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Auto-scaling Resources with Balsam Elastic Queue

A Balsam Site does not automatically use your computing allocation by default. Instead, we must use the BatchJob API or balsam queue CLI to explicitly request compute nodes at a given Balsam Site. This follows the principle of least surprise: as the User, you explicitly decide when and how resources are spent. This is not a limitation since we can remotely submit BatchJobs to any Site from a computer where Balsam is installed!

However, we can opt-in to enable the elastic_queue plugin that creates BatchJob submissions on our behalf. This can be a useful service in bursty or real-time workloads: instead of micro-managing the queues, we simply submit a stream of Jobs and allow the Site to provision resources as needed over time.

Enabling the Elastic Queue Plugin

Auto-scaling is enabled at a Site by setting the elastic_queue configuration appropriately inside of the settings.yml file.

You should find this line uncommented by default:

elastic_queue: null

Following this line is a commented-block showing an example elastic_queue configuration. We want to comment out the elastic_queue: null line and uncomment the configuration; setting each of the parameters appropriately for our use case.

Once the Plugin has been configured properly (see below), we must restart the Balsam Site Agent to load it:

$ balsam site sync

# Or if the Site isn't already running:
$ balsam site start

Disabling the Elastic Queue Plugin

To disable, comment out (or delete) the current elastic_queue configuration in settings.yml and replace it with the line:

elastic_queue: null

Then restart the Balsam Site to run without the elastic queue plugin:

$ balsam site sync

Configuring the Elastic Queue

The configuration is fairly flexible to enable a wide range of use cases. This section explains the YAML configuration in chunks.

Project, Queue, and Submit Frequency

Firstly, service_period controls the waiting period (in seconds) between cycles in which a new BatchJob might be submitted to the queue. The submit_project, submit_queue, and job_mode are directly passed through to the new BatchJob.

The max_queue_wait_time_min determines how long a submitted BatchJob should be enqueued before the elastic queue deletes it and tries re-submitting. When using backfill to grab idle nodes (see next section), it makes sense to set a relatively short waiting time of 5-10 minutes. Otherwise, this duration should be increased to a reasonable upper threshold to avoid deleting BatchJobs that have accrued priority in the queues.

The elastic queue will maintain up to max_queued_jobs in the queue at any given time. This should be set to the maximum desired (or allowed) number of simultaneously queued/running BatchJobs at the Site.

elastic_queue:
     service_period: 60
     submit_project: "datascience"
     submit_queue: "balsam"
     job_mode: "mpi"
     use_backfill: True
     min_wall_time_min: 35
     max_wall_time_min: 360
     wall_time_pad_min: 5
     min_num_nodes:  20
     max_num_nodes: 127
     max_queue_wait_time_min: 10
     max_queued_jobs: 20

Wall Time and Backfilling

Many HPC systems use backfilling schedulers, which attempt to place small Jobs while draining nodes for larger Jobs to start up at a determined future time. By opportunistically sizing jobs to fit into these idle node-hour windows, Balsam effectively "fills the gaps" in unused resources. We enable this dynamic sizing with use_backfill: True.

The interpretation of min_wall_time_min and max_wall_time_min depends on whether or not use_backfill is enabled:

  • When use_backfill is False: min_wall_time_min is ignored and BatchJobs are submitted for a constant wallclock time limit of max_wall_time_min.
  • When use_backfill is True: Balsam selects backfill windows that are at least as long as min_wall_time_min (this is to avoid futile 5 minute submissions when all Jobs take at least 30 minutes). The wallclock time limit is then the lesser of the scheduler's backfill duration and max_wall_time_min.
  • Finally, a "padding" value of wall_time_pad_min is subtracted from the final wallclock time in all BatchJob submissions. This should be set to a couple minutes when use_backfill is True and 0 otherwise.
elastic_queue:
     service_period: 60
     submit_project: "datascience"
     submit_queue: "balsam"
     job_mode: "mpi"
     use_backfill: True
     min_wall_time_min: 35
     max_wall_time_min: 360
     wall_time_pad_min: 5
     min_num_nodes:  20
     max_num_nodes: 127
     max_queue_wait_time_min: 10
     max_queued_jobs: 20

Node Count

Finally, the min_num_nodes and max_num_nodes determine the permissible range of node counts in submitted BatchJobs. When operating with the use_backfill=True constraint, backfill windows smaller than min_num_nodes will be ignored. Otherwise, BatchJob submissions use min_num_nodes as a lower bound. Likewise, max_num_nodes gives an upper bound on the BatchJob's node count.

The actual submitted BatchJob node count falls somewhere in this range. It is determined from the difference between how many nodes are currently requested (queued or running BatchJobs) and the aggregate node footprint of all runnable Jobs.

elastic_queue:
     service_period: 60
     submit_project: "datascience"
     submit_queue: "balsam"
     job_mode: "mpi"
     use_backfill: True
     min_wall_time_min: 35
     max_wall_time_min: 360
     wall_time_pad_min: 5
     min_num_nodes:  20
     max_num_nodes: 127
     max_queue_wait_time_min: 10
     max_queued_jobs: 20

Therefore, the elastic queue automatically controls the size and number of requested BatchJobs as the workload grows. We can think of each BatchJob as a flexibly-sized block of resources, and the elastic queue creates multiple blocks (one per service_period) while choosing their sizes. If one BatchJob does not accommodate the incoming volume of tasks, then multiple BatchJobs of the maximum size are submitted at each iteration.

When the incoming Jobs slow down and the backlog falls inside the (min_num_nodes, max_num_nodes) range, the BatchJobs reduce down to a single, smaller allocation of resources. As utilization decreases and launchers become idle, the nodes are released according to the launcher's idle_ttl_sec configuration (also in settings.yml).