eee 243b applied computer programming timing considerations

EEE 243BApplied Computer Programming

Timing considerations

Winter 2005Maj JGA Beaulieu & Capt MWP

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Review

What benefits does reuse have?

What is the definition of a component?


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Outline

Normal flow Periodic tasks Cyclic executives Aperiodic tasks Scheduling


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Normal flow

So far in this course, we have studied programs that are controlled through programming language structures

Our programs follow a structure that is dictated through if-else, while,do-while, switch,… statements

But is this all there is to controlling the behaviour of programs?


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Normal flow

Each instruction or set of instruction takes some time to execute; even if it is only microseconds

There may be some programs for which we may want to control the elapsed time between various instructions (functions or tasks)

This control of time is common among applied programs (robotics, process control, sampling,…)


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Normal flow

The first time we played God with our robots (and face it, we are Gods to our Bots) is when we delayed their activities

We used the sleep() or msleep functions to enlist the help of the brickOS operating system to impose a timeout to our programs


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Normal flow

So if we do not impose some form of time control, our programs will execute following the control flow logic; in this case, the only time limitation is the

execution time of the instructions

Most applications that we use from day to day follow this kind of “static” scheduling of tasks where no timing behaviour is specified


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Periodic tasks

Closely related to static task execution, is the concept of periodic tasks

Some systems require some tasks (functions) to execute at given intervals to ensure that the system behaves in a specific manner

Most control systems include periodic tasks Periodic tasks are “clockwork” maintenance

events


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Periodic tasks

The period of a task is dictated by control requirements and stability concerns

Systems with periodic tasks can be implemented in several ways

Some operating systems allow programmers to specify the periodicity of tasks. This spec may be: Actual period that the task will take (run every 10 msecs) Priority of a task is determined by its required frequency

(period). A task with high frequency requirement will have a high priority so that it executes often

Release time, execution time and deadline


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Cyclic executives

Perhaps the easiest way to implement periodic tasks is the cyclic executive

In the implementation of a cyclic executive, the programmer specifically directs or “hand controls” the timing behaviour of every task

At the base of the cyclic executive is a loop with counters (or a set of loops) that execute “tasks” at given frequencies

Cyclic executives do not require much support from the operating system; the entire schedule is embedded in the program itself


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Cyclic executives

Cyclic executives mean that the programmer knows the hardware intimately The programmer must know how long each

instruction takes to execute in order to tweak the cycles or loops that contain each tasks

Most of the time, the programmer will measure the execution times empirically with tests

Delays are used to tweak loop frequencies


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A

Cyclic executives

A B BC B BCA A

Period of A

Period of B

Period of C

Delay/idle time

time0


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Cyclic executives…while (1){

//tasks at high frequency each time the loop executes

if (!(count%2)){ //tasks at half frequency every second loop //some delay}

if (!(count%4)){ //tasks at quarter freq and delay}

count++;sleep(x); //idle time that regulates the

//highest frequency loop }…


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Aperiodic tasks

Very few systems operate on a set of “pure” periodic tasks

In order to be responsive, most applied systems allow for tasks to occur at unknown times; hence the tasks are aperiodic

Aperiodic tasks occur in a spontaneous fashion through a computer’s interrupt facilities

The operating system must have a mechanism to handle interrupts


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Aperiodic tasks

Timing behaviour for aperiodic tasks is, in general, unpredictable, because you cannot know “when” an event will occur: When a temperature will be reached Data comes on a port Robot hits an obstacle…


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Aperiodic tasks

Aperiodic tasks can, however, be modelled as pseudo periodic tasks

In many systems, it is possible to estimate the frequency of aperiodic events

When you model aperiodic tasks, you goal is only to see if you will have “enough room” in your schedule so that the system can respond to a maximum number of events


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Scheduling

Scheduling is a software engineering discipline that deals with several problems: Efficiency: maximises system resources Throughput: execute as many tasks as possible

for a given time Fairness: ensure that users on a multi-user

system get their fair share of system resources Safety: ensures that all tasks in a real-time

system meet their deadlines Response time: ensures that the system is

responsive to requests


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Scheduling

All the programs that we have written so far run in a single task and have a single thread

In some systems, it is possible for a program to have several tasks that can execute in parallel

These tasks can be full processes or execution threads; where the processor is shared in some fashion (round-robin, priority based, …)


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Quiz Time

How would you implement a cyclic executive?

eee 243b applied computer programming timing considerations

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