INSE - Lectures 19 & 20

SE for Real-Time & SE for Concurrency

Really these are two topics – but rather tangled together

Double-lecture structure

Real-time – concepts Real-time – system characteristics Concurrency – concepts Concurrency – processes Real-time scheduling of concurrency

Real-Time - Concepts

“Real-time system”

Some computer systems have to do certain outputs before particular actual times they have to “meet deadlines”

Forms of deadline fixed lead time after some stimulus

event; or variable lead time (from a stimulus)

before some outside world event happens; or

clocked deadlines (usually at regular outside-world intervals); or

continuous (e.g. keeping something up-to-date)

Kinds of Stimuli

One classification: periodic – e.g. “will come between

4 and 6 seconds after the last one” uniform – e.g. “averaging 10 per

minute” = probability of 1/6th in any

second sporadic – a burst of stimuli, then a

lull

Severity of deadlines The consequences of missing a deadline vary

from systems to systemor even between different sorts of deadline in a

single system Common classifications:

hard – missing the deadline is some sort of catastrophe

soft – missing the deadline is embarrassing or irritating

optimum – we want to minimize the average response time

Challenge: meeting deadlines

How long will a “task” take? Will/can it complete between stimulus

and deadline? Should concurrent tasks

run “in parallel”? or run one at a time “to completion”?

when should each task be started?

Challenge: how much memory?

Peak memory demand? is there enough RAM?

Preventing “memory leakage”

Challenge: testing

How to generate test stimuli? How to test for having met a deadline? How to test for memory issues? How to test without perturbation – i.e.

without the test mechanism affecting speed or size

Challenge: Human issues

How to find staff who can visualize the timing issues?

Real-Time System Characteristics

Real-time systems are often also…

safety-critical or security-critical; embedded (including control

systems)

Real time system size…

…varies enormously: from tiny embedded systems (e.g.

in a washing machine) to the world-wide “cashcard”

system

Real-time system “modes”

Many real-time systems have a number of “modes” – and will be in one at a time.

E.g. for a washing machine: idle; washing; rinsing; draining; spinning.

Clashing deadlines

Suppose another stimulus comes in while the task for another kind of stimulus is processing should the new task wait for the

other to end? should the two tasks run “simultaneously”?

I.e. “queued” or “concurrent”?

Priority of a deadline

A concurrent deadline might have a prioritywill depend on how soon the

deadline it is, how severe the deadline is, etc;

used in deciding which deadline’s process gets the CPU

Mode-change => change of priority?

Jitter vs stability

A system might be prone to timing races between tasks may cause long-term behaviour

changes – jitter otherwise the system is stable

Concurrency – Concepts

A non-concurrent program…

… is a sequence of instruction (“the code”), and

a set of data.

There is a single continuous “thread of control” through the instructions.

Two generalizations:

1) non-continuous;

2) more than one “thread of control”

These lead to two different forms of concurrency (but we can also have the two

forms together)

Non-continuous programming The program runs to complete a “task”,

which then sleeps or dies; Later the program re-awakens, and runs

another task.

The program consists of code for the various tasks – the tasks might run in any order; the tasks usually share their data.

Interrupt-driven programming

Non-continuous programming where the tasks are started by interrupts the interrupts may be hardware-

generated; the interrupts may be user-

generated.

Uses of i/r-driven programming

include… inside an Operating System for the

various OS processes controlled by a scheduler in the OS’s

kernel in embedded systems

(kernel without full OS) in GUI programming for user events.

Concurrency (=parallelism)

= multiple continuous “threads of control”(possibly scattered across >1 CPU)

Hybrids

1) Multiple non-continuous tasks at the same time

2) Continuous threads and also non-continuous tasks at the same time

Concurrency – processes

Process = a concurrent thread of control the code it executes though the data it operates on

States of a process…

… typically include running – the/a CPU is executing the

process’s thread of control ready – it could run, but hasn’t currently

been scheduled blocked – it has done something that

stops it running for now suspended – another process has

stopped it

Inter-process communication

When processes share resources they need to collaborate not conflict.

An example of conflict (1)

Suppose 2 processes share an integer variable V, & both try to add one to it…

Initially the variable contains (say) 27…V=27

An example of conflict (2)

Suppose 2 processes share an integer variable V, & both try to add one to it…

.. then process A takes a copy of V’s value…V=27

A’s copy = 27

An example of conflict (3)

Suppose 2 processes share an integer variable V, & both try to add one to it…

.. then process B takes a copy of V’s value…V=27

A’s copy = 27

B’s copy = 27

An example of conflict (4)

Suppose 2 processes share an integer variable V, & both try to add one to it…

.. then process A adds one to it’s copy of V’s value…V=27

A’s copy = 28

B’s copy = 27

An example of conflict (5)

Suppose 2 processes share an integer variable V, & both try to add one to it…

.. then process B adds one to it’s copy of V’s value…V=27

A’s copy = 28

B’s copy = 28

An example of conflict (6)

Suppose 2 processes share an integer variable V, & both try to add one to it…

.. then process A writes it’s copy back to V itself…V=28

A’s copy = 28

B’s copy = 28

An example of conflict (7)

Suppose 2 processes share an integer variable V, & both try to add one to it…

.. then process B also writes it’s copy back to V itself…V=28 (still)

A’s copy = 28

B’s copy = 28

An example of conflict (8)

Suppose 2 processes share an integer variable V, & both try to add one to it…

.. we have added one to 27, twice…V=28

and the result is 28, not 29!!!

Preventing conflict (1)

Put each of the “add-one” instructions into a mutual exclusion zone:

wait till V is unlocked,

then lock it

V = V+1

unlock V

Preventing conflict (2)

Implementing that wait+lock/ unlock capability needs either interrupts off or that the wait+lock and unlock

operations be operating system calls

Cooperation

So the “mutual exclusion” technique will give us co-operation rather than conflict

But mutual exclusion can lead to “deadly embrace” avoiding that calls for careful

design, rather than indiscriminate use of mutual exclusion

Challenge: Human issues

How to find staff who can visualize the conflict/coperation/deadly embrace issues?

Real-Time scheduling of Concurrency

If a system is both real-time and concurrent then

concurrent => we need scheduling real-time => the scheduling needs to enable meeting deadlines

Techniques of r/t scheduling

Very dependant on the application The notes describe assorted

detailed techniques

After this double lecture

There are a lot of real-time jobs out there are you right for them?

© C Lester 1997-2014