1. Real-Time Process Scheduling Concepts, Design and Implementations
CS 314 Operating Systems
Real-Time Process Scheduling
Concepts, Design and Implementations
Department of Computer Science
Southern Illinois University Edwardsville
Spring, 2019
Dr. Hiroshi Fujinoki
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2. CS 314 Operating Systems Real-Time Systems
= (Computer) systems that are designed to execute “real-time tasks”
“Real-time tasks” are activities (or sets of activities) that should (or must)
be executed within some time limits.
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3. CS 314 Operating Systems Real-Time Systems - as embedded-systems
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4. CS 314 Operating Systems Real-Time Systems
Example: NASA Mars Rover Landing Mission
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5. CS 314 Operating Systems Real-Time Systems
Real-time process schedulers schedule processes based on process’
“execution deadlines”
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6. CS 314 Operating Systems Real-Time Systems
= (Computer) systems that are designed to execute “real-time tasks”
“Real-time tasks” are activities (or sets of activities) that should (or must)
be executed within some time limits.
The real-time systems that should execute tasks in time limits
The real-time systems that must execute tasks in time limits
“soft real-time systems”
“hard real-time systems”
 Determinism
“Time limits”
 Responsiveness
 Response Time
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7. CS 314 Operating Systems     Determinism & Responsiveness
Response Time
Determinism
Responsiveness    
What are going here? t0 t1 t2
Time
A processor is assigned
by the short-term scheduler
A task arrives
(for execution)
This task is
completed
Context Switching
 Another task is being executed
 Save the process state (and the processor
registers) of this process to memory
 Process scheduler overhead (the scheduler needs to select who is next)
 Load the process state (and its registers) of the next process
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8. CS 314 Operating Systems Determinism & Responsiveness Response Time
What makes meeting this deadline difficult?
 Accurately predicting how long a task takes is sometime difficult.
 A higher-priority task (than this) can preempt this task
 Unpredictable interrupts can (will) happen
- Hardware interrupt
(at clock-ticks from real-time hardware clock)
- Software interrupt
(“signal” message to a process on “wait”)
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9. CS 314 Operating Systems Determinism & Responsiveness Determinism
Time needed before the OS initiates a required task (since a task
is requested).
“Initiates” here means the task
is assigned a processor by the
short-term scheduler (not by the
long-term scheduler).
Important metric for “start deadline”
Responsiveness
Time the OS completes an initiated task
Response-Time
= Determinism + Responsiveness
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10. CS 314 Operating Systems
Four Different Classes of Real-Time Scheduling
Table-Driven
Priority-Driven
 Static Table-Driven
Scheduling
 Static Priority-Driven
Static
Scheduling
Create a schedule
first
Real-Time
Scheduling
Follow the schedule
without a change
Dynamic
Scheduling
Planning-Based
Best-Effort
 Dynamic Planning-Based
Scheduling
 Dynamic Best-Effort
Can create a schedule
first
Dynamically update
the schedule
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11. CS 314 Operating Systems
Four Different Classes of Real-Time Scheduling
= To be discussed
 Static Table-Driven Scheduling
(a) Completion-Time Earliest-Deadline
(b) Start-Time Earliest-Deadline
 Static Priority-Driven Scheduling
Rate Monotonic Scheduling (RMS)
 Dynamic Planning-Based Scheduling
 Dynamic Best-Effort Scheduling
(a) Priority-Driven Non-Preemptive
(b) Priority-Driven Immediate Preemption
(c) Priority-Driven Immediate Preemption on (periodic)
preemption-points
(d) Round-Robin
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12. CS 314 Operating Systems  Static Table-Driven Scheduling
- For periodic real-time tasks
Event interval should be known
for each real-time process
Task-processing time should be known
Process
Arrival Time
Execution Time
Ending Deadline
When to
Start
  
