Scheduling jobs

Overview

Teaching: 45 min
Exercises: 30 min

Questions

• What is a scheduler and why are they used?

• How do we submit a job?

Objectives

• Submit a job and have it complete successfully.

• Understand how to make resource requests.

• Submit an interactive job.

An HPC system might have thousands of nodes and thousands of users. How do we decide who gets what and when? How do we ensure that a task is run with the resources it needs? This job is handled by a special piece of software called the scheduler. On an HPC system, the scheduler manages which jobs run where and when.

Job scheduling roleplay (optional)

Your instructor will divide you into groups taking on different roles in the cluster (users, compute nodes and the scheduler). Follow their instructions as they lead you through this exercise. You will be emulating how a job scheduling system works on the cluster.

notes for the instructor here

The scheduler used in this lesson is PBS Pro. Although PBS Pro is not used everywhere, running jobs is quite similar regardless of what software is being used. The exact syntax might change, but the concepts remain the same.

Running a batch job

The most basic use of the scheduler is to run a command non-interactively. Any command (or series of commands) that you want to run on the cluster is called a job, and the process of using a scheduler to run the job is called batch job submission.

In this case, the job we want to run is just a shell script. Let’s create a demo shell script to run as a test.

Creating our test job

Using your favourite text editor, create the following script and run it. Does it run on the cluster or just our login node?

#!/bin/bash

echo 'This script is running on:'
hostname
sleep 120

If you completed the previous challenge successfully, you probably realise that there is a distinction between running the job through the scheduler and just “running it”. To submit this job to the scheduler, we use the qsub command.

[yourUsername@cirrus-login0 ~]$ qsub -A tc001 -q R387726 example-job.sh

387775.indy2-login0

And that’s all we need to do to submit a job. Our work is done – now the scheduler takes over and tries to run the job for us. While the job is waiting to run, it goes into a list of jobs called the queue. To check on our job’s status, we check the queue using the command qstat -u yourUsername.

[yourUsername@cirrus-login0 ~]$ qstat -u yourUsername

indy2-login0: 
                                                            Req'd  Req'd   Elap
Job ID          Username Queue    Jobname    SessID NDS TSK Memory Time  S Time
--------------- -------- -------- ---------- ------ --- --- ------ ----- - -----
387775.indy2-lo yourUser workq    example-jo  50804   1   1    --  96:00 R 00:00

We can see all the details of our job, most importantly that it is in the “R” or “RUNNING” state. Sometimes our jobs might need to wait in a queue or have an error. The best way to check our job’s status is with qstat. Of course, running qstat repeatedly to check on things can be a little tiresome. To see a real-time view of our jobs, we can use the watch command. watch reruns a given command at 2-second intervals. Let’s try using it to monitor another job.

[yourUsername@cirrus-login0 ~]$ qsub -A tc001 -q R387726 example-job.sh
[yourUsername@cirrus-login0 ~]$ watch qstat -u yourUsername

You should see an auto-updating display of your job’s status. When it finishes, it will disappear from the queue. Press Ctrl-C when you want to stop the watch command.

Customising a job

The job we just ran used all of the scheduler’s default options. In a real-world scenario, that’s probably not what we want. The default options represent a reasonable minimum. Chances are, we will need more cores, more memory, more time, among other special considerations. To get access to these resources we must customise our job script.

Comments in UNIX (denoted by #) are typically ignored. But there are exceptions. For instance the special #! comment at the beginning of scripts specifies what program should be used to run it (typically /bin/bash). Schedulers like PBS Pro also have a special comment used to denote special scheduler-specific options. Though these comments differ from scheduler to scheduler, PBS Pro’s special comment is #PBS. Anything following the #PBS comment is interpreted as an instruction to the scheduler.

Let’s illustrate this by example. By default, a job’s name is the name of the script, but the -N option can be used to change the name of a job.

Submit the following job (qsub -A tc001 -q R387726 example-job.sh):

#!/bin/bash
#PBS -N new_name

echo 'This script is running on:'
hostname
sleep 120

[yourUsername@cirrus-login0 ~]$ qstat -u yourUsername

indy2-login0: 
                                                            Req'd  Req'd   Elap
Job ID          Username Queue    Jobname    SessID NDS TSK Memory Time  S Time
--------------- -------- -------- ---------- ------ --- --- ------ ----- - -----
387778.indy2-lo yourUser workq    new_name    51536   1   1    --  96:00 R 00:00

Fantastic, we’ve successfully changed the name of our job!

Setting up email notifications

Jobs on an HPC system might run for days or even weeks. We probably have better things to do than constantly check on the status of our job with qstat. Looking at the man page for qstat, can you set up our test job to send you an email when it finishes?

