Reference Model of Real Time System

MODEL OF
REAL-TIME
SYSTEM
PRESENTED
BY:
MANOJ
KHADKA
1

ACCORDING TO REFERENCE MODEL, A
SYSTEM IS CHARACTERIZED BY THREE
ELEMENTS:-
(1)a workload model that describes the applications supported by the system,
 Temporal parameters
 Precedence constraints and dependencies
 Functional parameters
(1)a resource model that describes the system resources available to the applications,
(2)algorithms that deﬁne how the application system uses the resources at all times
2

PROCESSORS AND
RESOURCES
servers and active resources;
computers, transmission links,
disks, and database server are
examples of processors
 Every job must have one or more
processors in order to execute
and make progress toward
completion
resources are memory, sequence
numbers, mutexes, and
database lock
 A job may need some resources in
addition to the processor in
order to make progress
3

Temporal Parameters of Real-Time
Workload
Its temporal parameters that tell us its timing constraints
and behavior. Its interconnection parameters describe how
it depends on other jobs and how other jobs depend on it.
Release time (ri)
Relative deadline(Di)
Absolute deadline(di)
Execution time(ei)
4

Fixed, Jittered, and Sporadic
Release Times
Fixed: we do know the actual release time ri of each job .
Jitter: we do not know the actual release time ri of each job Ji ; only that ri is in a range [ri− , ri+ ].
Sporadic/aperiodic : they are released at random time instants.
5

APERIODIC AND SPORADIC TASKS
 a task is aperiodic if the jobs in it have either soft deadlines or no deadlines.
 Tasks containing jobs that are released at random time instants and have
hard deadlines are sporadic tasks.
6

PERIODIC TASK MODEL
 each computation or data transmission that is executed
repeatly at regular or semiregular time intervals in order
to provide a function of the system on a continuing basis
is modeled as a period task
 such as digital control, real-time monitoring, and constant
bit-rate voice/video transmission
 Four tuples (φi(phase), pi(periods), ei, Di(relative deadline))
7

PRECEDENCE CONSTRAINTS
• jobs are said to have precedence constraints if they are constrained to execute in some order
Example:
• For example, in a radar surveillance system, the signal-processing task is the producer of track records,
while the tracker task is the consumer. In particular, each tracker job processes the track records
generated by a signal-processing job. The designer may choose to synchronize the tasks so that the
execution of the kth tracker job does not begin until the kth signal-processing job completes.
8

PRECEDENCE GRAPH AND TASK
GRAPH
• Jobs are either precedence constrained or independent
• Precedence relation:
->partial ordering operator <
->Ji < Jk : Ji is a predecessor of Jk , Jk a successor of Ji
->directed graph : G=(J,<)
• Task graph is an extended precedence graph
9

TASK GRAPH
• gives us a general way to describe the application system
• the vertices in a task graph represent jobs
• shown as circles and squares
• numbers in the bracket above each job give its feasible interval
• edges in the graph represent dependencies among jobs
• If all the edges are precedence edges, representing precedence constraints, then the graph is a
precedence graph
10

DATA DEPENDENCY
• a situation in which a program statement (instruction) refers to the data of a preceding statement
• there is a data-dependency edge from a vertex Ji to vertex Jk in the task graph if the job Jk consumes
data generated by Ji or the job Ji sends messages to Jk
• A parameter of an edge from Ji to Jk is the volume of data from Ji to Jk
11

OTHER DEPENDENCIES
• Temporal Dependency
• AND/OR Precedence Constraints
• Conditional Branches
• Pipeline Relationship
12

TEMPORAL DEPENDENCY
• In task graph, temporal distance constraints among jobs are represented by temporal-dependency edge
• There is a temporal dependency edge from a vertex Ji to Jk if the job Jk must be completed within a
certain time after Ji completes.
• The value of this parameter is infinite if jobs have no temporal distance constraints or there is no
temporal dependency edge among the jobs
13

