CM Timing Requirements

In order to manage (or store) an audio/video stream, a computer has to digitalize it, and this implies dividing the stream in frames. Hence, in order to properly reproduce the stream each frame must be played ``at the correct time''. Saying this in a more formal way, some temporal constraints must be respected, otherwise the quality of reproduced stream decreases. In particular, a frame played too late is equivalent to a lost frame.

According to these considerations, it is easy to understand that applications managing CM streams have new requirements, that can be better fulfilled by the operating system if it:

• provides support for periodic activities. Since a task must process frames periodically, this activity can be easily implemented by using a periodic task model. A periodic task is a stream of instances (or jobs), that are activated at the beginning of each period. Conventional systems do not provide any explicit support for periodic activities.
• terminates each task instances before the next activation. In this way each frame can be hanlded in real-time before the next one, and the CM implicit timing is respected. This can be done by using timing information to schedule the tasks, but conventional schedulers do not do so.

As a result of these considerations, classical time-sharing systems perform in an acceptable manner when the avaible resources are much more than required, but the quality of the reproduced stream degrades in an unpredictable manner whenever the system load increases too much.

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