Supporting Phases

  An important feature of any communications language is that it support phases: without such support, all the communications must be merged into a single phase. For scheduled routing, such merging would result in disastrously little bandwidth available to each operator, as all the operators would have to share the available scheduled bandwidth.

As is discussed in the next chapter, some existing communication languages support fixed, user-specified phases. That is, the communications language explicitly indicates the boundaries between phases. However, for the HLL compiler to attempt to derive phase boundaries is for it to take on itself the job of the communications compiler. Determining phase boundaries depends on message traffic, the effects of scheduling multiple operators in one phase, and the overhead for changing out of a phase with a given set of operators in it, among other things. Accordingly, making such decisions is beyond the scope of the HLL compiler.

It is also worth observing that since the notion of a `phase' does not contribute much to compiling for a dynamic-routing target, it would be inappropriate to require phases to be specified in a language intended to be used for dynamic routing as well as scheduled routing.