Resource requests

But what about more important changes, such as the number of nodes, cores and memory for our jobs? One thing that is absolutely critical when working on an HPC system is specifying the resources required to run a job. This allows the scheduler to find the right time and place to schedule our job. If you do not specify requirements (such as the amount of time you need), you will likely be stuck with your site’s default resources, which is probably not what we want.

The following are several key resource requests:

• -l select=<nnodes>:ncpus=<ncores per node> - how many nodes does your job need and how many cores per node? Note that there are 36 cores per node on Cirrus.

• -l walltime=<hours:minutes:seconds> - How much real-world time (walltime) will your job take to run?

• -l place=scatter:excl - Reserve your nodes just for yourself. (If you are using full nodes, you should include this as it stops other users from interferring with the performance of your job.)

Note that just requesting these resources does not make your job run faster! We’ll talk more about how to make sure that you’re using resources effectively in a later episode of this lesson.

Submitting resource requests

Submit a job that will use 1 full node and 5 minutes of walltime.

Job environment variables

When PBS Pro runs a job, it sets a number of environment variables for the job. One of these will let us check what directory our job script was submitted from. The PBS_O_WORKDIR variable is set to the directory from which our job was submitted. Using the PBS_O_WORKDIR variable, modify your job so that it prints (to stdout) the location from which the job was submitted.

Resource requests are typically binding. If you exceed them, your job will be killed. Let’s use walltime as an example. We will request 30 seconds of walltime, and attempt to run a job for two minutes.

#!/bin/bash
#PBS -l 0:0:30

echo 'This script is running on:'
hostname
sleep 120

Submit the job and wait for it to finish. Once it is has finished, check the log file.

[yourUsername@cirrus-login0 ~]$ qsub -A tc001 -q R387726 example-job.sh
[yourUsername@cirrus-login0 ~]$ watch qstat -u yourUsername
[yourUsername@cirrus-login0 ~]$ cat example-job.sh.e387798 

=>> PBS: job killed: walltime 33 exceeded limit 30

Our job was killed for exceeding the amount of resources it requested. Although this appears harsh, this is actually a feature. Strict adherence to resource requests allows the scheduler to find the best possible place for your jobs. Even more importantly, it ensures that another user cannot use more resources than they’ve been given. If another user messes up and accidentally attempts to use all of the CPUs or memory on a node, PBS Pro will either restrain their job to the requested resources or kill the job outright. Other jobs on the node will be unaffected. This means that one user cannot mess up the experience of others, the only jobs affected by a mistake in scheduling will be their own.

Cancelling a job

Sometimes we’ll make a mistake and need to cancel a job. This can be done with the qdel command. Let’s submit a job and then cancel it using its job number (remember to change the walltime so that it runs long enough for you to cancel it before it is killed!).

[yourUsername@cirrus-login0 ~]$ qsub -A tc001 -q R387726 example-job.sh
[yourUsername@cirrus-login0 ~]$ qstat -u yourUsername

38759.indy2-login0

indy2-login0: 
                                                            Req'd  Req'd   Elap
Job ID          Username Queue    Jobname    SessID NDS TSK Memory Time  S Time
--------------- -------- -------- ---------- ------ --- --- ------ ----- - -----
38759.indy2-log yourUser workq    example-jo  32085   1   1    --  00:10 R 00:00

Now cancel the job with it’s job number. Absence of any job info indicates that the job has been successfully cancelled.

[yourUsername@cirrus-login0 ~]$ qdel 38759
... Note that it might take a minute for the job to disappear from the queue ...
[yourUsername@cirrus-login0 ~]$ qstat -u yourUsername

...(no output from qstat when there are no jobs to display)...

Other types of jobs

Up to this point, we’ve focused on running jobs in batch mode. PBS Pro also provides the ability to start an interactive session.

There are very frequently tasks that need to be done interactively. Creating an entire job script might be overkill, but the amount of resources required is too much for a login node to handle. A good example of this might be building a genome index for alignment with a tool like HISAT2. Fortunately, we can run these types of tasks as a one-off with qsub.

qsub (with the right options) can submit a job and then wait for it to start so we can use the compute node resources interactively. Let’s demonstrate this by submitting an interactive job that uses a single core:

[yourUsername@cirrus-login0 ~]$ qsub -IVl select=1:ncpus=1 -A tc001 -q R387726

You should be presented with a bash prompt. Note that the prompt will likely change to reflect your new location, in this case the worker node we are logged on. You can also verify this with hostname.

When you are done with the interactive job, type exit to quit your session.

Key Points

• The scheduler handles how compute resources are shared between users.

• Everything you do should be run through the scheduler.

• A job is just a shell script.

• If in doubt, request more resources than you will need.