Embedded Software for SoC - Grupo de Mecatrônica EESC/USP

266 Chapter 20
The first outcome leads to a valid schedule constructed so far. If, after
returning from the recursive calls we find that the schedule is not valid we
would re-explore the path again. To exhaustively explore the subgraph that
exists after a given node v all the ECSs enabled in v must be explored. This
is controlled by procedure proc1.
The latter two outcomes do not produce a valid schedule. Hence, after
returning from the recursive call, a re-exploration of new paths will be done
providing that there are still paths that have not been exhaustively explored.
4. REDUCING THE SIZE OF A SCHEDULE
Scheduling deals with an unbounded reachability space stemming from source
transitions of the PN given as the specification. Termination criteria make
the space under analysis bounded, but it is still very large in general. The
efficiency of a scheduling procedure essentially depends on the ways of compacting
the explored space. Compression techniques introduced in this section
explore the reachability space at a coarser granularity level by identifying
sequences of transitions fireable at given markings rather than firing a single
transition at a time.
4.1. Trace-based compression
The motivation behind a trace-based compression is given by a simple
example of a multi-rate producer-consumer system (see Figure 20-2).
In this system a single iteration of the producer requires the consumer to
iterate twice. A corresponding schedule is shown in Figure 20-2(b). Clearly
if a scheduler can identify that it needs to fire twice the sequence of transitions
corresponding to a single iteration of the consumer, then it could fire
this sequence as a whole without firing individual transitions of the sequence.
This would avoid storing internal markings of the consumer’s iteration and
would compress a schedule (see Figure 20-2(c)).