HPC/Submitting and Managing Jobs

Processing flow of a typical job.

Introduction

To accept and run jobs, Carbon uses the Torque Resource Manager, which is a variant of PBS. A separate software component called a scheduler chooses the nodes and the start time for jobs according to the job parameters, resource availability, and configuration. Carbon uses the Moab Workload Manager as scheduler. Further, the resources used by each job (primarily the CPU time) are managed by the Gold Allocation Manager.

The adjacent figure shows schematically how a typical job is processed. Unless otherwise told, Torque will execute the job on your behalf on the first node allocated by Moab, in your $HOME directory, and typically using your login shell. It is the job's responsibility to:

• Change to a suitable working directory,
• Recognize and use the other nodes of the job.
  • This is usually done by directing an MPI program to start on the nodes listed in the job's $PBS_NODEFILE.
  • A job is perfectly free to run more than one program, such as several MPI programs in sequence.

You must configure modules and $PATH in your $HOME/.bashrc file to help mpirun and other mechanisms locate your programs on each compute node. Otherwise, mpirun will fail, typically as soon as more than one node is involved.

See also: directory layout.

Submitting and managing jobs

Job Script

A job is described to Torque by a job script, which in most cases is a shell script beginning with PBS directives which are lines ignored by the shell but interpreted by Torque.

• Example Job Script.
• Interpretation of PBS directives ends at the first executable statement in the script.

Advanced usage

• The job script may be written in any scripting language, such as Perl or Python. The interpreter is specified in the first script line in Unix hash-bang syntax #!/usr/bin/perl, or using the qsub -S path_list option.
• The default directive token #PBS can be changed or unset entirely with the qsub -C option; see qsub, sec. Extended Description.

Submission

To submit a job, use Torque's qsub command on a login node.

 qsub [-A accountname] [options] jobfile

To refresh your memory, consult the following:

 man qsub
 qsub --help

The main queue on Carbon is batch and need not be specified.

All job routing decisions are handled by the scheduler. In particular, short jobs are accommodated by a daily reserved node and by backfill scheduling, i.e. "waving forward" while a big job waits for full resources to become available.

Advanced usage

• You can submit follow-up jobs from compute nodes within a job script, but it may be better to leverage job dependencies.
• Requesting Resources
• Moab's msub command does not work in Carbon's current server constellation.

Using the debug queue

For testing job processing and your job environment, use qsub -q debug on the command line, or the follwing in a job script:

#PBS -q debug

The debug queue accepts jobs under the following conditions

nodes ≤ 2
ppn ≤ 4
walltime ≤ 1:00:00

Checking job status

Query your jobs using commands similar to those on other HPC clusters. You can use either Torque commands or Moab commands. Both access the same information, but through different routes, and offer different display styles.

To get the regular output format:

qstat [-u $USER]
showq [-u $USER]

To get an alternate format (e.g. showing job names):

qstat -a [-u $USER]
showq -n [-u $USER]

You can always use -u $USER to show only your jobs.

Getting extra information

Show the nodes where a job runs:

qstat -n [-1] jobnum

Give full information such as submit arguments and run directories, and do not wrap long lines ("-1" is the digit "one"):

qstat -f [jobnum] [-1]

To diagnose problems with "stuck" jobs, use Moab's checkjob command. The last few lines of output give past events of the job and its current status.

checkjob -v jobnum

Peeking

To inspect the output of a running job, or to review its script file, use qpeek:

qpeek jobnum
qpeek -eo jobnum
qpeek -s jobnum

Read its manual page for full documentation:

qpeek --man

This command is a Carbon re-implementation of a contributed script to Torque.

Changing jobs after submission

Use qalter to modify attributes of a job. Specify the attributes to be changed on the command line. Other attributes will not be affected. If any of the specified attributes cannot be modified for any reason, none of that job's attributes will be modified.

qalter [-l resource] […] jobnumber …
qalter -A accountspec jobnumber …

• Most attributes can be changed only before the job starts. Others, notably mail options and job dependencies can be meaningfully adjusted after start.

Advanced usage

• Extending the walltime resource of an already running job requires Torque administrator permission. You yourself can extend prior to running, and shorten always.
• You cannot modify the job script itself after submission. You will have to remove and resubmit the job.

Contact me (stern) to arrange for these interventions.

Removing jobs

To retract a queued job or terminate an already running job:

 qdel jobnumber

Interactive node access

There are two ways to interactively access nodes:

• Requesting an interactive job

qsub -I [-l nodes=…]

This is a special Torque job submission that will wait for the requested nodes to become available, and then you will be dropped into a login shell on the first core of the first node. $PBS_NODEFILE is set as in regular (non-interactive) jobs.

• Side-access a node that already runs a job, such as to debug or inspect running processes or node-local files.

You can use ssh to interactively access any compute node on which a job of yours is running (use qsub -n jobnum to find out the node names). As soon as a node no longer runs at least one of your jobs, your ssh session to that node will be terminated. You may find the following commands helpful:

/usr/sbin/lsof -u $USER
/usr/sbin/lsof -c command_name
/usr/sbin/lsof -p pid

- list open files; man page.

ps -fu $USER

- list your own processes.

psuser

- a Carbon-specific wrapper for ps to list user-only processes (yours and other regular users', excluding system processes).