Sometimes the release time of a job may be later than that of its successors, or its deadline may be earlier than that specified for its predecessors then concept of effective release time or effective deadline comes to play.
Effective release time
If a job has no predecessors then its effective release time is equals to its release time. When it has predecessors then its effective release time is the maximum of its release time and the effective release times of its predecessors
If a job has no successors then its effective deadline is equals to its deadline. When It if has successors then its effective deadline is the minimum of its deadline and the effective deadline of its successors.
To generate more accurate real time system on multiprocessor environment, effective release time and deadlines must be considered but there no unnecessary on single processor with preemptable jobs.
It is feasible to schedule any set of jobs according to their actual release times and deadline when feasible to schedule according to effective release times and deadlines. Schedule use effective release times and deadlines as if all jobs independent then ignore the all precedence constraints.
Consider the following example whose task graph is given in the following figure. Effective Release time
- The numbers in brackets next to each job are its given release time and
- Because J1 and J2 have no predecessors, their effective release times are their given release times, 2 and 0 respectively.
- The given release time of J3 is 1, but the latest effective release time of its predecessors is 2 (that of J1) so its effective release time is 2.
- The effective release times of J4, J5, J6, J7 are 4, 2, 4, 6 respectively.
- J6 and J7 have no successors so their effective deadlines are their given deadlines, 20 and 21
- Sincethe effective deadlines of the successors of J4 and J5 are later than the given deadlines of J4 and J5, the effective deadlines of J4 and J5 are equal to their given deadlines, 9 and 8
- However the given deadline of J3 (12) is larger than the minimum value (8) of its successors, so the effective deadline of J3 is 8.
- Similarly the effective deadlines of J1 and J2 are 8 and 7 respectively.