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This chapter is from the book

1.4 The Minimal Effort Required

Parallel and distributed programming come with a cost. Although there are many benefits to writing parallel and distributed programming, there are also some challenges and prerequisites. We discuss some challenges in Chapter 2. We mention the prerequisites here. Before a program is written or a piece of software is developed, it must first go through a design process. For parallel and distributed programs, the design process will include three issues: decomposition, communication, and synchronization.

1.4.1 Decomposition

Decomposition is the process of dividing up the problem and the solution into parts. Sometimes the parts are grouped into logical areas (i.e., searching, sorting, calculating, input, output, etc.). In other situations the parts are grouped by logical resource (i.e., file, communication, printer, database, etc.). The decomposition of the software solution amounts to the WBS (work breakdown structure). The WBS determines which piece of software does what. One of the primary issues of concurrent programming is identifying a natural WBS for the software solution at hand. There is no simple or cookbook approach to identifying the WBS. Software development is the process of translating concepts, ideas, patterns of work, rules, algorithms, or formulas into sets of instructions and data that can be executed or manipulated by a computer. This is largely a process of modeling. Software models are reproductions in software of some real-world task, process, or ideal. The purpose of the model is to imitate or duplicate the behavior and characteristics of some real-world entity in a particular domain. This process of modeling uncovers the natural WBS of a software solution. The better the model is understood and developed the more natural the WBS will be. Our approach is to uncover the parallelism or distribution within a solution through modeling. If parallelism doesn’t naturally fit, don’t force it. The question of how to break up an application into concurrently executing parts should be answered during the design phase and should be obvious in the model of the solution. If the model of the problem and the solution don’t imply or suggest parallelism and distribution then try a sequential solution. If the sequential solution fails, that failure may give clues to how to approach the parallelism.

1.4.2 Communication

Once the software solution is decomposed into a number of concurrently executing parts, those parts will usually do some amount of communicating. How will this communication be performed if the parts are in different processes or different computers? Do the different parts need to share any memory? How will one part of the software know when the other part is done? Which part starts first? How will one component know if another component has failed? These issues have to be considered when designing parallel or distributed systems. If no communication is required between the parts, then the parts don’t really constitute a single application.

1.4.3 Synchronization

The WBS designates who does what. When multiple components of software are working on the same problem, they must be coordinated. Some component has to determine when a solution has been reached. The components’ order of execution must be coordinated. Do all of the parts start at the same time or does some work while others wait? What two or more components need access to the same resource? Who gets it first? If some of the parts finish their work long before the other parts, should they be assigned new work? Who assigns the new work in such cases? DCS (decomposition, communication, and synchronization) is the minimum that must be considered when approaching parallel or distributed programming. In addition to considering DCS, the location of DCS is also important. There are several layers of concurrency in application development. DCS is applied a little differently in each layer.

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