courseware basics of real-time scheduling jan madsen informatics and mathematical modelling...

courseware

Basics of Real-Time Scheduling

Jan Madsen

Informatics and Mathematical ModellingTechnical University of Denmark

Richard Petersens Plads, Building 321DK2800 Lyngby, Denmark

[email protected]

Jan Madsen 2SoC-MOBINET courseware

WAP application

Need structure and to perform multiple processes

- send requests to server- translate data received from server

- render data on display

- operating the user’s control panel

- logically different operations

- performed at different rates

• Tasks:

• Why?


Why use multiple processes?

Processes are a neat way to handle complex systems

• Processes introduce structure into the system design– understand each process independently– analyze communication between processes

to determine system behavior

• Timing is easier to specify• Communication between processes

provide test points• Suitable for team effort


What is a process?

A process is a state and a procedure State:

Running Suspended Deactivated

Procedure: Sequence of separate steps

A process may interact with its surroundings


Process execution

deadlineactivation

period


Process execution

• Periodic

– the period is the time between successive executions

– the process rate is the inverse of its period

• Aperiodic

– non-periodic process, typical an event handler

• Sporadic

– hard real-time aperiodic process

– usually, a minimum interarrival constraint is imposed


Example: A software processes

A process includes: code memory which belongs to the process connections to other processes

A process is a unique execution of a program In most embedded systems, processes never die!


CPUMemory

Executing on a single CPU

P1 code & data

P2 code & data

P3 code & data

P1 state

P2 state

P3 state

PC

registers


Execution of multiple processes

• On a single CPU

– A Pentium processor

• On a distributed homogeneous system

– A network of Pentium processors

• On a distributed heterogeneous system

– A network of differing processors (CPUs and ASICs)


About this lecture

Huge literature on scheduling theory Aim is to give an overview of problems and

possible solutions to the multi-task scheduling problem related to RTOS

Structure of lecture: Some basic concepts and teminology Uni-processor scheduling

Rate-monotonic scheduling Earlies-deadlines-first scheduling

Multi-processor scheduling


PART I

Scheduling Terminology and Basic Concepts


Design flow

P1P2

P3

Partitioning/clustering

Break processes toincrease parallelism

Cluster processes toreduce communication

Allocation

PE1 PE2

MappingSchedulingCommunication

P1

P3 P2

PE1

PE2P1 P2 & P3


Scheduling

1

34

2

a b c

1 2 os

3

4

schedulinga b c


Terminology

Application

Architecture

constraints

Scheduler Schedule

•Architecture

•Application

•Scheduling

•Constraints


Architecture

Components Processors

CPU, DSP, ASIC, ...

Communication Bus, network, ...

Devices sensor, actuator, display, ...

Architecture may be fixed or (re-)configurable mem device

devicea b c

os1 2

3

4


Application

Task A module which can be envoked to perform a

particular function A schedulable entity

Characterized by its: Timing constraints Precedence contraints Exclusion constraints Resource requirements

1

34

2


Timing constraints

r1

r1 = time at which task becomes released (or active)

e1

e1 = worst case execution time (WCET)

d1 = deadline, task should complete before this!

d1s1

s1 = time at which task starts its execution

T1

T1 = period, minimum time between task releases

1


Task execution

Periodic the period is the time between successive executions

Aperiodic non-periodic task, typical an event handler

Sporadic hard real-time aperiodic task typical, a minimum interarrival constraint is imposed


Precedence constraints

pi precede pj if pi must finish before pj begins

process interrelate with each other to achieve: synchronization exchange datapi

pj

pi

pj

blocked


Exclusion constraints

pi exclude pj if no execution of pj is allowed between the time that pi starts its computation and the time that pi complete its computation

Prevent simultaneous access to shared resources data memory devices

pi

pj

pi

pj

pj blocked


Resource constraints

R = r1, r2, …, rm

Example: R(pi) = {r1, r3} resource r1 and r3 is required by pi during

its execution

may be used to implement a critical section enforce PE assignment of processes


Scheduling approaches

Classical scheduling theory Uniprocessor systems RMS EDF

Distributed real-time scheduling Deterministic (static) scheduling Dynamic scheduling


Scheduling

Allocation Determine number and type of processors/resources

Assignment Binding tasks to processors

Scheduling Determine execution order

a

b

b

a

1

2 1

2

Task view

1

2

Architecture view

a

b


Scheduling principles

Off-line A scheduler performing its job before the scheduled

system is put into operation

On-line A scheduler performing its job at run-time, when the

system is running

Typically list-based When a task is released (ready) it is placed in a list Scheduler select which task from the list to execute next Selection based on some criteria – scheduling policy


Scheduling areas

High-level Synthesis: fine grained, operation/instruction level schedule done off-line single time constraint

Real-time operating systems (RTOS): coarse grained, programs/sub-programs scheduling done on-line multiple time constraints


SoC scheduling

Similarity to RTOS Time constraints Context switching Task synchronization Task communication

Difference to RTOS Large design space Variable granularity Wide range of metrics

Time, power, cost, ... Flexibility, reliability, ...

High degree of optimization Dedicated hardware


Non-preemptive scheduling

Preemptive scheduling

Preemption vs. Non-preemption

1

2

1

2


Metrics

Minimize:

c i

w ci i

}{maxmax deL iii

lateness, tardiness

Schedule length

p1 p2 p3 p4 p5

p1p2 p3 p4 p5

d1 d2 d3 d4 d5

courseware basics of real-time scheduling jan madsen informatics and mathematical modelling...

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