[kepler-dev] web service actor
Edward A Lee
eal at eecs.berkeley.edu
Thu Apr 1 07:16:32 PST 2004
At 02:18 AM 4/1/2004 -0800, Bertram Ludaescher wrote:
>Apropos breaking the model of computation: I'm not sure whether I
>asked that before, but what are the "recommended" (and maybe not so
>recommended) means to "drain" a running process network.
>Clearly a PN is easy to "drain" or "starve" from the "head", i.e.,
>source side, by slowing down or stopping token production.
>For some applications it may be desirable to also slow down or even
>stop the computation at an arbitrary point downstream. I've heard of
>this notion of "backpressure" that can be applied in the opposite
>direction of the token flow to slow down (and possibly pause/stop) a
>Is such a thing already in Ptolemy II?
This is a deep question.
The Ph.D. Thesis of Tom Parks addresses much of this,
and gives the principles behind the way Pt II schedules PN
models. It's at:
The bottom line is:
- almost anything you ask about a process network is undecidable.
- Determining whether particular actors are "starved" by any
given action is tantamount to solving the halting problem...
- lots of schemes exist in the literature for "demand-driven",
"data-driven" and "fair" execution. They are all "wrong" in
that it's easy to construct models where their execution is
clearly not what you want.
- The question of whether the execution policy is "right" can
be posed as determining whether a finite execution is
an approximation to the infinite denotational semantics.
Comparing schemes amounts to determining how to define
"approximation" and how to define progress towards the
(unachievable) infinite behavior.
Edward A. Lee, Professor
518 Cory Hall, UC Berkeley, Berkeley, CA 94720
phone: 510-642-0455, fax: 510-642-2739
eal at eecs.Berkeley.EDU, http://ptolemy.eecs.berkeley.edu/~eal
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