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Short term scheduling

Short term scheduling concerns with the allocation of CPU time to processes in order to meet some pre-defined system performance objectives. The definition of these objectives (scheduling policy) is an overall system design issue, and determines the ``character'' of the operating system from the user's (i.e. the buyer's) point of view, giving rise to the traditional distinctions among ``multi-purpose, time shared'', ``batch production'', ``real-time'' systems, and so on.

From a user's point of view, the performance criteria can be stated as follows:

When the overall system performance is considered, additional scheduling criteria must be taken into account:

When the interaction between users' needs and overall system performance is considered, additional criteria to be accounted for arise:

The design of the short term scheduler is one of the critical areas in the overall system design, because of the immediate effects on system performance from the user's point of view. It's usually one of the trickiest as well: since most processor architectures support their own task switching facilities, the implementation of the process switch mechanism is generally machine-dependent. The result is that the actual process switch software is usually written in the assembly language of a particular machine, whether the operating system is meant to be portable across different machines or not.

Independently of the adopted scheduling policy, three basic issues in short term scheduler design can be individuated:


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Next: Alternative scheduling methods Up: Scheduling - I Previous: Types of scheduling -

Franco Callari