Hardware Reference
In-Depth Information
9.4.3
IMPLEMENTATION ISSUES
The elastic compression algorithm can be efficiently implemented on top of a real-
time kernel as a routine (elastic manager) that is activated every time a new task is
created, terminated, or there is a request for a period change. When activated, the
elastic manager computes the new periods according to the compression algorithm
and modifies them atomically.
To avoid any deadline miss during the transition phase, it is crucial to ensure that all
the periods are modified at opportune time instants, according to the following rule
[BLCA02]:
The period of a task
τ
i
can be increased at any time, but can only be reduced at
the next job activation.
Figure 9.30 shows an example in which
τ
1
misses its deadline when its period is
reduced at time
t
=15(i.e., before its next activation time (
t
=20). Note that
the task set utilization is
U
p
=29
/
30 before compression, and
U
p
=28
/
30 after
compression. This means that although the system is schedulable by EDF in both
steady state conditions, some deadline can be missed if a period is reduced too early.
τ
1
0
10
15
20
τ
2
0
3
6
9
12
15
18
t = 14
Figure 9.30
A task can miss its deadline if a period is reduced at an arbitrary time instant.
An earlier instant at which a period can be safely reduced without causing any deadline
miss in the transition phase has been computed by Buttazzo et al. [BLCA02] and later
improved by Guangming [Gua09].
Search WWH ::
Custom Search