7 Multitasking

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Multitasking


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Multitasking

A method by which multiple tasks, also

known as processes, share common

processing resources such as a CPU.
http://en.wikipedia.org/wiki/Computer_processhttp://en.wikipedia.org/wiki/Central_processing_unithttp://en.wikipedia.org/wiki/Central_processing_unithttp://en.wikipedia.org/wiki/Computer_process


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mul t ip rogramming systems

the running task keeps running until it

performs an operation that requires

waiting for an external event(e.g.

reading from a tape) or until the

computer's scheduler forcibly swaps the

running task out of the CPU.

Multiprogramming systems are designedto maximize CPU usage.


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t ime-sharingsystems

the running task is required to relinquish

the CPU, either voluntarily or by an

external event such as a hardware

interrupt

Time sharing systems are designed to

allow several programs to execute

apparently simultaneously.
http://en.wikipedia.org/wiki/Time-sharinghttp://en.wikipedia.org/wiki/Hardware_interrupthttp://en.wikipedia.org/wiki/Hardware_interrupthttp://en.wikipedia.org/wiki/Hardware_interrupthttp://en.wikipedia.org/wiki/Hardware_interrupthttp://en.wikipedia.org/wiki/Time-sharinghttp://en.wikipedia.org/wiki/Time-sharinghttp://en.wikipedia.org/wiki/Time-sharing


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Time Sharing

Cooperative multitasking

Preemptive multitasking


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Cooperative multitasking

When computer usage evolved from batch mode

to interactive mode, multiprogramming was no

longer a suitable approach. Each user wanted to

see his program running as if it was the onlyprogram in the computer.

The use of time sharing made this possible, with

the concession that the computer would not

seem as fast to any one user as it really wouldbe if it were running only that user's program.


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Disadvantages

A cooperatively multitasked system must

rely on each process to regularly give time

to other processes on the system

(voluntarily).

A single stalled process could halt the

entire system

Irregular use of system resources


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Preemptive multitasking

Preemptive multitasking allows the

computer system to more reliably

guarantee each process a regular "slice"

of operating time. It also allows the systemto rapidly deal with important external

events like incoming data, which might

require the immediate attention of one oranother process.


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real-t imesystems

some waiting tasks are guaranteed to be

given the CPU when an external event

occurs.

Real time systems are designed to control

mechanical devices such as industrial

robots, which require timely processing.


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Multi-threading

Block Multi-threading

Interleaved Multi-threading

Simultaneous Multi-threading


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Block/Cooperative/Coarse-grained

Multi-threading The simplest type of multi-threading is where

one thread runs until it is blocked by an event

that normally would create a long latency stall.

For example:

Cycle i : instruction j from thread A is issued

Cycle i+1: instruction j+1 from thread A is issued

Cycle i+2: instruction j+2 from thread A is issued, load instructionwhich misses in all caches

Cycle i+3: thread scheduler invoked, switches to thread B

Cycle i+4: instruction k from thread B is issued

Cycle i+5: instruction k+1 from thread B is issued


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Interleaved Multi-threading

A higher performance type of

multithreading is where the processor

switches threads every CPU cycle.

Cycle i : an instruction from thread A is issued

Cycle i+1: an instruction from thread B is issued

Cycle i+2: an instruction from thread C is issued

Source: -en.wikipedia.org


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Simultaneous Multi-threading

The most advanced type of multi-threading

applies to superscalarprocessors.

Cycle i : instructions j and j+1 from thread A; instruction k from thread B allsimultaneously issued

Cycle i+1: instruction j+2 from thread A; instruction k+1 from thread B;

instruction m from thread C all simultaneously issued

Cycle i+2: instruction j+3 from thread A; instructions m+1 and m+2 from

thread C all simultaneously issued
http://en.wikipedia.org/wiki/Superscalarhttp://en.wikipedia.org/wiki/Superscalar


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From scalar to superscalar

Each instruction executed by a scalar processormanipulates one or two data items at a time.

By contrast, each instruction executed by a

vector processoroperates simultaneously onmany data items.

A superscalar processor is a hybrid of the two -each instruction processes one data item but

with multiple redundant functional units withineach CPU so that multiple instructions can beprocessing separate data items concurrently.

Source: - en.wikipedia.org
http://en.wikipedia.org/wiki/Vector_processorhttp://en.wikipedia.org/wiki/Vector_processor

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