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1.4 Choosing Models of Computation Introduction The rich variety of concurrent models of computation outlined in the previous section can be daunting to a designer faced with having to select them. Most designers today do not face this choice because they get exposed to only one or two. This is changing, however, as the level of abstraction and domain-specificity of design software both rise. We expect that sophisticated and highly visual user interfaces will be needed to enable designers to cope with this heterogeneity. An essential difference between concurrent models of computation is their modeling of time. Some are very explicit by taking time to be a real number that advances uniformly, and placing events on a time line or evolving continuous signals along the time line. Others are more abstract and take time to be discrete. Others are still more abstract and take time to be merely a constraint imposed by causality. This latter interpretation results in time that is partially ordered, and explains much of the expressiveness in process networks and rendezvous-based models of computation. Partially ordered time provides a mathematical framework for formally analyzing and comparing models of computation [82]. A grand unified approach to modeling would seek a concurrent model of computation that serves all purposes. This could be accomplished by creating a melange, a mixture of all of the above, but such a mixture would be extremely complex and difficult to use, and synthesis and simulation tools would be difficult to design. Another alternative would be to choose one concurrent model of computation, say the rendezvous model, and show that all the others are subsumed as special cases. This is relatively easy to do, in theory. It is the premise of Wright [6] and Metropolis [47], for example. Most of these models of computation are sufficiently expressive to be able to subsume most of the others. However, this fails to acknowledge the strengths and weaknesses of each model of computation. Wright, for example, uses rendezvous, which is very good at resource management, but very awkward for loosely coupled dataoriented computations. Asynchronous message passing is the reverse, where resource management is awkward, but data-oriented computations are natural 1 . Thus, to design interesting systems, designers need to use heterogeneous models. In Metropolis [47], the model of computation assigns to each actor its own thread of control, and leaves it to the designer to define the interactions between actors. These can be specialized on a per-connection basis. While this is extremely expressive, and with discipline on the part of the designer can be used to realize a variety of more specialized models of computation, undisciplined usage could lead to incomprehensible models. The approach used in Ptolemy II is to provide in the infrastructure an abstract semantics, rather than a unifying model of computation. It is “abstract” in the sense that it is not a complete model of computation. For example, the abstract semantics of the actor package asserts that actors “fire,” but it says nothing about when they fire. This makes it possible to define actors that can operate in several domains (we call these domain polymorphic actors). 1.5 Ptolemy II Architecture Ptolemy II offers a unified infrastructure for implementations of a number of models of computa- 1. Consider the difference between the telephone (rendezvous) and email (asynchronous message passing). If you are trying to schedule a meeting between four busy people, getting them all on a conference call would lead to a quick resolution of the meeting schedule. Scheduling the meeting by email could take several days, and may in fact never converge. Other sorts of communication, however, are far more efficient by email. 24 Ptolemy II
Introduction tion. The overall architecture consists of a set of packages that provide generic support for all models of computation and a set of packages that provide more specialized support for particular models of computation. Examples of the former include packages that contain math libraries, graph algorithms, an interpreted expression language, signal plotters, and interfaces to media capabilities such as audio. Examples of the latter include packages that support clustered graph representations of models, packages that support executable models, and domains, which are packages that implement a particular model of computation. Ptolemy II is modular, with a careful package structure that supports a layered approach. The core packages support the data model, or abstract syntax, of Ptolemy II designs. They also provide the abstract semantics that allows domains to interoperate with maximum information hiding. The UI packages provide support for our XML file format, called MoML, and a visual interface for constructing models graphically. The library packages provide actor libraries that are domain polymorphic, meaning that they can operate in a variety of domains. And finally, the domain packages provide domains, each of which implements a model of computation, and some of which provide their own, domain-specific actor libraries. 1.5.1 Core Packages The core packages are shown in figures 1.12,1.13, 1.14, and 1.15.These are UML package diagrams. The name of each package is in the tab at the top of each box. Subpackages are contained within their parent package. Dependencies between packages are shown by dotted lines with arrow heads. For example, actor depends on kernel which depends on kernel.util. Actor also depends on data and graph. The role of each package is explained below. actor This package supports executable entities that receive and send data through ports. It includes both untyped and typed actors. For typed actors, it implements a sophisticated type system that supports polymorphism. It includes the base class Director that is extended in domains to control the execution of a model. actor.lib This subpackage and its subpackages contain domain polymorphic actors. The actor.lib package is discussed further in section 1.5.4. actor.parameters This subpackage provides specialized parameters for specifying locations, ranges of values, etc. actor.process This subpackage provides infrastructure for domains where actors are processes implemented on top of Java threads. actor.sched This subpackage provides infrastructure for domains where actors are statically scheduled by the director, or where there is static analysis of the topology of a model associated with scheduling. actor.util This subpackage contains utilities that support directors in various domains. Specifically, it contains a simple FIFO Queue and a sophisticated priority queue called a calendar queue. copernicus This subpackage contains the “actor specialization” infrastructure (Java code generation). data This package provides classes that encapsulate and manipulate data that is transported between actors in Ptolemy models. The key class is the Token class, which defines a set of polymorphic methods for operating on tokens, such as add(), subtract(), etc. Heterogeneous Concurrent Modeling and Design 25
Page 1 and 2: PTOLEMY II HETEROGENEOUS CONCURRENT
Page 3 and 4: VOLUME 1 INTRODUCTION TO PTOLEMY II
Page 5 and 6: Volume 1 Contents Introduction to P
Page 7 and 8: 2.10.3. Constructing Modal Models 8
Page 9 and 10: 7. MoML 191 7.1. Introduction 191 7
Page 11 and 12: 1 Introduction Author: Edward A. Le
Page 13 and 14: Introduction heterochronous dataflo
Page 15 and 16: Introduction A major emphasis in Pt
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Page 19 and 20: Introduction provides levels of vis
Page 21 and 22: Introduction is a guard, and the se
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Page 25 and 26: Introduction clients to update thei
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Page 39 and 40: Introduction 1.5.2 Overview of Key
Page 41 and 42: Introduction Actor interfaces are k
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Page 45 and 46: Introduction actor actor.gui data.e
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Page 49 and 50: Introduction 1.5.8 Future Capabilit
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Page 55 and 56: 2 Using Vergil Authors: Edward A. L
Page 57 and 58: Using Vergil 2.2.2 Executing a Pre-
Page 59 and 60: Using Vergil The Lorenz attractor m
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Page 69 and 70: Using Vergil 2.4 Hierarchy Ptolemy
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Using Vergil instances. This proble
