Hardware Reference
In-Depth Information
Note that the simple guarantee scheme is able to avoid domino effects by sacrificing
the execution of the newly arrived task. Basically, the acceptance test acts as a filter
that controls the load on the system and always keeps it less than one. Once a task is
accepted, the algorithm guarantees that it will complete by its deadline (assuming that
no task will exceed its estimated worst-case computation time). The acceptance test,
however, does not take task importance into account and, during transient overloads,
always rejects the newly arrived task, regardless of its value. In certain conditions
(such as when tasks have very different importance levels), this scheduling strategy
may exhibit poor performance in terms of cumulative value, whereas a robust algo-
rithm can be much more effective.
always accepted
task
Ready queue
RUN
(a)
accepted
Guarantee
task
Ready queue
RUN
Routine
rejected
(b)
scheduling
policy
task
Planning
Ready queue
RUN
reclaiming
policy
rejection
policy
reject queue
(c)
Figure 9.11
Scheduling schemes for handling overload situations: best effort (a), with
acceptance test (b), and robust (c).
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