Running a Job: the Steps To Take¶
Here are the basic steps to run a job with HTCondor.
- Work Decomposition
Typically, users want High Throughput computing systems when they have more work than can reasonably run on a single machine. Therefore, the computation must run concurrently on multiple machines. HTCondor itself does not help with breaking up a large amount of work to run independently on many machines. In many cases, such as Monte Carlo simulations, this may be trivial to do. In other situations, the code must be refactored or code loops may need to be broken into separate work steps in order to be suitable for High Throughput computing. Work must be broken down into a set of jobs whose runtime is neither too short nor too long. HTCondor is most efficient when running jobs whose runtime is measured in minutes or hours. There is overhead in scheduling each job, which is why very short jobs (measured in seconds) do not work well. On the other hand, if a job takes many days to run, there is the threat of losing work in progress should the job or the server it runs on crashes.
- Prepare the job for batch execution.
To run under HTCondor a job must be able to run as a background batch job. HTCondor runs the program unattended and in the background. A program that runs in the background will not be able to do interactive input and output. Create any needed input files for the program. Make certain the program will run correctly with these files.
- Create a description file.
A submit description file controls the all details of a job submission. This text file tells HTCondor everything it needs to know to run the job on a remote machine, e.g. how much memory and how many cpu cores are needed, what input files the job needs, and other aspects of machine the job might need.
Write a submit description file to go with the job, using the examples provided in the Submitting a Job section for guidance. There are many possible options that can be set in a submit file, but most submit files only use a few. The complete list of submit file options is in condor_submit.
- Submit the Job.
Submit the program to HTCondor with the condor_submit command. HTCondor will assign the job a unique Cluster and Proc identifier as integers separated by a dot. You use this Cluster and Proc id to manage the job later.
- Manage the Job.
After submission, HTCondor manages the job during its lifetime. You can monitor the job’s progress with the condor_q. On some platforms, you can ssh to a running job with the condor_ssh_to_job command, and inspect the job as it runs.
HTCondor can write into a log file describing changes to the state of your job – when it starts executing, when it uses more resources, when it completes, or when it is preempted from a machine. You can remove a running or idle job from the queue with condor_rm.
- Examine the results of a finished job.
When your program completes, HTCondor will tell you (by e-mail, if preferred) the exit status of your program and various statistics about its performances, including time used and I/O performed. If you are using a log file for the job, the exit status will be recorded in there. Output files will be transfered back to the submitting machine, if a shared filesystem is not used. After the job completes, it will not be visible to the condor_q command , but is queryable with the condor_history command.