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Fine-tuning Job Specifications - Specifying Walltime

From ALICE Documentation

Specifying Walltime

One of the most important and also easiest parameters to measure is the duration of your program. This information is needed to specify the walltime. The time utility executes and times your application. You can just add the time command in front of your normal command line, including your command line options. After your executable has finished, time writes the total time elapsed, the time consumed by system overhead, and the time used to execute your executable to the standard error stream.

The calculated times are reported in seconds.

Test the time command:

  $ time sleep 75
  real 1m15.005s
  user 0m0.001s
  sys 0m0.002s

It is a good practice to correctly estimate and specify the run time (duration) of an application. Of course, a margin of 10% to 20% can be taken to be on the safe side. It is also wise to check the walltime on different compute nodes or to select the “slowest” compute node for your walltime tests. Your estimate should appropriate in case your application will run on the “slowest” (oldest) compute nodes. The walltime can be specified in a job scripts as:

  #sbatch --time=3:00:00:00>

r on the command line

  $ sbatch --time=3:00:00:00

It is recommended to always specify the walltime for a job.