OS-Priorities


Priorities may be assigned automatically by the system or they may be assigned externally.

STATIC VS DYNAMIC PRIORITIES:

Static priorities do not change. Static priority mechanisms are easy to implement and have relatively low overhead. They are not responsive to changes in environment, changes that might make it desirable to adjust a priority.

Dynamic priority mechanisms are responsive to change. The initial priority assigned to a process may have only a short duration after which it is adjusted to a more appropriate values. Dynamic priority schemes are more complex to implement and have greater overhead than static schemes.

PURCHASED PRIORITIES:

An operating system must provide competent and reasonable service to a large community of users but must also provide for those situations in which a member of the user community needs special treatment. A user with a rush job may be willing to pay a premium, ie., purchase priority, for a higher level of service. This extra charge is merited because resources may need to be withdrawn from other paying customers. If there were no extra charge, then all users would request the higher level of service.
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