AND/OR PRECEDENCE
CONSTRAINTS
• A job with one or more than one immediate predecessor that must wait until all of its predecessor are
executed is AND jobs
• The dependencies among theme AND precedence constraints
• These are represented by unfilled circles in the task graph
• OR jobs are the job that can begin execution at or after its release time provided one or more of its
immediate predecessor has been completed
• It is represented by square vertices
14

CONDITIONAL BRANCHES
• all the immediate successors of a job must be completed before the job to be executed
• This convention makes it inconvenient for us to represent conditional execution of job
• Let us consider the program shown below:
15

A Program with conditional branches 16

PIPELINE RELATIONSHIP
• In task graph, the vertices are granules of producer and consumer
• Each consumer granule can begin execution when previous granules of job and the producer job have
been completed
• We represent the pipeline relationship between edges with the dotted lines
• There is an pipeline edge from Ji to Jk if o/p of Ji is piped into Jk and execution of Jk can proceed as long
as there are data for it to proceed
17

FUNCTIONAL PARAMETERS
• Main function parameters include:-
• Preemptivity of jobs
• Criticality of jobs
• Optional execution
• Laxity type and laxity function
18

PREEMPTIVITY OF JOBS
• Preemptable:-Can be suspended and resumed form the suspension point
• Non-preemptable jobs:-it must be implemented from start time to completion time without
preemption
19

CRITICALITY OF THE JOBS
• Criticality of jobs:-positive number indicating how critical the job is
• Value increases with importance of job
• Here we don’t use the terms priority and weight
20

OPTIONAL EXECUTION
• If an optional job or optional portion completes late or is not executed at all, the system still functions
satisfactorily
• Non-optional job:- mandatory
21

LAXITY TYPE AND LAXITY FUNCTIONS
• Laxity type of a job means whether its timing constraints are hard or soft.
• Laxity type is supplemented by a usefulness function
• This function gives the usefulness of the result produced by the job as a function of its tardiness
• The tardiness is the difference between the completion time and deadline of the job.
22

RESOURCE PARAMETER OF JOBS AND PARAMETER OF
RESOURCES
• Preemptivity of resource
• Resource Graph
24

PREEMPTIVITY
• If a job can use every unit of resource in a interleaved fashion, the resource is called preemptable
• A resource is non-preemptable if each unit of the resource is constrained to be used serially
25

RESOURCE GRAPH
• A graph that is used to describe the configuration of the resources
• Vertex Ri for every processor or resource Ri in the system
• Attributes of the vertex are the parameters of the resources
• p- processor R- passive resources
• Parameters
 Type
 No. of available units
26

SCHEDULING HIERARCHY
• Task Graph
 Representation of application system
 Processor time
 Resource requirement
 Timing constraints & dependencies of the job
• Resource graph
 Amount of attributes
 Resource attributes
 Usage rules
• Scheduling and resource access control algorithm used by the operating system
28

SCHEDULER AND SCHEDULE
• Scheduler
 Implements scheduling algorithm
 Schedule jobs
 Allocate resources
Example :-processor assignments on jobs(or vice-versa)
• Schedule
 A schedule is an assignment of all jobs in the system on the available processors.
29

VALID SCHEDULES
• Every processor is assigned at most one job at any time.
• Every job is assigned at most one processor at any time.
• No job is scheduled before its release time.
• The total amount of processor time assigned to every job is equal to its maximum or actual execution
time.
• All the precedence and resource usage constraints are satisfied.
30

FEASIBILITY, OPTIMALITY AND PERFORMANCE MEASURES OF
SCHEDULING
• A valid schedule is also a feasible schedule if every job meets its timing constraints or completes by its
deadline
• A job is a schedulable according to a scheduling algorithm, if the scheduler always produces a feasible
schedule
• Miss Rate: The percentage of jobs that are executed but completed too late
• Loss Rate: The percentage of jobs that are not executed at all or discarded
The other performance measures are maximum and average tardiness, response time, miss/loss and invalid rates
31

BIBLIOGRAPHY
• https://www.kullabs.com/classes/subjects/units/lessons/notes/note-detail/4508
32

Reference Model of Real Time System

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Reference Model of Real Time System