A(1)
A(2)
A(3)
B(1)
B(2) 20 40 10 60 50 25
100
  
The short-term
scheduler
pre-calculates
when to start
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13. CS 314 Operating Systems  Static Table-Driven Scheduling
Scheduling happens when:
 A new task arrives
Completion-time (response time) deadline OR
Earliest-Deadline First
 A task is completed
Preemptive
The short-term scheduler
pre-calculates this in advance
No priority
B1 preempted
B2 preempted
Time 50
100 A1 B1 A2 B1 B1 A3 A3 B2 A4 B2 B2 A5 10 20 30 40 60 70 80 90
110
120
A1(20)
B1(50)
B1(50)
B1(50)
B1(50)
B2(100)
A5(100)
B2(100)
A2(40)
A3(60)
B1(50)
A3(60)
A4(80)
B2(100)
A3(60)
B2(100)
A5(100)
B2(100)
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14. CS 314 Operating Systems  Static Priority-Driven Scheduling
Performs both short/long-term
scheduling
Rate-Monotonic-Scheduling
Priority-based scheduling
(for the short-term scheduling)
Concept of “rate” in RMS
For periodic real-time tasks
Rate = 1/P P P
Idle Time T1 T2 T3
Time
C (Running Time)
RMS
Assign the highest priority to the process that has the largest rate
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15. CS 314 Operating Systems  Static Priority-Driven Scheduling
Short-term Scheduling
Long-term Scheduling
For each new process i
Short-Term
Priority H L
Rate (=1/P)
Calculate the utilization (Ui) as
Ui = Ci/Pi
RMS-Feasibility Test
Then, the RMS long-term scheduler
rejects the new process if:
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16. CS 314 Operating Systems  Static Priority-Driven Scheduling Processes
Preemptive priority-based scheduling
Processes
Running Time (C)
Task Period (P)
Rate (1/P)
Priority
Process 1
Process 2
Process 3 20 40
100
100
150
350
0.01
0.006
0.002 H L M
Long-Term Feasibility Test: =
 1.0 =
 n(21/n-1)
(These three processes are schedulable for MRS)
Assign Short-Term Priority:
Since 0.2 (P1) < (P2) < (P3)
Process 1 
Process 2
Process 3
Highest Priority
Lowest Priority
Medium Priority
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17. CS 314 Operating Systems  Static Priority-Driven Scheduling
RMS uses this sufficient condition
 Static Priority-Driven Scheduling
Processes
Running Time (C)
Cycle (T)
Rate (1/T)
Process 1
Process 2 20
100 30
145
Process 3 68
150
0.200
0.206
0.453
Necessary Condition =
= 0.859
 1.0
Sufficient Condition =
 n(21/n-1)
= 0.779
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18. CS 314 Operating Systems  Dynamic Best-Effort Scheduling
Task N has the highest priority
(a) Priority-Driven Non-Preemptive
Meeting responsiveness for task N?
Meeting determinism for task N? 
(Because of non-preemptive)
Although N has a
higher priority, N
can’t start since 1
does not give up 
(Because of non-preemptive)
Task-N is
completed at t2 t2
Determinism(N)
Responsiveness(N) tX
Task-1 releases
the processor at t1 t1
Responsiveness(1)
Task 1
Task N t0
Time N
Task-N with a
higher priority
arrives at t0
 Determinism is out-of-control
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19. CS 314 Operating Systems  Dynamic Best-Effort Scheduling
(b) Priority-Driven Immediate Preemptive
Although N has a
higher priority, N
can’t start since 1
does not give up
Task-N is
completed at t2
Determinism
Responsiveness t0 t1 t2
Task 1
Task N
Time
Task-N with a
higher priority
arrives at t0
Task-1 releases
the processor at t1
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20. CS 314 Operating Systems  Dynamic Best-Effort Scheduling
(b) Priority-Driven Immediate Preemptive (continued)
Meeting determinism (N)?
Meeting responsiveness (N)?  
N’s determinism tR
Task-N-1 is
completed at tR
Task-N is
completed at tC tC
Let N preempts 1
immediately
= 0
Responsiveness(N) tX t0
Task-N with a
higher priority
arrives at t0
Task-1 Responsiveness
Task 1
Task N
Task (N-1)
Task 1
Time
 Responsiveness is out-of-control
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21. after Task-N is requested
CS 314 Operating Systems
 Dynamic Best-Effort Scheduling
(c) Priority-Driven Preemption on (periodic) preemption-points C
= a hardware-clock interrupt
Hardware-clock interrupt is a special interrupt
caused by the hardware-clock periodically at
a fixed time interval C C t C t C t tS t0
Task-N with a
higher priority
arrives at t0
tC1
tC2
tC3
Responsiveness
Task 1
Task N
Time tX
The first HC interrupt
after Task-N is requested
Determinism(N):
Responsiveness(N):
tX – t0
guaranteed up to t
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22. CS 314 Operating Systems  Dynamic Best-Effort Scheduling
(c) Priority-Driven Immediate Preemption on (periodic) preemption-points
Although N has a
higher priority, N
can’t start since 1
does not give up
If non-preemptive
Responsiveness
Task N
Task-1 releases
the processor at t1 t1
Task-N is
completed at t2 t2
Determinism
Task 1 t0
Time
Task-N with a
higher priority
arrives at t0
 Determinism is out-of-control
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23. CS 314 Operating Systems  Dynamic Best-Effort Scheduling
If (task time) < (time-slice)
this is the responsiveness
(d) Round-Robin
Time-Slice
Determinism
Task 1
Task 2
Task 3
Task (N-1)
  
