Distributed Operating System

Distributed Operating System 

• A distributed operating system is one that looks to its users like an ordinary centralized operating system but runs on multiple independent CPUs. 
• The key concept here is transparency. The use of multiple processors should be invisible to the user. 
• Another way of expressing the same idea is to say that user views the system as virtual uniprocessor but not as a collection of distinct machines. 

• Distributed operating systems have many aspects in common with centralized ones but they also differ in certain ways. 
• Distributed operating system, for example, often allow programs to run on several processors at the same time, thus requiring more complex processor scheduling algorithms in order to achieve maximum utilisation of CPU's time.

• Fault-tolerance is another area in which distributed operating systems are different. 
• Distributed systems are considered to be more reliable than uniprocessor based system. 
• They perform even if certain part of the hardware is malfunctioning. 

Advantages of Distributed Operating Systems

There are three important advantages in the design of distributed operating system:

1. Major breakthrough in microprocessor technology: Micro- processors have become very much powerful and cheap, compared with mainframes and minicomputers, so it has become attractive to think about designing large systems consisting of small processors. These distributed systems clearly have a price/performance advantages over more traditional systems.

2. Incremental Growth: The second advantage is that if there is a need of 10 per cent more computing power, one should just add 10 per cent more processors. System architecture is crucial to the type of system growth, however, since it is hard to give each user of a personal computer another 10 per cent.

3. Reliability: Reliability and availability can also be a big advantage; a few parts of the system can be down without disturbing people using the other parts; On the minus side, unless one is very careful, it is easy for the communication protocol overhead to become a major source of inefficiency.
Protection: In a true distributed system there is a unique UID for every user, and that UID should be valid on all machines without any mapping. In this way no protection problems arise on remote access to files; a remote access can be treated like a local access with the same UID. There is a difference between network operating system and distributed operating system in implementing protection issue. In networking operating system, there are various machines, each with its own user to UID mapping but in distributed operating system there is a single systemwide mapping that is valid everywhere.

Program Execution: In the most distributed case the system chooses a CPU by looking at the processing load of the machine, location of file to be used etc. In the least distributed case, the system always run the process on one specific machine

An important difference between network and distributed operating system is how they are implemented. A common way to realise a networking operating system is to put a layer of software on top of the native operating system of the individual machines.