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  • Where in the queue is my job, and why?
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The simplest queuing algorithm is "first come, first served". The queuing of jobs on the HPC cluster is a little more sophisticated as we pursue several goals:

  • Minimal queuing times, especially for short jobs. Nobody wants to spend 4 hours in the queue for a 1-hour job. 
  • Efficient use of the available resources. If there is a job ready which can use hardware that would otherwise be idle, run it, even if it's not next in the queue.
  • Fair use of resources. If you've made heavy use of the cluster recently, jobs belonging to a user who has had less CPU time will get higher priority. 
    At NYU "recently" means "the last 24 hours", so users with large workloads are not excessively penalized.

    Should you need more resources than the fair share allocations because of critical deadlines such as a grant application, a publication deadline, or class use, please email to make special arrangements.

  • Special consideration for HPC Stakeholders. NYU HPC uses a "condo" model in which we manage HPC resources owned by specific schools and departments, in exchange for allowing the rest of the NYU HPC community to use those resources when the owners are not. 

Moab supports these goals by calculating a priority for each submitted job and placing the job in the queue according to its priority. The schedule of which job will run where and when is built from the job queues. When a job finishes earlier than scheduled (due to an overestimated walltime request), Moab attempts to fill the newly-available space by scanning the queue for the first job which will fit without delaying an already-scheduled, higher priority job. In this way low-priority jobs with smaller resource requirements can jump ahead and be run early.

You can take advantage of this by requesting CPU, walltime and memory resources as accurately as possible. Be careful not to request too little though, or your job may exceed the request and be killed.

Monitoring jobs with qstat

To see the status of a single job - or a list of specific jobs - pass the Job IDs to qstat, as in the following example: 

$ qstat 3593014 3593016
Job id Name User Time Use S Queue
------------- ---------------- --------------- -------- - -----
3593014 model_scen_1 ab123 7:23:47 R s48
3593016 model_scen_1 ab123 7:23:26 R s48

Most of the fields in the output are self-explanatory. The second-last column "S" is the job status, which can be :

  • Q meaning "Queued"
  • H meaning "Held" - this may be the result of a manual hold or of a job dependency
  • R meaning "Running"
  • C meaning "Completed". After the job finishes, it will remain with "completed" status for a short time before being removed from the batch system.

Other, less common job status flags are described in the manual (man qsub).

The qstat command is described in more detail here.

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