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A time based job is a job, which has a start date and start time as start condition.

On each batch server the so called time scheduler (which is just an ABAP program named SAPMSSY2) runs periodically with a fixed interval.
This interval is given by the server specific profile parameter rdisp/btctime.

The time scheduler selects all time based jobs which in status released, whose start time has come or already passed, and which don’t have a target server or which have as target the server where this time scheduler is running. The time scheduler starts as many time based jobs as it can start, However, the time scheduler cannot start more jobs than there are free batch work processes on the server, where it is running.

For example, if you launch many jobs simultaneously using a Logon Group which includes A, B, C and D server. When the time scheduler run on A, it will start jobs as many as the number of free BGD work processes at that moment, and when the time scheduler run on B, it will do the same, the same as C and D.  Actually, there is no special load distribution configuration/mechanism at SAP background processing system side at all. (unless you specify the jobs to one specific background server. )

Since the time scheduler was executed only every rdisp/btctime seconds, The consequence was, that the batch system often could not process the whole batch workload without delays – especially if there were many short running jobs in the system. Therefore, SAP introduced an additional mechanism(note 923228). With the introduction of the improvement described in note 923228, The event scheduler starts the time scheduler, too. The event scheduler runs after each job end, because after each job end the event SAP_END_OF_JOB is triggered. And a job end means also, that a batch work process is free again. So the time scheduler is triggered each time, when a batch work process is free again. With this mechanism we could basically maximize the capacity of the batch system.

923228 - Background job sched.: using processes that have become free