The SLAC workflow engine has been extended to allow multiple workflow engine jobs to run inside a "pilot" job. The main intent is to allow multiple (possible single thread) jobs to run on hosts supporting many threads, for example on CORI at NERSC, although the implementation is not specific to any particular machine.
Implementation
The workflow engine works by having "JobDaemons" for each type of batch system (LSF, Condor, SLURM etc). In the case of pilot jobs we use a special daemon called the PilotJobDaemon, which does not submit jobs to a batch system, but instead keeps the jobs in an internal queue until a pilot job comes and pulls them out of the queue to execute them. For the most part, using a PilotJobDaemon is transparent to the workflow engine, and the developer of specific workflows.
For DESC we have set up two JobDaemons at NERSC, both running under the shared "desc" account:
- SlurmJobDaemon which submits jobs to SLURM. The "jobsite" which must be used to submit jobs to this daemon is "CORI".
- PilotJobDaemon which queues up jobs until a pilot job fetches them and runs them. The "jobsite" which must be used to submit jobs to this daemon is "CORIP".
To a first approximation, the only change to the workflow to switch from one to the other is to change the jobsite. Since the jobsite can be set dynamically it is possible to have one workflow which submits jobs to either site (or even both sites).
In the current implementation the pilot jobs are not submitted automatically, although this may change in future. Currently to submit the default pilot job simply login as user "desc" (separate instructions needed?) and run the following:
sbatch ~/jobcontrol/runPilot.sh
It is possible to customize the pilot for specific applications, for example you might want to setup some specific environment which will be inherited by all jobs running in the pilot, or you might want to customize some of the options supported by the job daemon. In either case you can create your own modified copy of the runPilot.sh script, and/or specify an option on the command line.
The options supported by the JobControlPilot are:
-c N : The total number of cores of share among all running jobs (default: 64)
-h VAL : The host from which this pilot will attempt to pull jobs (default: corigrid.nersc.gov)
-i N : The time after which the pilot will die if no work is provided (seconds) (default: 300)
-m N : The total memory (in kB) of this machine to share among all running jobs (default: 64000000)
-o : True if OK to overwrite existing files (default: false)
-p N : The port that the pilot will attempt to pull jobs from (default: 0)
-r N : The maximum runtime for the job (seconds) (default: 172800)
-s VAL : The service name of the pilot service (default: PilotJobProvider)
-u VAL : The user name under which the pilot service is running (default: desc)