1. Introduction to Real-Time Systems
4/26/2017
Introduction to Real-Time Systems
Fred Kuhns ()
CS523S: Operating Systems

2. What is a Real-Time System?
Real-time systems have been defined as: "those systems in which the correctness of the system depends not only on the logical result of the computation, but also on the time at which the results are produced";
J. Stankovic, "Misconceptions About Real-Time Computing," IEEE Computer, 21(10), October 1988.
Fred Kuhns ()
CS523S: Operating Systems

3. Real-Time Characteristics
Real-time systems often are comprised of a controlling system, controlled system and environment.
Controlling system: acquires information about environment using sensors and controls the environment with actuators.
Timing constraints derived from physical impact of controlling systems activities. Hard and soft constraints.
Periodic Tasks: Time-driven recuring at regular intervals.
Aperiodic: event-driven.
Fred Kuhns ()
CS523S: Operating Systems

4. Typical Real-Time System
Con trolled System
sensor
actuator
Controlling
System
Environment
Fred Kuhns ()
CS523S: Operating Systems

5. CS523S: Operating Systems
Some Definitions
Timing constraint: constraint imposed on timing behavior of a job: hard or soft.
Release Time: Instant of time job becomes available for execution. If all jobs are released when the system begins execution, then there is said to be no release time
Deadline: Instant of time a job's execution is required to be completed. If deadline is infinity, then job has no deadline. Absolute deadline is equal to release time plus relative deadline
Response time: Length of time from release time to instant job completes.
Fred Kuhns ()
CS523S: Operating Systems

6. CS523S: Operating Systems
Hard versus Soft
Hard: failure to meet constraint is a fatal fault. Validation system always meets timing constraints.
Deterministic constraints
Probabilistic constraints
Constraints in terms of some usefulness function.
Soft: late completion is undesirable but generally not fatal. No validation or only demonstration job meets some statistical constraint. Occasional missed deadlines or aborted execution is usually considered tolerable. Often specified in probabilistic terms
Fred Kuhns ()
CS523S: Operating Systems

7. Validating Constraints
Validation: Demonstration by a provably correct, efficient procedure or by exhaustive simulation and testing. Involves three steps:
timing constraints of each application and corresponding components are consistent,
each component can meet its timing constraints if executed alone and required resources are available,
The underlying scheduling algorithm(s), all timing constraints are met
Fred Kuhns ()
CS523S: Operating Systems

8. Developing a Reference Model
Modeling the system to focus on timing properties and resource requirements. Composed of three elements:
workload model - describes applications supported by system
Temporal parameters
Precedence constraints and dependencies
Functional parameters
resource model - describes system resources available to applications
Modeling resources (Processors and Resources)
Resource parameters
algorithms - defines how application uses resources at all times.
Scheduling Hierarchy
Fred Kuhns ()
CS523S: Operating Systems

9. Tasks and Jobs: Definitions
Task (Ti): Set of related jobs jointly provide function.
Job (Jij): Unit of work, scheduled and executed by system. characterized by the following parameters:
Temporal parameters: timing constraints and behavior
Functional parameters: intrinsic properties of the job.
Resource parameters: resource requirements.
Interconnection parameters: how it depends on other jobs and how other jobs depend on it
Fred Kuhns ()
CS523S: Operating Systems

10. CS523S: Operating Systems
Resources
Resources can be divided into passive and active:
Active resources == Processors (Pi): they execute jobs.
Every job must have one or more processors
Same type if functionally identical and used interchangeably.
Passive resource == Resource (Ri):
job may require Resources in addition to processor.
reusable resources are not consumed
Fred Kuhns ()
CS523S: Operating Systems

11. Job/Task Temporal Parameters
Hard real-time: number and parameters of tasks are known at all time.
for Job Ji:
ri - release time, may know range [r-, r+] (jitter). For aperiodic/sporadic release or interrelease time is a random variable.
di - absolute deadline
Di - relative deadline
(ri, di] - feasible interval
ei - Execution time. May know range [e-, e+]. Most deterministic models use e+.
Fred Kuhns ()
CS523S: Operating Systems

