Information Technology Reference
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Deferrable Server a Deferrable Server (DS) is a periodic task with period T DS
and capacity C DS . The DS is used to provide high priority service to aperiodic
tasks. It is ready at the start of its period and services aperiodic task arrivals,
subject to preemption by higher priority tasks, until it exhausts its execution
time C DS or the end of its period is reached Strosnider et al. ( 1995 ). Unlike the
polling server which loses any unused execution time when there is no aperiodic
work remaining, the DS execution time C DS , is available for servicing aperiodic
arrivals throughout its entire period. It loses any unused execution time at the
end of its period when its full capacity C DS is restored. The DS task is scheduled
as if it was a periodic task with period if DS . Aperiodic tasks that arrive when the
DS execution time, C DS , has been exhausted can be serviced at background
priority. In general, the DS task is assigned a priority according to the rate
monotonic algorithm based on its period, T DS , relative to the other periodic
tasks. While the DS task can execute at any priority level, assigning the DS task
the highest priority allows one to guarantee that the deadlines of aperiodic alerts
are met as well as enhancing the responsiveness of the soft deadline aperiodic
tasks. At intermediate priority levels, the DS is less capable of providing
responsive aperiodic service. Moreover, DS capacity can be lost because of
higher priority preemptions even when aperiodic tasks are ready for processing
Strosnider et al. ( 1995 ).
￿
Background Server a Background Server (BS) schedules aperiodic tasks in
background (when no periodic task is running) and schedule of periodic tasks is
not changed. BS Treats aperiodic tasks as lowest-priority tasks and have the
following advantages:
￿
Simple,
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Aperiodic tasks has no impact on the schedulability of periodic tasks
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In contrast, the disadvantages of the background scheduling are the following:
Aperiodic tasks have very long response times when the utilization of
periodic tasks is high,
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Acceptable only if the system is not busy or the aperiodic tasks can tolerate
long delays
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Sporadic Server Sporadic Server (SS) is a real-time scheduling algorithm used
to control the execution of processes/threads on a system. This scheduler allows
one to set the maximum amount of time a process may receive in a speci
￿
ed
time window. The basic idea is that the server is given a time budget at the
server
is priority that it consumes when executing on the processor, and which is
replenished according to some rules:
'
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bounds the CPU time consumed by a process
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guarantees CPU time given to a process
'
-
conforms process
s execution to simpler model
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