BASIC CONCEPTS - Hard Real-Time Computing Systems: Predictable Scheduling Algorithms and Applications - page 27

C

i

τ

i

t

a s f d i

i i i

Figure 2.4 Typical parameters of a real-time task.

Arrival time a i is the time at which a task becomes ready for execution; it is also
referred as request time or release time and indicated by r i ;

Computation time C i is the time necessary to the processor for executing the
task without interruption;

Absolute Deadline d i is the time before which a task should be completed to
avoid damage to the system;

Relative Deadline D i is the difference between the absolute deadline and the
request time: D i = d i − r i ;

Start time s i is the time at which a task starts its execution;

Finishing time f i is the time at which a task finishes its execution;

Response time R i is the difference between the finishing time and the request

time: R i = f i − r i ;

Criticality is a parameter related to the consequences of missing the deadline
(typically, it can be hard, firm, or soft);

Value v i represents the relative importance of the task with respect to the other
tasks in the system;

Lateness L i : L i = f i − d i represents the delay of a task completion with respect

to its deadline; note that if a task completes before the deadline, its lateness is
negative;

Tardiness or Exceeding time E i : E i = max (0 ,L i ) is the time a task stays active
after its deadline;

Laxity or Slack time X i : X i = d i − C i is the maximum time a task can be

delayed on its activation to complete within its deadline.

a i −

Some of the parameters defined above are illustrated in Figure 2.4.