Status: draft. Verified steps and screenshots to come.
In this lesson you will start a program on a compute node from your web browser, check that it's running, and end it properly. Time: 10 minutes, plus any wait in the queue · Branch: Start here · Program: the desktop app (AMA27 desktop app name)
| Before this | Data | You'll need |
|---|---|---|
| Nothing | None | An account on the cluster, and whatever second factor it asks for at sign-in: AMA27 account instructions, to come |
Note
Cluster details. Highlighted values are placeholders. AMA27's queue names, module versions, paths and addresses go in when the cluster is deployed.
Why this matters
The visualization lessons need a program running on a compute node, with a window you can use. Open OnDemand gives you both from a web browser. You ask for part of a node, and ParaView or a Linux desktop appears in a tab. The data stays on the cluster, and you install nothing on your own computer.
Key idea
An OnDemand session is a job. Open OnDemand asks the scheduler for a compute node on your behalf, exactly as a batch script would. The form you fill in is that job request. You book a queue, a length of time and a number of cores. The node is then yours until the time runs out or you end the session. Closing the browser tab does neither: it closes your view, while the job keeps running and keeps using your time. Deleting the session is what ends it. → Glossary: interactive session
Steps
Step 1: Log in
Go to the cluster's OnDemand site, AMA27 OnDemand address, and sign in with AMA27 sign-in method.
You should see: the OnDemand dashboard, with a row of app icons (pinned apps) and a menu bar with Files, Jobs, Clusters and Interactive Apps.
Screenshot: the OnDemand dashboard
Screenshot to come: The OnDemand dashboard: pinned app icons, with Files, Jobs, Clusters and Interactive Apps in the menu bar
Step 2: Choose an app
Choose Interactive Apps and the desktop app (AMA27 desktop app name), or click its icon on the dashboard. The other interactive apps, ParaView among them, ask for the same kind of form.
You should see: a form asking what the session needs.
Screenshot: the Interactive Apps menu
Screenshot to come: The Interactive Apps menu open, listing the apps, with the desktop app marked
Step 3: Fill in the form
Each field is part of an ordinary Slurm job request. These values suit the visualization lessons in this set:
| Field | What it means | Slurm equivalent | Use for these lessons |
|---|---|---|---|
| Queue | Which group of nodes to run on, and its time limit | --partition |
A queue meant for short work that shares nodes, so it usually starts quickly: AMA27 queue name |
| Number of hours (or Time Limit) | How long the session may run | --time |
1, the default. You can start another when it ends |
| Number of cores | How many cores the program gets | a core request | 4 |
| Memory | How much memory, written with a G |
--mem |
16G |
| Desktop Environment (Desktop only) | The look of the Linux desktop | none | AMA27 desktop environment |
| Nodes | How many nodes | --nodes |
1. Leave it |
| Account | Which Slurm account to charge | --account |
Leave blank unless your group told you otherwise |
| Email Notification | Email when the session starts | --mail-type |
Optional, and useful when the queue is busy |
Note
Apps differ: not every one shows every field, and some name them differently. AMA27's forms, to check.
You should see: your values in the form, and a Launch button at the bottom.
Screenshot: the filled-in form
Screenshot to come: The desktop app's form filled in with the values from the table, and the Launch button at the bottom
Step 4: Launch and wait
Click Launch. The session card on My Interactive Sessions moves through Queued, then Starting, then Running.
You should see: a green Running card, after anything from seconds to a few minutes. It has a button that opens the desktop, and sliders for image quality.
Screenshot: the Running session card
Screenshot to come: A green Running card on My Interactive Sessions, with its Launch button and image-quality sliders
Step 5: Connect
Click the card's launch button (AMA27 desktop app name). If it asks, sign in again.
You should see: a Linux desktop in a new browser tab. It's running on one of the cluster's compute nodes, not on your computer.
Screenshot: the desktop in the browser
Screenshot to come: A Linux desktop in a browser tab, running on one of the cluster's compute nodes
Step 6: Check where you are
Open a terminal on the desktop: Applications ▸ Terminal Emulator.
Run in: the OnDemand desktop terminal
echo "Job $SLURM_JOB_ID on $(hostname) with $(nproc) cores"
You should see: a job number, a compute node's name and the number of cores you asked for, in this shape:
Job 1234567 on <compute-node-name> with 4 cores
Step 7: End the session
Close the desktop's browser tab. Then, on My Interactive Sessions, click Delete on the card and confirm.
You should see: the card disappears. The node is free for the next person, and your time stops counting.
Screenshot: the card's Delete button
Screenshot to come: The session card on My Interactive Sessions, with its Delete button marked
Check your understanding
You close the browser tab of a running session and go to lunch. What happens, and what should you have done?
The session keeps running, and keeps using time, until its time limit ends. Closing the tab only closes your view of it. Delete the session on My Interactive Sessions when you're done.
Your session sits in "Queued" for twenty minutes. Name two changes to the form that would probably help.
Ask for less: fewer cores, less memory or fewer hours make it easier to find room (see backfill). Or choose a queue that shares nodes, or one meant for short tests: AMA27 queue names and their limits.
If something goes wrong
| What you see | Why | What to do |
|---|---|---|
| The card stays Queued for a long time | The queue is busy, or you asked for a lot | Delete it and relaunch with fewer cores, less memory or fewer hours |
| The card jumps straight to Completed | The session failed to start | Click the Session ID link on the card and open output.log; the last lines say why |
| The desktop tab is blank or gray | The desktop is still starting | Wait 30 seconds, then reload the tab |
| Sign-in fails with "Disk quota exceeded" | Your home directory is full | Open a shell from the Clusters menu, check your usage (AMA27 quota command), and free some space |
| You can't find your data in the app | Apps start in your home directory | See Get your data where the app can see it |
Next
Credits
The structure follows TACC's TAP guide: form fields explained alongside their Slurm equivalents, and troubleshooting by symptom. No text is copied. Written for SBSC; text CC-BY 4.0.