Job Scheduling & Priority Policy

Overview

The DSI Cluster uses a three-tier Quality of Service (QoS) system to balance competing demands for compute resources. This system ensures that interactive development work is always accessible while maximizing hardware utilization for batch computation.

Every job submitted to the cluster runs under one of three QoS tiers. Each tier offers a different tradeoff between scheduling speed, preemption protection, and fairshare cost.

For practical how-to examples, see the Submitting Batch Jobs guide and the Interactive Sessions guide.

QoS Tiers

general — Default Tier

All jobs use the general tier unless you explicitly request otherwise. This is the cheapest tier in terms of fairshare cost, but jobs are preemptable — they can be interrupted and requeued if a higher-priority job needs the resources.

Use this for: Hyperparameter sweeps, training runs, batch experiments, anything that can tolerate interruption.

Preemption protection: Jobs are guaranteed at least 5 minutes of runtime before becoming eligible for preemption. When preempted, you receive a 5-minute warning signal before the job is killed and automatically requeued. See Handling Preemption below.

protected — Non-Preemptable Tier

The protected tier guarantees that your job will run to completion without interruption. It is designed for users who are learning the cluster or running work where preemption would be disruptive. It costs 4x the fairshare of the general tier and is limited to one job at a time with a shorter wall time.

Use this for: Learning and experimentation when you don’t want to deal with preemption, short training runs that must complete without interruption, or any job where implementing checkpointing is impractical.

Not for: Long-running batch work or sweeps. The 1-job limit and 2-hour wall time are enforced — requests that exceed these limits are rejected immediately. For bulk work, use general (cheaper, more jobs allowed). For active development, use interactive (longer wall time, preempts general jobs).

interactive — Development Tier

The interactive tier is designed for active, hands-on development: debugging code, running notebooks, testing configurations. It preempts general jobs and schedules with the highest priority. It is tightly limited to prevent misuse.

Use this for: Interactive development, debugging, Jupyter notebooks, any task where you need a human sitting at a terminal with real-time feedback.

Not for: Batch training, sweeps, or any unattended computation. The 1-session limit and 4-hour wall time are enforced — requests that exceed these limits are rejected immediately. There is no GPU cap on interactive sessions; however, the 8x fairshare cost means large interactive allocations are expensive and should be used only when genuinely necessary (e.g., testing sharding strategies across multiple GPUs).

How to Specify a QoS Tier

Add --qos=<tier> to your job submission command:

# Batch job on general tier (default — you can omit --qos entirely)
sbatch --qos=general train.sh

# Non-preemptable job (guaranteed to complete, 2h max)
sbatch --qos=protected --time=02:00:00 train.sh

# Interactive development session
salloc --qos=interactive --partition=general --gres=gpu:1 --time=04:00:00

If you do not specify --qos, your job defaults to general.

Preemption Policy

What Is Preemption?

Preemption is the process by which a higher-priority job reclaims resources from a lower-priority job. When your job is preempted:

1. You receive a warning signal (SIGUSR1) indicating preemption is imminent.
2. You have a 5-minute grace period to save your work (checkpoint).
3. After the grace period, your job is killed and automatically requeued.
4. When resources become available, your requeued job starts again from the beginning of the script.

Preemption Hierarchy

interactive ──preempts──> general

Only interactive jobs can preempt, and only general jobs can be preempted. Specifically:

• interactive jobs can preempt general jobs.
• general jobs cannot preempt anything — including other general jobs. Two general jobs coexist normally on the cluster: one does not kick the other out, even if one has higher fairshare priority. If GPUs are fully allocated, new general jobs simply wait in the queue.
• protected jobs cannot be preempted and cannot preempt other jobs. They run to completion.

In other words, preemption is a one-way interaction between exactly two tiers (interactive → general). Every other pairing is queue-based only.

Preemption Protections

• Minimum runtime guarantee: Jobs run for at least 5 minutes before becoming eligible for preemption. A job that just started will not be immediately preempted.
• Grace period: When preemption occurs, you receive a 5-minute warning before your job is killed. This is your window to save a checkpoint.
• Automatic requeue: Preempted jobs are automatically placed back in the queue and will restart when resources are available.
• Opt out entirely: If preemption is a concern, use the protected tier. Your job is guaranteed to run to completion (up to the 2-hour wall time).

Handling Preemption

To handle preemption gracefully, your job should implement checkpoint/restart logic. The standard contract is:

1. Submit your job with --signal=B:USR1@300 and --requeue
2. In your script, trap SIGUSR1 and save a checkpoint
3. Exit with code 99 after checkpointing
4. On startup, check for an existing checkpoint and resume from it

Jobs that do not implement checkpointing will simply restart from the beginning when requeued. This is fine for short jobs, but for longer training runs, implementing checkpointing is strongly recommended.

See the Submitting Batch Jobs guide for complete examples including a wrapper script and PyTorch checkpointing code.

Priority & Fairshare

How Priority Is Calculated

Your job’s scheduling priority is determined by a weighted combination of factors:

How Fairshare Works

Fairshare ensures that no single user (or research group) can monopolize the cluster indefinitely. The system tracks your recent resource consumption and adjusts your scheduling priority accordingly:

• New or light users have high fairshare priority — their jobs schedule quickly.
• Heavy users see their fairshare priority decline — their new jobs queue behind lighter users.
• Recovery is automatic: Usage decays with a half-life of 2 days. After a few days of lighter usage, your fairshare priority recovers.

The Fairshare Cost of Premium Tiers

This is intentional: premium tiers provide better service (guaranteed completion or preemption rights), and the cost discourages using them as the default.

Practical impact: Occasional use of interactive for development or protected for a short guaranteed job barely affects your fairshare. The system rewards users who put most of their work in the general tier and reserve premium tiers for when they truly need them.

Fair Usage Expectations

Job Arrays for Sweeps

If you are running hyperparameter sweeps or parameter scans, use job arrays with a concurrency throttle:

sbatch --array=0-999%20 --qos=general sweep.sh

The %20 limits your sweep to 20 concurrent tasks. This is more efficient for Slurm to manage than submitting 1000 individual jobs, and the throttle prevents you from consuming excessive resources.

Choose the Right Tier

• Most work belongs in general. It’s cheap, and preemption is handled gracefully if you implement checkpointing.
• Use protected when you need guaranteed completion and are running a short job (under 2 hours) where preemption would be disruptive. Good for learners and one-off experiments.
• Use interactive only for active development. The 1-session and 4-hour limits are enforced.

Queue Flooding

The cluster enforces per-user limits on job submission and priority accrual to prevent any single user from overwhelming the scheduling system. These limits are generous enough for legitimate use but prevent extreme queue flooding.

Lab Partitions

The QoS tier system applies across all partitions, including lab-specific partitions (veitch, clab, litian, ai+s, SSLab, complementary-ai, Monsoon, DDRI). Lab partitions provide an additional scheduling advantage on dedicated hardware through partition-level priority.

If you have access to a lab partition, you benefit from both your QoS tier priority and your partition priority. For example, a general-tier job on the veitch partition has higher partition priority than a general-tier job on the general partition, because the veitch partition has a higher partition priority weight.