Then the actors are ordered such that each actor only fires when the scheduler has determined that enough tokens will be present on its input ports to allow it to fire. In cases where the dataflow graph is cyclic, a valid firing vector exists, but no actor can fire, since they all depend on the output of another actor. This situation is known as deadlock. Deadlock must be prevented in SDF by manually inserting delay actors, which represent initial tokens on each relation. Such delay actors are responsible for creating tokens during initialization that will prevent deadlock. These actors set the tokenInitProduction parameter of their output ports to represent the number of tokens they will create during initialization. The SDFScheduler uses this parameter to break the dependency in a cyclic graph.
Note that this scheduler only ensures that the number of firings is minimal. Most notably, it does not attempt to minimize the size of the buffers that are associated with each relation. The resulting schedule is a linear schedule (as opposed to a looped schedule) and is not suitable for multiprocessing environments.
Any actors may be scheduled by this scheduler, which will, by default, assume homogeneous behavior for each actor. (i.e. each output port produces one token for each firing, and each input port consumes one token on each firing, and no tokens are created during initialization.) If this is not the case then parameters named tokenConsumptionRate, tokenProductionRate, and tokenInitProduction must be set. The SDFIOPort class provides easier access to these parameters.
Note that reconstructing the schedule is expensive, so the schedule is locally cached for as long as possible, and mutations under SDF should be avoided.
Note that this scheduler supports actors with 0-rate ports as long as the graph is not equivalent to a disconnected graph. This scheduler is somewhat conservative in this respect.
Disconnected graphs are supported if the SDF Director parameter allowDisconnectedGraphs is true. @see ptolemy.actor.sched.Scheduler @see ptolemy.domains.sdf.lib.SampleDelay @author Stephen Neuendorffer and Brian Vogel @version $Id: SDFScheduler.java,v 1.239 2007/12/07 06:27:56 cxh Exp $ @since Ptolemy II 0.2 @Pt.ProposedRating Green (neuendor) @Pt.AcceptedRating Green (neuendor)
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