Definition of CPU scheduling:
The assignment of physical processors to processes allows processors to complete work. The process of determining that for which process which processor should be assign this process is called processor scheduling or CPU scheduling.
When one or more than one process is run able, the operating system must choose which one first. The division of the operating system concerned with this decision is called the scheduler, and algorithm it uses is called the scheduling algorithm.
Objectives of scheduling (objectives)
Many objectives must be regarded as in the design of a scheduling discipline. In exacting, a scheduler should consider fairness, efficiency, reaction time, turnaround time, throughput, etc., Some of these objectives depends on the system one is using for instance batch system, cooperative system or real-time system, etc. but there are also some objectives that are desirable in all systems.
General objectives
• Fairness is vital under all circumstances. A scheduler makes sure that each process gets its fair share of the CPU and no process can undergo indefinite postponement. Note that giving equivalent or equivalent time is not fair. Think of security control and payroll at a nuclear plant.
• The scheduler has to assure that system''s policy is enforced. For instance, if the local policy is safety then the safety control processes must be talented to run whenever they want to, even if it means delay in payroll processes.
• Scheduler should remain the system (or in exacting CPU) demanding cent percent of the time when possible. If the CPU and all the Input / Output devices can be kept running all the time, more work obtains done per second than if some components are idle.
• A scheduler should reduce the time batch users must remain for an output.
• A scheduler should exploit the number of jobs processed per component time.
• A scheduler should reduce the response time for interactive user.
A little thought will show that some of these objectives are opposing. It can be shown that any scheduling algorithm that supports some class of jobs hurts another class of jobs.