the instance and if idle AND close to the end of the current hour, remove it from the ComputeFleet ASG. It specifically calls the TerminateInstanceInAutoScalingGroup API call, which will remove an instance as long as the size of the ASG is larger than the desired capacity. That is what handles the scale down of the cluster, without affecting any running jobs and also enables an elastic cluster with a fixed base number of instances.
The value of the auto scaling is the same for HPC as with any other workloads, the only difference here is CfnCluster has code to specifically make it interact in a more intelligent manner. If a static cluster is required, this can be achieved by setting initial_queue_size and max_queue_size parameters to the size of cluster required and also setting the maintain_initial_size parameter to true. This will cause the ComputeFleet ASG to have the same value for minimum, maximum and desired capacity.