Time sharing Operating System

Time sharing concept was introduces to provide fast response time to interactive users of a computer system. Response time is the time since submission of a computational task by a user till its results are reported to the user.

Timesharing systems give the illusion to each user of having a machine to oneself.

To provide good response times to all users, the timesharing supervisor should ensure that no program should be allowed to monopolize the CPU. To ensure this following two provisions are made,

·        A program consume only a limited amount of CPU time in one go.

·        Programs must assign variable priorities, not assign fixed priorities

Most timesharing systems use the time slicing scheduling. The techniques of Round Robin (RR) scheduling and time slicing are used to implement above provisions.

RR scheduling policy processes all user jobs in such a manner that a program finishing its turn on the CPU gets another turn after all other programs have had their turn in the CPU. The first program in the scheduling list is the highest priority program, once it executed its time slice then put to the last of the scheduling list where it has the lowest priority. Thus, the program priorities are time dependent. Each program is allowed to use a short CPU time, known as time slice. All the programs in the system get equal opportunity to execute in the CPU.

A timesharing system may use thousands of users simultaneously. The total main memory available in a computer may not be able to occupy all these programs at a time. Thus, at any instant, the timesharing operating system keeps only a few programs in main memory and rest are stored on the disk storage. The memory resident programs include the active programs and some of the ready programs, which will get the CPU attention very shortly. The process of temporarily transferring programs from the main memory to the disk storage and back is known as swapping. We will discuss swapping later.

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