The Implementation Mechanisms Design Space in Java

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6 The Implementation Mechanisms Design Space
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61 OVERVIEW 62 UE MANAGEMENT 63 SYNCHRONIZATION 64 COMMUNICATION Up to this point, we have focused on designing algorithms and the high level constructs used to organize parallel programs With this chapter, we shift gears and consider a program's source code and the low level operations used to write parallel programs What are these low level operations, or implementation mechanisms, for parallel programming Of course, there is the computer's instruction set, typically accessed through a high level programming language, but this is the same for serial and parallel programs Our concern is the implementation mechanisms unique to parallel programming A complete and detailed discussion of these parallel programming "building blocks" would fill a large book Fortunately, most parallel programmers use only a modest core subset of these mechanisms These core implementation mechanisms fall into three categories:
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UE management Synchronization Communication
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Within each of these categories, the most commonly used mechanisms are covered in this chapter An overview of this design space and its place in the pattern language is shown in Fig 61
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Figure 61 Overview of the Implementation Mechanisms design space and its place in the pattern language
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In this chapter we also drop the formalism of patterns Most of the implementation mechanisms are included within the major parallel programming environments Hence, rather than use patterns, we provide a high level description of each implementation mechanism and then investigate how the mechanism maps onto our three target programming environments: OpenMP, MPI, and Java This mapping will in some cases be trivial and require little more than presenting an existing construct in an API or language The discussion will become interesting when we look at operations native to one programming model, but foreign to another For example, it is possible to do message passing in OpenMP It is not pretty, but it works and can be useful at times We assume that the reader is familiar with OpenMP, MPI, and Java and how they are used for writing parallel programs Although we cover specific features of these APIs in this chapter, the details of using them are left to the appendixes
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61 OVERVIEW
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Parallel programs exploit concurrency by mapping instructions onto multiple UEs At a very basic level, every parallel program needs to (1) create the set of UEs, (2) manage interactions between them and their access to shared resources, (3) exchange information between UEs, and (4) shut them down in an orderly manner This suggests the following categories of implementation mechanisms
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UE management The creation, destruction, and management of the processes and threads
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used in parallel computation
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Synchronization Enforcing constraints on the ordering of events occurring in different UEs This is primarily used to ensure that shared resources are accessed by a collection of UEs in such a way that the program is correct regardless of how the UEs are scheduled Communication The exchange of information between UEs
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62 UE MANAGEMENT
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Let us revisit the definition of unit of execution, or UE A UE is an abstraction for the entity that carries out computations and is managed for the programmer by the operating system In modern parallel programming environments, there are two types of UEs: processes and threads A process is a heavyweight object that carries with it the state or context required to define its place in the system This includes memory, program counters, registers, buffers, open files, and anything else required to define its context within the operating system In many systems, different processes can belong to different users, and thus processes are well protected from each other Creating a new process and swapping between processes is expensive because all that state must be saved and restored Communication between processes, even on the same machine, is also expensive because the protection boundaries must be crossed A thread, on the other hand, is a lightweight UE A collection of threads is contained in a process Most of the resources, including the memory, belong to the process and are shared among the threads The result is that creating a new thread and switching context between threads is less expensive, requiring only the saving of a program counter and some registers Communication between threads belonging to the same process is also inexpensive because it can be done by accessing the shared memory The mechanisms for managing these two types of UEs are completely different We handle them separately in the next two sections 621 Thread Creation/Destruction Threads require a relatively modest number of machine cycles to create Programmers can reasonably create and destroy threads as needed inside a program, and as long as they do not do so inside tight loops or time critical kernels, the program's overall runtime will be affected only modestly Hence, most parallel programming environments make it straightforward to create threads inside a program, and the API supports thread creation and destruction
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