Hardware Reference
In-Depth Information
Similarly, the methods developed under EDF consist in modifying a parameter based
on the tasks' relative deadlines.
The rest of this chapter presents the following resource access protocols:
1. Non-Preemptive Protocol (NPP);
2. Highest Locker Priority (HLP), also called Immediate Priority Ceiling (IPC);
3. Priority Inheritance Protocol (PIP);
4. Priority Ceiling Protocol (PCP);
5. Stack Resource Policy (SRP);
Although the first four protocols have been developed under fixed priority assign-
ments, some of them have been extended under EDF. 2 The Stack Resource Policy,
instead, was natively designed to be applicable under both fixed and dynamic priority
assignments.
7.3
TERMINOLOGY AND ASSUMPTIONS
Throughout this chapter, we consider a set of n periodic tasks, τ 1 2 ,...,τ n , which
cooperate through m shared resources, R 1 ,R 2 ,...,R m . Each task is characterized
by a period T i and a worst-case computation time C i . Each resource R k is guarded
by a distinct semaphore S k . Hence, all critical sections on resource R k begin with a
wait ( S k ) operation and end with a signal ( S k ) operation. Since a protocol modifies
the task priority, each task is characterized by a fixed nominal priority P i
(assigned,
for example, by the Rate Monotonic algorithm) and an active priority p i
P i ),
which is dynamic and initially set to P i . The following notation is adopted throughout
the discussion:
( p i
B i
denotes the maximum blocking time task τ i
can experience.
z i,k
denotes a generic critical section of task τ i
guarded by semaphore S k .
Z i,k
denotes the longest critical section of task τ i guarded by semaphore S k .
2 The Priority Inheritance Protocol has been extended for EDF by Spuri [Spu95], and the Priority Ceiling
Protocol has been extended for EDF by Chen and Lin [CL90].
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