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
,τ
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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