Disabling resource limits for your job-scheduler

By default, an Alces Flight Compute cluster enables limits on the resources your jobs can use on the compute nodes of your cluster. These limits are designed to help protect the compute nodes from being overloaded, and help to promote efficient usage of your compute cluster. It is important to train HPC cluster users in requesting the right resources for their jobs, as this is a fundamental principle when working on shared HPC facilities, where users do not have the benefit of exclusive use.

Experienced HPC cluster users working on a personal Alces Flight Compute cluster may prefer to disable the job-scheduler resource limits and manage compute node resources themselves. These instructions provide a guide to disabling the default limits for your compute cluster job scheduler.


Once scheduler resource limits are disabled, users must manually ensure that the compute node instances selected have enough resources to run the jobs submitted to the job-scheduler queue.


Requesting more CPU cores for your job

There is a default CPU allocation of 1 core for a job. To override this, specify the number of cores to be used either;

  • On the command line when submitting the job script with the -n X flag (where X is the number of cores to be used). For example, sbatch -n 8 myjobscript.sh to submit myjobscript.sh requesting 8 cores.
  • In the job script by adding #SBATCH -n X underneath the shebang (#!) line, replacing X with the number of cores to be requested.

Disabling memory resource limits

By default, SLURM has no limit on the amount of memory a user can request per core (it is set to UNLIMITED as can be seen with scontrol show config | grep Mem).

Disabling the default runtime limit

The default configuration for a job’s runtime on any partition in SLURM is unlimited. This can be checked from the command-line with the command scontrol show part all |grep MaxTime.

For more information, see SLURM default resources.

Open-grid scheduler

By default, your Alces Flight Compute cluster sets limits for the following properties of your compute jobs:

  • Number of CPU cores
  • Amount of memory per requested slot (measured per CPU core, or complete node)
  • Run time (measured in seconds of node occupancy)

Requesting more CPU cores for your job

The default number of CPU cores for your job is one. This setting is used in the absence of a Parallel Environment (PE) request and cannot be sensibly over-ridden. Users can however the job-scheduler to request a default parallel-environment by adding a job-scheduler instruction into their local $HOME/.sge_request file. For example; if a user wanted to ensure that all jobs requested the SMP parallel environment with 2 CPU-cores, they could add the following into their $HOME/.sge_request file:

-pe smp 2


Job-scheduler instructions provided as parameters to the qsub command, or included in job-scripts are prioritised over the contents of your $HOME/.sge_request file.

Use the command man sge_request for more information on configuring default settings for job submitted on your compute cluster.

Disabling memory resource limits

Users can follow the process below to disable memory limits for jobs running via the cluster job-scheduler.

  1. Use the sudo -s command on the cluster login node to become the root user.
  2. Run the command EDITOR=nano qconf -mc
  3. Using the nano editor, change the settings for the h_vmem entry so that:
    • requestable is set to NO
    • consumable is set to NO
    • default is set to 0 (zero)
  4. Press CTRL+X to exit the nano editor

Once memory limits are disabled, cluster users can submit jobs without a -l h_vmem setting, allowing jobs to use as much memory as is available on their cluster compute nodes.


A job submitted via the job-scheduler that uses more memory than your compute node can provide may be automatically killed by Linux, or could in extreme cases cause your compute node to stop responding. Users must manually manage memory resources if memory resource limits are disabled in the cluster job scheduler.

Disabling the default runtime limit

Submitted jobs automatically have a default runtime applied, unless overridden with the -l h_rt= parameter to qrsh or qsub on the command-line, or included as a job-scheduler instruction in your job-script.

Users can optionally disable the default runtime limit for their cluster by following these steps:

  1. Edit the system-wide grid-scheduler defaults file; for example, to use the nano editor, use the command:

    nano $GRIDSCHEDULERDIR/etc/common/sge_request

  2. Remove the h_rt=24:00:00 section at the end of the file

  3. Save the file by pressing CTRL+X

Once the default runtime limit is disabled, cluster users can submit jobs without a -l h_rt= setting, allowing jobs to run forever until they complete or are terminated by the user.


A job submitted via the job-scheduler without a run-time limit will run forever until it naturally completes or is terminated by the user. Users should review their jobs to ensure that suitable infrastructure has been selected for long-running jobs, and may wish to setup billing alerts via their platform provider to ensure that run-away jobs are identified and stopped promptly.