12. CS523S: Operating Systems
Periodic Task Model
Jobs repeated at regular or semi-regular intervals modeled as periodic.
Task Ti is a serious of periodic Jobs Jij.
pi - period, minimum interrelease interval between jobs in Task Ti. Must be bounded from below.
ei - maximum execution time for jobs in task Ti.
rij - release time of the jth Job in Task i (Jij in Ti).
i - phase of Task Ti, equal to ri1.
H - Hyperperiod = Least Common Multiple of pi for all i: H = lcm(pi), for all i.
ui - utilization of Task Ti.
U - Total utilization = Sum over all ui.
Fred Kuhns ()
CS523S: Operating Systems

13. Aperiodic and Sporadic Tasks
A periodic or Sporadic task is a stream of aperiodic or sporadic jobs.
Jobs with a task have similar statistical behavior and timing requirements
Assumed system is stationary within the hyperperiod
Aperiodic: jobs have soft or no deadlines. Want responsiveness.
Sporadic: jobs have hard deadlines
Fred Kuhns ()
CS523S: Operating Systems

14. Precedence Constraints and Task Graphs
Jobs are either precedence constrained or independent.
Precedence relation: partial ordering operator <
Ji < Jk : Ji is predecessor of Jk, Jk is successor of Ji
directed graph G = (J,<)
Task graph is an extended precedence graph.
Fred Kuhns ()
CS523S: Operating Systems

15. Functional Parameters
Preemptivity
Preemption: suspend job then dispatch different job to processor. Cost includes context switch overhead.
Non-preemptable task - must be run from start to completion.
Criticalness - positive integer indicating the relative importance of a job. Useful during overload.
Optional Executions - jobs or portions of jobs may be declared optional. Useful during overload.
Laxity - Laxity type => hard or soft timing constraints. Supplemented by a usefulness function. Useful during overload.
Fred Kuhns ()
CS523S: Operating Systems

16. CS523S: Operating Systems
Resource Parameters
Job resource parameters indicate processor and resource requirements.
Preemptivity of resources
non-preemptive: serial access, typical case.
preemptive - jobs can interleave access.
Resource Graph:
vertex for processors (Pi) and resources (Ri)
is-a-part-of edge: Ri -> Rk, Rk is a part of Ri
accessibility edge: Pi -> Pk, cost of Pi accessing Pk
Fred Kuhns ()
CS523S: Operating Systems

17. Schedules and Scheduling
Jobs scheduled and allocated resources based on a set of scheduling algorithms and access control protocols.
Scheduler: Module implementing scheduling algorithms
Schedule: assignment of all jobs to available processors, produced by scheduler.
Valid schedule:
every processor assigned to at most one job at a time
every job assigned to at most one processor at a time
no job scheduled before its release time
Total amount of processor time assigned to every job is equal to its maximum or actual execution time
Fred Kuhns ()
CS523S: Operating Systems

18. CS523S: Operating Systems
Definitions
Feasible schedule: Every job starts at or after release time and completes by deadline
Schedulable: set of jobs schedulable according to an algorithm if the it always produces a feasible schedule.
Optimal: Scheduling algorithm optimal if it always produces a feasible schedule if such a schedule exists
Tardiness: Zero if completion time <= deadline, otherwise > 0 (complete - deadline).
Lateness: difference between completion time and deadline, can be negative if early.
Fred Kuhns ()
CS523S: Operating Systems

19. CS523S: Operating Systems
Performance Measures
Miss rate: percentage of jobs executed but completed late
Loss rate: percentage of jobs discarded
Invalid rate: sum of miss and loss rate.
makespan : If all jobs have same release time and deadline, then makespan = response time of the last job to execute.
Max or average response times
Max or average tardiness/lateness
Fred Kuhns ()
CS523S: Operating Systems