g. Behind the Curtain

Let’s now look at what really happens in AWS when you submit a job and reveal a bit of the magic behind your auto-scaling computational resources.

At the heart of AWS ParallelCluster exists an auto-scaling group. It is a logical group of instances that can scale up and scale down based on a series of criteria. In the case of AWS ParallelCluster we have three processes controlling the scaling of the cluster. These processes will:

  • Check the queue for any pending job and compute the total number of instances needed to process the queued jobs.
  • Check if instances are busy over a cooldown period (600 seconds or 10 mins) and emit a heartbeat if they are not doing anything.
  • If no jobs are waiting in the queue and some instances are sitting idle then they will be terminated and the auto-scaling group reduced.

More details can be found in the documentation of AWS ParallelCluster.

Let’s check in more details how this works.

Auto-scaling Groups

We will now take a look at auto-scaling groups

  1. Open the AWS Console, select the EC2 Dashboard, then go to Auto Scaling Groups at the bottom left.
  2. You should see an auto-scaling group with Desired at 0, Min at 0 and Max at 8. The Min and Max value correspond to the min_queue_size and max_queue_size of your cluster configuration file. ParallelCluster Create
  3. Go back to your AWS Cloud9 environment and launch a new job with the commands below. It will just run a wait for 5 minutes.

    cat > sleep_script.sbatch << EOF
    #!/bin/bash
    #SBATCH --job-name=hello-world-job
    #SBATCH --ntasks=2
    #SBATCH --output=%x_%j.out
    
    sleep 300
    EOF
    sbatch sleep_script.sbatch
  4. Go back to the EC2 Dashboard and Auto Scaling Groups, refresh the fields with the circling arrows if necessary. You should see that an instance just appeared on the desired field instead of 0. It corresponds to the 2 physical cores or c4.xlarge equivalent that we requested. ParallelCluster Create

  5. On the EC2 Dashboard click on Instances on the left side of the window. You should see your compute instances labeled as Compute. ParallelCluster Create

Now you have a better understanding on how AWS ParallelCluster operates. If you are interested, please look at the documentation as there is more to explore.