Processor Architectures and Boolean Algebra: Question 3

Worked solution

Part (a): What a virtual machine is

A virtual machine is software that behaves, from the point of view of the operating system and applications running on it, as if it were an entirely separate physical computer. It runs its own guest operating system on top of a real, physical host machine, with a virtualisation layer (a hypervisor) responsible for sharing out the host’s actual physical resources. Processor time, memory, storage. Between however many virtual machines are running.

To meet the university’s need, several virtual machines could be created on the one physical server, each one configured with a different guest operating system. Because each virtual machine behaves as an independent computer, users get the experience of several different operating systems running side by side, even though there is only one physical machine underneath. [2 marks: 1 for explaining a virtual machine as software presenting a separate simulated computer with its own guest OS on shared host hardware, 1 for applying this to running several different guest operating systems on the one physical server]

Part (b): One benefit and one limitation

Benefit (either of, or an equivalent valid point):

• Isolation: if one virtual machine crashes, misbehaves, or is compromised (for example, by malware), the other virtual machines and the underlying host are not directly affected, because each virtual machine is kept separate from the others.
• Cost/resource efficiency: several virtual machines, each running a different operating system, can share the cost of a single set of physical hardware, instead of the university needing to buy and maintain a separate physical machine for every operating system it wants to run.

Limitation (either of, or an equivalent valid point):

• Performance overhead: instructions from software running inside a virtual machine must pass through an extra virtualisation layer before reaching the real hardware, so a program typically runs a little slower inside a virtual machine than it would running directly on the physical hardware.
• Resource contention: because several virtual machines share the same underlying physical processor and memory, if too many virtual machines are active at once they can compete for those resources, reducing the performance available to each one.

[2 marks: 1 for a valid, correctly explained benefit, 1 for a valid, correctly explained limitation, distinct from the benefit]

Part (c): Characteristics of a massively parallel computer

A massively parallel computer is distinguished from an ordinary multi-core computer by:

1. Scale: it uses a very large number of individual processors. Often hundreds, thousands, or even more. Each of which typically has its own memory, rather than the two or three processor cores found in a general-purpose computer.
2. Concurrent division of one problem: these very many processors are connected together, usually via a dedicated high-speed interconnection network, and work at the same time, each one handling a separate part of the same overall problem (such as a different region of the climate model), so that a single very large task is divided into many smaller parts that all execute concurrently.

[2 marks: 1 for the very large number of separate processors, typically each with its own memory, 1 for these processors being connected together and working concurrently on different parts of the same overall problem]

Final answers

• (a) A virtual machine is software presenting a separate simulated computer, with its own guest OS, on top of shared host hardware; the university can run one guest OS per virtual machine on its single physical server.
• (b) Benefit: isolation or cost/resource efficiency. Limitation: performance overhead or resource contention.
• (c) A massively parallel computer has a very large number of individual processors (often each with its own memory), connected together and working concurrently on different parts of the same problem.