A2 · Practice questions · Parallel processing architectures

Split the work.

Six original Cambridge-style questions on the four basic computer architectures and the characteristics of massively parallel computers, with mark-scheme style solutions.

Original questions All questions on this page are original work, written in the Cambridge AS & A Level style. They are not from past papers. They test the same concepts and skills the syllabus rewards.

How to earn the marks

Name it, then explain it.

Expand the acronym: single or multiple instruction, single or multiple data.
Say what happens: how many processors, and whether they run the same or different instructions on the same or different data.
Massively parallel: very many processors, simultaneously, different parts of the same program, connected by a network.
Examples: GPU or array processor for SIMD; multicore processor for MIMD.

Recall

[4 marks]

Describe the SIMD and MIMD computer architectures.

SIMD: single instruction, multiple data. ✓
One instruction is executed on many data items at the same time (each processing unit has its own data item), e.g. an array processor or a GPU. ✓
MIMD: multiple instruction, multiple data. ✓
Several processors each execute their own instructions on their own data at the same time, e.g. a multicore processor. ✓

Explain

[4 marks]

Describe the characteristics of a massively parallel computer.

A very large number of processors (or computers). ✓
They work simultaneously / at the same time. ✓
Each works on a different part of the same program or problem. ✓
They communicate through a high-speed interconnection network; each processor usually has its own memory. ✓

Apply

[3 marks]

A photo-editing program brightens every pixel of a 24-megapixel image by adding the same value to each one. State which architecture would be most suitable, and justify your choice.

SIMD. ✓
The same instruction (add the value) is applied to every pixel. ✓
Many pixels can be processed at the same time, one by each processing unit, so the whole image is processed much faster. ✓

Explain

[2 marks]

Explain the difference between SISD and MISD.

SISD: a single processor executes a single instruction stream on a single data stream. ✓
MISD: several processors execute different instruction streams on the same single data stream. ✓

Calculate

[3 marks]

A simulation takes 60 seconds on one processor. 90% of the work can be divided equally between processors. Calculate the time taken on 10 processors, and the speed-up to one decimal place.

Sequential part: 10% of 60 s = 6 s. ✓
Parallel part: 54 s ÷ 10 = 5.4 s, so the total is 6 + 5.4 = 11.4 s. ✓
Speed-up: 60 ÷ 11.4 = 5.3 times. ✓

Analysis

[3 marks]

A school wants to buy a massively parallel computer to run its student records database faster. Explain why this is unlikely to be a good choice.

Massively parallel computers only help with problems that can be split into many independent parts processed at the same time. ✓
Database tasks such as updating a record are small and largely sequential, and need consistent shared data. ✓
The cost, the specialist software and the communication overhead would far outweigh any benefit; a standard (multicore) server is suitable. ✓

Mark this once you have attempted all six and checked your working. It records a Practiced badge on the topic and adds a one-time bonus. Revealing the solutions alone does not count.