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Using Vergil quick tour for more de
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Using Vergil uses the index in the
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Using Vergil AddSubtract actor, and
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Using Vergil If you look inside the
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Using Vergil If you look inside the
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Using Vergil transition (or right c
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Using Vergil Once you have created
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Using Vergil The grid is turned off
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Authors: Edward A. Lee Xiaojun Liu
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Expressions classes to create a gen
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Expressions The % operation is a mo
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Expressions attribute, which includ
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Expressions 3.3.5 State Machines Ex
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Expressions whose value is an array
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Expressions from it. For instance,
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Expressions defined. For example, >
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Expressions length is given by the
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Expressions Thus, for example, you
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Expressions number. With one additi
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Expressions value. For example, fix
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Expressions In the example in figur
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Expressions A.2 Basic Mathematical
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Expressions A.3 Matrix, Array, and
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Expressions A.5 Signal Processing F
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Expressions TABLE 8: Functions perf
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4 Actor Libraries Authors: Elaine C
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Actor Libraries properly in the CT
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Actor Libraries ing that multiple c
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Actor Libraries TypedAtomicActor «
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Actor Libraries SequenceScope (exte
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Actor Libraries UnsignedByteArrayTo
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Actor Libraries MobileFunction (ext
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Actor Libraries LogicFunction (exte
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Actor Libraries Sleep (extends Tran
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Actor Libraries VariableFIR (extend
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Actor Libraries Integrator: Integra
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Actor Libraries Figure 4.4 shows th
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5 Designing Actors Authors: Christo
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Designing Actors /** Javadoc commen
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Designing Actors although with most
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Designing Actors import ptolemy.act
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Designing Actors makes such actors
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Designing Actors same parameter val
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Designing Actors morphic actor comp
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Designing Actors To get data polymo
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Designing Actors public class Seque
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Designing Actors be used in domains
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Designing Actors alComm actor. Beca
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Designing Actors attribute has valu
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Designing Actors import ptolemy.act
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6 Coding Style Authors: Christopher
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Coding Style /* One line descriptio
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Coding Style Copyright (c) 2004 The
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Coding Style @version $Id: NamedObj
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Coding Style } We use instead the i
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Coding Style * @exception MyExcepti
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Coding Style + "it does not impleme
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Authors: Edward A. Lee Steve Neuend
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MoML cutes it forever, until you hi
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MoML A second difference between ou
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MoML
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MoML 7.3.3 Names and Classes Most M
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MoML Here, the element named
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MoML implement. 7.3.7 Doc Element S
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MoML It is also sometimes necess
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MoML then there will be a link t
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MoML found. For example, if the cla
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MoML it cannot collide with the nam
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MoML The rendering of the icon is d
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MoML 7.4.5 Deleting Entities, Relat
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MoML This unlinks a port from the s
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MoML model. The EntityLibrary class
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MoML Suppose that using incremental
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MoML Generate a sine wave.
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MoML
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MoML Number of iterations in
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MoML -P- The f
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MoML -P- The phase,
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8 Custom Applets Authors: Edward A.
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Custom Applets html code, see the J
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Custom Applets public class Tutoria
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Custom Applets then the result of
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Custom Applets 8.3.5 Using Model Pa
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Custom Applets ptolemy/domains/doma
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References [1] G. Agha, Actors: A M
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[33] S. A. Edwards, “The Specific
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[67] E. Kohler, The Click Modular R
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[95] J. Liu, X. Liu, T. J. Koo, B.
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[132]W. R. Sutherland, "The on-Line
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Glossary abstract syntax ..........
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Ptolemy II ........................
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Index Symbols - in UML 41 # in UML
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coin flips 142 Color class 173 colo
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entity in XML 198 EntityLibrary cla
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Director class 159 inverse FFT 124
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CompositeEntity class 206 nextPower
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safety 34 SampleDelay actor 78, 138
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type constraint 155 type constraint
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