Different users may have different preferences for how processing time should be allocated among a set of applications. Not all applications are always of equal importance to a user. For example, a user may want to ensure that an important video teleconference be played at the highest image and sound quality possible, at the sacrifice if need be of the quality of a television program that the user was just watching to pass the time. However, current practice, as typified by UNIX, provides little in the way of predictable controls to bias the allocation of resources in accordance with user preferences. For instance, in UNIX timesharing, all that a user is given is a nice knob whose setting is poorly correlated to the scheduler’s externally observable behavior .
As users may have different preferences for how processing time should be allocated among a set of applications, SMART provides two parameters to predictably control processor allocation: priority and share. These parameters can be used to bias the allocation of resources to provide the best performance for those applications which are more important to the user. The user can specify that applications have different priorities, meaning that the application with the higher priority is favored whenever there is contention for resources. The system will not degrade the performance of a higher priority application to execute a lower priority application. For instance, suppose we have two real-time applications, one with higher priority than the other, and
the lower priority application has a computation with a more stringent time constraint. If the lower priority application needs to execute first in order to meet its time constraint, the system will allow it to do so as long as its execution does not cause the higher priority application to miss its time constraint.
Among applications at the same priority, the user can specify the share of each application, resulting in each application receiving an allocation of resources in proportion to its respective share whenever there is contention for resources. Shares only affect the allocation of resources among applications with equal priorities.
