Operating System Design/Scheduling Processes/Priority Scheduling

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In Priority Scheduling, each process is given a priority, and higher priority methods are executed first, while equal priorities are executed First Come First Served or Round Robin.

There are several ways that priorities can be assigned:

• Internal priorities are assigned by technical quantities such as memory usage, and file/IO operations.
• External priorities are assigned by politics, commerce, or user preference, such as importance and amount being paid for process access.(The latter usually being for mainframes)

SPN is a form of priority scheduling based on External Priorities.

[edit] Analogy

Internal: At a store, the person with the right change and only two items gets to check out first. External: At a store, the cashier is bribed by the most important customer, and thus can check out first.

[edit] Implementation

[edit] Advantages and Disadvantages

A process can sometimes become starved, live locked, or indefinitely blocked. One way to fix this is by aging the priority, so that the longer a process has waited for CPU time, the higher it's priority is.