Getting started on clusters

This document presumes zero prior knowledge of cluster computing. If instead you're an intermediate user (e.g. you have an account and have run a few jobs before but need a reminder) the table of contents is your friend.

This document gives you all the information you need to choose a system, log in to a cluster/phat node, write a script, submit it via qsub to the scheduler, and find the output. As such, these notes cover hardware and software. (If you're a sysadmin, you may be interested in this page instead.)

Prerequisites

1. Get a cluster account. You can email admin at cs dot earlham dot edu or a current CS faculty member to get started. Your user account will grant access to all the servers below, and you will have a home directory at ~username that you can access when you connect to any of them.
   1. Note: if you have a CS account, you will use the same username and password for your cluster account.
2. Connect through a terminal via ssh to username@hopper.cluster.earlham.edu. If you intend to work with these machines a lot, you should also configure your ssh keys.

Cluster systems to choose from

The cluster dot earlham dot edu domain consists of clusters (a collection of physical servers linked through a switch to perform high-performance computing tasks with distributed memory) and jumbo servers (nee "phat nodes"; a system comprising one physical server with a high ratio of disk+RAM to CPU, good for jobs demanding shared memory).

Our current machines are:

• whedon: newest cluster; 8 compute nodes; Torque-only pending an OS upgrade
• layout: cluster; 4 compute nodes, pre-whedon, features NVIDIA GPGPU's and multiple CUDA options
• lovelace: newest jumbo server
• pollock: jumbo server, older than lovelace but well-tested and featuring the most available disk space

To get to, e.g., whedon, from hopper, run ssh whedon.

If you're still not sure, click here for more detailed notes.

Cluster software bundle

The cluster dot earlham dot edu servers all run a supported CentOS version.

All these servers (unless otherwise noted) also feature the following software:

• Slurm (scheduler): submit a job with sbatch jobname.sbatch, delete it with scancel jobID. Running a job has its own doc section below.
• Environment modules: run module avail to see available software modules and module load modulename to load one; you may load modules in bash scripts and qsub jobs as well.

The default shell on all these servers is bash.

The default Python version on all these servers is Python 2.x, but all have at least one Python 3 module with a collection of widely-used scientific computing libraries.

Using Slurm

Slurm is our batch scheduler.

You can check that it's working by running: srun -l hostname

You can submit a job in a script with the following: sbatch my_good_script.sbatch

Here's an example of a batch file:

#!/bin/sh
#SBATCH --time=1
#SBATCH --job-name hello-world
#SBATCH --nodes=5
#SBATCH --mail-type=BEGIN,END,FAIL 
#SBATCH --mail-user=excellent_email_user@earlham.edu

echo "queue/partition is `echo $SLURM_JOB_PARTITION`"
echo "running on `echo $SLURM_JOB_NODELIST`"
echo "work directory is `echo $SLURM_SUBMIT_DIR`"

/bin/hostname
srun -l /bin/hostname
sleep 10
srun -l /bin/pwd

Interactive and command line interfaces also exist. After submitting a job slurm captures anything written to stdout and stderr by the programs and when the job completes puts it in a file called slurm-nnn.out (where nnn is the job number) in the directory where you ran sbatch. Use more to view it when you are looking for error messages, output file locations, etc.

Conversion from Torque to Slurm

To submit a job to PBS, you'll need to write a shell script wrapper around it and submit it through qsub on your system of choice. For example (change the specific options):

Commands

Torque	Slurm	Description
qsub	sbatch	run/submit a batch job
qstat	squeue	show jobs currently in the queue
qdel	scancel	cancel a job
pbsnodes -a	scontrol show nodes	show nodes in the cluster

Environment Variables

Torque	Slurm	Description
$PBS_QUEUE	$SLURM_JOB_PARTITION	the queue/partition you are in
cat $PBS_NODEFILE	$SLURM_JOB_NODELIST	there's no equivalent of the nodes file but there is an environment variable that stores that information
$PBS_O_WORKDIR	$SLURM_SUBMIT_DIR	working directory from which the command was run

Example script

#!/usr/bin/bash

#SBATCH --job-name hello-world
#SBATCH --nodes=5
#SBATCH --mail-type=BEGIN,END,FAIL 
#SBATCH --mail-user=excellent_email_user@earlham.edu

echo "queue is `echo $SLURM_JOB_PARTITION`"
echo "running on `echo $SLURM_JOB_NODELIST`"
echo "work directory is `echo $SLURM_SUBMIT_DIR`"

srun -l echo "hello world!"

About qsub

Before Slurm we used Torque and its associated software, including qsub. This is now deprecated and should not be used on the Earlham CS cluster systems.