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Approaches to Real-Time Scheduling
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 Clock-Driven Scheduling approach also known as Time-Driven
 Scheduling is none Priority Preemptive
 Typically implemented using preemptive priority-based scheduling
 Decisions on what processes runs when is made a priori and not
during system execution
 Scheduler schedules jobs based on time
 Scheduling overhead during run-time is very low
 Each node stores a schedule table locally and executes according
to the schedule table
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 Weighted Round-Robin scheduling
 Used for scheduling time-share applications
 Used for scheduling real-time traffic in high-speed switched
networks
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 Priority-Driven Scheduling
 Never leaves resource idle if there are any ready processes in
the system
 Most scheduling algorithms used in non real-time systems are
priority-driven
 Operating systems use a priority driven scheduler to
schedule tasks
 FIFO, LIFO, SETF (Shortest Execution Time First),
LETF (Longest Execution Time First)
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 Priority-Driven scheduling
 Preemptive
 Non-preemptive
 Dynamic priorities
 Static priorities
 Dynamic Systems
 Static Systems
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 Dynamic job dispatching to processors
 Once a ready job is at the head of the priority queue, it
can be dispatched to any processor
 Jobs can migrate from processor to processor when
preempted
 Clustering
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 Statically jobs are dispatched to pre-assigned processors or
subsystem
 Processes are pre-assigned to servers or processors a
priori
 In event of failure, processing is failed-over to hot
spare (idle) processors/servers
 Tasks are only moved among processors when system
is reconfigured
 Upon failure (fail-over)
 Operator initiated reconfiguration
 Processor affinity in multi-Core x86 processors, where
jobs are attached to processors in the system
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 There are no reliable techniques to validate timing constraints
of dynamic systems
 Static systems are far easier to test and validate and reliable
validation technique for these systems exist
 Most hard real-time systems are statically dispatched systems
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 Event-Driven Scheduling Algorithms
 Earliest Deadline First (EDF) Algorithm
 The earlier the deadline, the higher the priority
 Assumptions: Jobs do not contend for resources,
Jobs are scheduled on one processor, preemption of
lower priority jobs by higher priority jobs are allowed
 With these assumptions, the EDF algorithm is an
optimal priority preemptive scheduling algorithm
 An optimum scheduling algorithm is one that given
valid assumptions, can produce a feasible schedule if
a feasible schedule exists for the set of jobs
 In a preemptive system in which jobs do not contend
for resources, the EDF algorithm can produce a
feasible schedule of a set J of jobs on a processor if
and only if J has at least one feasible schedule
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 Latest Release Time (LRT) Algorithm
 Job with the latest release time has a higher priority to
be scheduled
 Schedules jobs backwards, starting from the latest
release time backwards to current time
 Priorities are based on release time of jobs, the later
the release time, the higher the priority
 LRT algorithm may leave the processor idle where
there are jobs ready for execution
 Priority-driven algorithms, can not leave the processor
idle when there are ready jobs
 LRT is not categorized as a priority-driven algorithm
 Like EDF algorithm, LRT algorithm is optimal when
preemption is allowed, jobs do not contend for
resources, on a processor
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 EDF algorithm does not require knowledge of the execution time for
jobs
 LRT algorithm requires knowledge of execution time of jobs
 This is a major disadvantage for the LRT algorithm
 When preemption is not allowed, EDF and LRT algorithms will no
longer be optimal
 No non-preemptive scheduling algorithm is optimal when jobs have
arbitrary release times, execution time, and deadlines
 When we introduce more than one processor, EDF algorithm is no
longer an optimal scheduling algorithm
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 Validating real-time performance of Priority-Driven systems
 Implementing Priority-Driven systems are easier than clockdriven
 Systems are easier to upgrade and modify
 Time-Driven schedulers, require knowledge of the release time
and execution times of tasks a priori
 Priority-Driven schedulers do not need this information and are
robust to tasks with varying release times
 Validating Priority-Driven systems is much harder than TimeDriven systems
 Timing characterization of Priority-Driven systems are
difficult since these systems are not as deterministic
as the Time-Driven systems
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 Validation of a system requires determination that all
jobs meet their deadlines as specified by the system
requirements
 Predictability of tasks
 Execution of jobs in a set of independent preempt-able
jobs is predictable if priority-driven scheduling is used
on a processor
 Validation of a set of independent, preemptive, static,
single processor priority-driven systems are relatively
easy since the execution of the jobs are predictable
 Maximum execution time is used to validate these
systems
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 On line and off line scheduling
 Pre-computed schedules, decisions on what processes runs
when is made before system execution
 On line scheduling, decisions on what processes runs when is
made during system execution
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