CSE 520: Real-Time Systems:Real-Time Scheduling (Single Core)
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Benefit of Scheduling Analysis
reduces development time by 50%
Consequences of a a deadline miss for a hard deadline
System fails if missed.
Consequences of a a deadline miss for a soft deadline
User may notice but system does not fail.
Goal of a hard deadline
guarantee no deadline miss.
Goal of a soft deadline
meet most deadlines, most of the time
General purpose system scheduling
- Fairness to all tasks; ergo no starvation
- Optimize throughput
- Optimize average performance
Embedded systems scheduling
- Meet ALL deadlines.
- Fairness or throughput in NOT important
- Hard Real-Time: worry about worst case performance.
- Map to a process or thread
- May be released several times
An instance of a task.
- Ideal: inter-arrival time = period
- General: inter-arrival time >= period.
Inter-arrival time does not have a lower bound.
period OF a task Ti.
Worst-case execution time OF a task Ti.
Job k OF task Ti.
Release time (math)
Time when a job is ready.
Response time Ri (math)
is equal to the finish time - release time
Absolute deadline (math).
Is equal to release time + Di (relative deadline)
A job Jik misses its deadline if
- Response time Ri > Di (relative deadline)
- Finish time > absolute deadline.
A task set is schedulable
if ALL jobs meet their deadlines.
Optimal scheduling algorithm
A task set is unschedulable under the optimal algorithm, and therefore unschedulable under any other algorithms.
Time required for scheduling.
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