Task N t0
Time 
Meeting responsiveness
Meeting determinism
Depends on what?
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24. CS 314 Operating Systems  Dynamic Best-Effort Scheduling
(d) Round-Robin (continued)
Time-Slice
Determinism
Task 1
Task 2
Task 3
Task (N-1)
  
Task N t0
Time
Time-Slice TN
Time-Slice TN
  
Time-Slice TN
  
Determinism
Task-N
finishes T1 T2 T3
TN-1
  
Time t0
Responsiveness
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25. CS 314 Operating Systems  Dynamic Best-Effort Scheduling
Determinism T1 T2 T3
TN-1
  
Time-Slice TN
Time t0
Responsiveness
Task-N
finishes
Meeting “determinism”
requirement easier if:
Meeting “responsiveness”
requirement easier if:
Reduce the degree of
multi-tasking
Same
Reduce the degree of
multi-tasking
Reduce Round-Robin Time Slice
Increase Round-Robin Time Slice
Tradeoff
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26. CS 314 Operating Systems Priority Inversion
A situation where a higher-priority task must wait for a lower-priority
task even in a preemptive priority-based scheduling.
Something that should not happen H
- A high-priority task L
- A low-priority task R
- Resource that requires exclusive access H L R
(H waits on R)
 Request
 Request
 Assigned
H requests R
H starts
H gets blocked H L t3
H gets R and ready
L starts first
L gets the processor t0 t1 t2 t4
L releases R
L locks on R
Time
Priority-Inversion
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27. CS 314 Operating Systems M Unbounded Priority Inversion H
- A high-priority task L
- A low-priority task R
- Resource that requires exclusive access H L R
 Request
 Assigned
 Request
(H waits on R) M
- A medium-priority task
H requests R H L t0 t1 t2 t3 t4
L releases R
L locks on R
Time
Priority-Inversion
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28. 1. Mars Pathfinder Priority Inversion Incident
On July 4th, 1997, the Mars Pathfinder landed to a media fanfare and began
to transmit data back to Earth. Days later and the flow of information and
images was interrupted by a series of total systems resets.

29. CS 314 Operating Systems Unbounded Priority Inversion M H
- A high-priority task L
- A low-priority task R
- Resource that requires exclusive access H L R
 Request
 Assigned
 Request
(H waits on R) M
- A medium-priority task
H requests R
M requests for execution
(requests for the processor) H L
 We never know
when L releases R M2 t3 M t4 t0 t1
L locks on R t2
L can not
release R
Priority-Inversion
Unbounded Priority-Inversion
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