Unit 1 Flashcards
Which of the following system configurations are capable of being concurrent, parallel, or neither?
(a) One processor, three activities
(b) Three processors, two activities
(c) Three processors, five activities
(d) Two processors, one activity
(a) Concurrent, but not parallel – the activities can share one processor in a pseudo parallel fashion.
(b) Parallel and therefore concurrent–each activity can progress simultaneously on one processor, with one idle.
(c) A mixture of parallel and pseudo-parallel activity is possible here – some processes must share one or more processors.
(d) No concurrency or parallelism is possible as there is only one activity – this can run on one of the processors while the other remains idle. If the activity could be split up into two or more subactivities then some concurrency and even parallelism would be possible.
Summarise the main benefits and costs of distributed systems.
The benefits include sharing of resources, which may be hardware, software or data. Distributed systems also offer advantages in scalability, fault-tolerance, interoperability and, if properly designed, can balance the local processing, centralised processing and communication activities of a system to suit the needs of the users.
The costs are the introduction of new ways for the system to fail – the network connections may fail or become unreliable. Ensuring security may also be more difficult when there are more points of access as compared to a centralised system. It may also be more complex to maintain possibly different types of software on a number of hosts and to ensure that they show interoperability.
Does the internet qualify as a distributed system according to the definition below?
Distributed computer systems have been defined in a number of different ways. It is generally agreed that a distributed system contains a number of distinct components at different locations, where each component is, in some sense, a computer system itself. These distinct components can communicate with one another via some sort of network, although there are a wide variety of technologies available for such communication. It is also sometimes suggested that a distributed system should appear to its users as a single coherent system providing a range of functions in such a way that the user may not know, or even need to know, that the system is distributed. This last requirement is called transparency and holds to varying degrees for systems that are generally agreed to be distributed systems.
The internet certainly satisfies the first two requirements above. Its components are computer systems – either individual computers, like PCs and web servers, or computer networks, depending on how detailed a view you take. These components are connected by computer networking and communicate using internet protocols such as TCP/IP. It is harder to say if the internet is a coherent, transparent system providing a range of functions – to naive users it may well be transparent, especially on a fast connection. At any rate, the internet is generally agreed to be a distributed system, so it must be the case!
Summarise the main benefits and costs of concurrent systems.
The benefits are more efficient use of hardware, increased responsiveness to the user, increased speed of computation (for parallel systems) and better modelling of systems that interact with a concurrent real world
The costs are increased complexity of both hardware and software, and new ways for systems to fail (such as deadlock), as compared to sequential systems.
Summarise the main benefits and costs of mobile systems.
The benefits are that users can communicate and access the other parts of a distributed system from a wider variety of locations. For example, linking a laptop computer to a local computing environment allows users to get the best of their personal computing facilities and any data stored there as well as using local facilities such as printers, scanners or file servers. For mobile systems that are wireless there are additional benefits of avoiding the cost and disruption of fixed wiring installation.
The costs are mainly, but not entirely, related to wireless mobile systems. There may be an increased security risk in allowing mobile users to link to a system whether wirelessly or not, and mobile components such as laptops are more easily stolen or lost than fixed components. Wireless communications such as radio signals are more vulnerable to interception and often are lower in bandwidth than wired systems. These drawbacks can be overcome, for example by encryption or by better hardware, but these have their own associated costs.
Draw a diagram (such as a Venn diagram, showing sets) to show how each of the following categories of computer system is distinct from, includes, or overlaps with the others.
- Sequential
- Concurrent
- Distributed
- Mobile
If you are not familiar with Venn diagrams, feel free to devise your own way to show this graphically.
There may be a number of possible ways to answer this. We take ‘mobile’ to mean a system at least some of whose components are mobile (like a mobile phone system which includes a considerable amount of fixed hardware as well as the mobile handsets).
In this interpretation, the sets for sequential and concurrent systems have no overlap. All distributed systems are (potentially) concurrent, and mobile systems may be distributed or could be independent (like a PDA). These independent mobile systems might or might not be capable of concurrency depending on their hardware and software design, although most such systems are likely to be concurrent nowadays. Our diagram is shown in Figure 11 – the size of the shapes and the size of their overlap is not significant.
