ICT301 Concurrent Systems

ICT301Concurrent Systems

Anthony Leewww.artofstudying.com

ICT301

• 10 credits module• 7 lectures – selected difficult topics• 6 tutorials – practical sessions • 3 TMAs

ICT301

• 10 credits module• 7 lectures – selected difficult topics• 6 tutorials – practical sessions • 3 TMAs• Lecturer and tutors• Electronic conferences• Examination• References Bacon

TanenbaumAndrews

ICT301

• Block 1 – The Foundations– Nature of concurrent systems

– Hardware and operating systems support

– Processes, multi-threading

Programming language needed

ICT301

• Block 2 – Inter-process Communications– Shared and non shared data

– Shared and non shared memory

– Problems and issues

Not fact-based• Requires understanding

ICT301

• Block 3 – Application Considerations– System viability

– Transactions

– Distributed databases

– Standards and mini case study

ICT301

Why study concurrent systems?

The Story of

“How to get Rich Quickly

Without Studying ICT301”

Concurrent Systems

Systems with several processes that:• contribute to overall objective

• independent

• cooperate among themselves – exchange data– synchronise actions

• compete for resources

• respond to external events – interact with external environment

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elevator 2 discharging passengers at level 1

level 1

level 2

level 3

level 4

level 5elevator 1 answers the call at level 4 to go up because someone will be getting off from it at this level

elevator 4 answers a call to go down

level 6

level 7 elevator 3 bypasses a call at level 4 to go up because elevator 1 will be stopping at that level

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level 1

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The system is dynamic and concurrent

Service zone

Service zone

Service zone

Service zone

Concurrent Systems

• How would the Java program looks like?

class ElevatorController

{ public static void main(String args[ ])

{

use for loop ?

}

}

Concurrent Systems

• How would the Java program looks like?

class ElevatorController

{ public static void main(String args[ ])

{

use for loop ?

}

}independentcooperate

exchange datasynchronise actions

compete for resourcesrespond to external events

interact with external environment

Concurrent Systems

• Inherently concurrent systems

– Concurrent by nature

• Potentially concurrent systems

– Not concurrent by nature

– But can be implemented as concurrent systems

Inherently Concurrent Systems

• Typical inherently concurrent systems

– Real-time and embedded systems

– Databases and transaction processing systems

– Operating systems



– Real-time and embedded systems• Hard real time systems

• Soft real time systems



Aircraft Flight Controls

Real time? Information up to date

Hard or soft? Response time critical

Aircraft Flight Controls

Real time? Information up to date

Hard or soft? Response time critical



– Real-time and embedded systems• Hard real time systems

• Soft real time systems



Banking: ATM system

Real time?

Hard or soft?

Balance inquiry Cash withdrawal






Recall our get-rich-quick story

Bank accountsATM GIRO











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Concurrent Systems

• So far, inherently concurrent systems• Next, potentially concurrent systems

Potentially Concurrent Systems

• Not concurrent by nature

• Advantageous to implement concurrent processing

• Usually in situations where

– large amount of data

– large amount of processing

– real-time need

– parallel processing possible


• Ways to implement concurrent processing

– Replicated code with partitioned data

– Partitioned code with duplicated data

– Pipelined processing

– Tree-structured algorithms







Program that coordinates the

processing

Codes that process data

Portion of data


Portion of data


Portion of data

. . . . . .






– Tree-structured algorithms Matrix operations

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Checked by checker

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stage 1

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Fields for Applications

– Artificial intelligence

– Graphical imaging

– Natural phenomena analysis

– Engineering analysis

– Mathematical modelling

Basic Computer Architecture

Recall operating systems

– inherently concurrent

– closer look at what it has to do

Disk

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Processor

Memory

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Keyboard

Typical architecture


• CPU execution cycle

– von Neumann architecture

update instruction pointer

execute instruction

fetch instruction

decode instruction

Machine instructions

Instruction 1Instruction 2Instruction 3Instruction 4Instruction 5

. . .

The big speed difference

Consider a 2 GHz CPU

– Two thousand million cycles per second

Consider human typing at 10 characters per second




Consider human typing at 10 characters per second

If CPU works at 1 cycle per second,

human would be typing at one character every six years.

Ratio is 200,000,000 to 1




Consider a hard-disk transferring data at 10 megabytes per second




Consider a hard-disk transferring data at 10 megabytes per second

For every byte read from disk, CPU does about 200 cycles

Ratio is 200 to 1

• Two implications arising from big difference in speed:

1. We need buffers

2. The CPU should be deployed elsewhere



1. We need buffers

So the CPU does not have to wait while IO operations takes place



1. We need buffers

2. The CPU should be deployed elsewhere

This means dividing its attention among many tasks



• Dividing CPU’s attention among many tasks

time-slicing

task 2task 3task 1task 2task 1

4321 765

. . .

. . .time interval



time-slicing

task 1task 1 . . .

4321 765 . . .time interval

task 2task 2 . . .task 3 . . .

task 1

task 2

task 3



And we need interrupts

Task 1 Interrupt service routine

Task 2



And we need interrupts

Time allocated to a task is up

Higher priority request from another program

Request to update the real time clock

Exceptions encountered in a program

Completion of an IO operation



Interrupts involve context switch

CPU

data of program

instruction pointer

Main memory

interrupt service routine

Before an interrupt

a program




CPU

data of interrupt service routine

instruction pointer

Main memory

interrupt service routine

After an interrupt

a program




saving the status of executing

program to main memory

CPU

instruction register

instruction pointer

data registersdata for

executing interrupt service

routine



An alternative to interrupts is polling




Twin sisters on a trip to Disneyland

Polly is highly energetic and vocal, keep asking “Are we there yet?” every 10 minutes

Molly is a daydreamer, busy with her own thoughts, has to be told when they arrive




Disk

Printer

Memory

Screen

Keyboard

Screen

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Disk

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Processor

Plotter

Finally . . .

Recall again our get-rich-quick story

Can it really happen?

With interrupts . . .

A possible scenario with interrupts

Consider the Java statement:

i = i + 1; // assume i has initial value

of 3

// assume i is global

A possible scenario with interrupts

Consider the Java statement:

i = i + 1;

Equivalent machine instructions could be

1. Copy value of i to CPU register i = 3

2. Add 1 to CPU register gives 4

3. Save value of CPU register i = 4

In program A

. . .

Copy value of i to register i = 3

Add 1 to registergives 4

Save value of register i = 4

. . .

i = i + 1 i = 3

In program B

. . .




. . . i = i + 2

i = 3

In program B

. . .




. . . i = i + 2

In program A

. . .




. . .

i = i + 1

What if we want to run both programs?

i = 3

In program B

. . .




. . .

In program A

. . .




. . .

Executing them sequentially

i = 3

i = 4

In program B

. . .Copy value of i to register i = 3



. . .

In program A

. . .




. . .

With interrupts

i = 3

In program B

. . .Copy value of i to register i = 3



. . .

In program A

. . .




. . .

Very important question!

Did we get the same result as executing the programs sequentially?

i = 3

What we have covered

• What are concurrent systems?

– Inherently concurrent

– Potentially concurrent

• Why operating systems?

– Background - how the system support concurrency

– As an example of a concurrent system

– Examined some basic ideas and issues

Documents

ICT301 Concurrent Systems