This matterial is related to real time embedded system

1. Real-Time Scheduling
COSC 513
Operating Systems
Yue Dou

2. Real-Time System
 Robotics
 Air traffic control
 Military command and control
systems
 Space station
 Undersea exploration

3. Correctness of Real-Time System
 Correctness of the real-time system
depends not only on the logical
result of the computation, but also
on the time at which the results are
produced

4. Used Terms
 Hard real-time task: must meet its
deadline ( deadline specifies either a start
time or a completion time)
 Soft real-time task: has an associated
deadline that is desirable but not
mandatory, it still makes sense to
schedule and complete the task even if it
has passed its deadline

5. Five Characteristics of Real-Time
Operating System
 Determinism : concerned with how long
an operating system delays before
acknowledging an interrupt
 Responsiveness : concerned with how
long after acknowledgment, it takes an
operating system to finish the interrupt
service routine
Determinism and responsiveness together
make up the response time to external
events which are critical for real-time
systems

6. Five Characteristics of Real-Time
Operating System (cont.)
 User control: allow the user fine-grained
control over task priority
 Reliability: a transient failure may cause
financial loss or major equipment damage
or even loss of life.
 Fail-soft operation: when data corruption
happen, system should attempt corrective
action and same time continues operation
perhaps at a reduced level of service

7. Two Popular Classes of
Scheduling Algorithms (1)
 Deadline Scheduling: the task which
has the earliest deadline, will be
scheduled first

8. Example of Deadline Scheduling
 A system that collects and processes
data from two sensors, A and B. The
deadline for collecting data from
sensor A must be met every 20 ms,
and that for B every 50 ms. It takes
10 ms to process each sample of
data from A and 25 ms to process
each sample of data from B.

9. Example of Deadline Scheduling
(cont.)

10. Two Popular Classes of
Scheduling Algorithms ( 2 )
 Rate Monotonic Scheduling ( RMS ):
used for resolving multitask
scheduling conflicts for periodic
tasks, it assigns priorities to tasks on
the basis of their periods ( rate = 1 /
periods )

11. High Rate, High Priority ( rate
monotonic )

12. Bound Equation
C1/T1 + C2/T2 +….+Cn/Tn <= n(21/n – 1)
Cn: processing time for each task
Tn: each task’s period
Cn/Tn: utilization of the processor

13. Example of Rate Monotonic
Scheduling
 P1: C1 = 20; T1= 100; C1/T1= 0.2
 P2: C2 = 40; T2=150; C1/T1= 0.267
 P3: C3= 100; T3=350; C1/T1= 0.286
Total utilization = 0.753
Since: 0.753 <= 3 (21/3 – 1) = 0.779
So, if RMS is used, all tasks will be
successfully scheduled