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AWS ParallelCluster Auto Scaling

The auto scaling strategy described here applies to HPC clusters deployed with one of the supported traditional job scheduler, either SGE, Slurm or Torque. In these cases AWS ParallelCluster directly implements the scaling capabilities by managing the Auto Scaling Group (ASG) of the compute nodes and changing the scheduler configuration accordingly. For HPC clusters based on AWS Batch, ParallelCluster relies on the elastic scaling capabilities provided by the AWS-managed job scheduler.

Clusters deployed with AWS ParallelCluster 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 the minimum size value of the ComputeFleet ASG and also the desired capacity value. The max_queue_size sets the maximum size value of the ComputeFleet ASG.


Scaling Up

Every minute, a process called jobwatcher runs on the master instance and evaluates the current number of instances required by the pending jobs in the queue. If the total number of busy nodes and requested nodes is greater than the current desired value in the ASG, it adds more instances. If you submit more jobs, the queue will get re-evaluated and the ASG updated up to the max_queue_size.

With SGE each job requires a number of slots to run (one slot corresponds to one processing unit, e.g. a vCPU). When evaluating the number of instances required to serve the currently pending jobs, the jobwatcher divides the total number of requested slots by the capacity of a single compute node. The capacity of a compute node that is the number of available vCPUs depends on the EC2 instance type selected in the cluster configuration.

With Slurm and Torque schedulers each job can require both a number of nodes and a number of slots per node. The jobwatcher takes into account the request of each job and determines the number of compute nodes to fulfill the new computational requirements. For example, assuming a cluster with c5.2xlarge (8 vCPU) as compute instance type, and three queued pending jobs with the following requirements: job1 2 nodes / 4 slots each, job2 3 nodes / 2 slots and job3 1 node / 4 slots, the jobwatcher will require three new compute instances to the ASG to serve the three jobs.

Current limitation: the auto scale up logic does not consider partially loaded busy nodes, i.e. each node running a job is considered busy even if there are empty slots.

Scaling Down

On each compute node, a process called nodewatcher runs and evaluates the idle time of the node. If an instance had no jobs for longer than scaledown_idletime (which defaults to 10 minutes) and currently there are no pending jobs in the cluster, the instance is terminated.

It specifically calls the TerminateInstanceInAutoScalingGroup API call, which will remove an instance as long as the size of the ASG is at least the minimum ASG size. That handles scale down of the cluster, without affecting running jobs and also enables an elastic cluster with a fixed base number of instances.

Static Cluster

The value of the auto scaling is the same for HPC as with any other workloads, the only difference here is AWS ParallelCluster 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.