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Preface

Preface

For a number of years, home computers have given the illusion of doing multiple tasks simultaneously. This has been achieved by switching between the running tasks many times per second. This gives the appearance of simultaneous activity, but it is only an appearance. While the computer has been working on one task, the others have made no progress. An old computer that can execute only a single task at a time might be referred to as having a single processor, a single CPU, or a single “core.” The core is the part of the processor that actually does the work.

Recently, even home PCs have had multicore processors. It is now hard, if not impossible, to buy a machine that is not a multicore machine. On a multicore machine, each core can make progress on a task, so multiple tasks really do make progress at the same time.

The best way of illustrating what this means is to consider a computer that is used for converting film from a camcorder to the appropriate format for burning onto a DVD. This is a compute-intensive operation—a lot of data is fetched from disk, a lot of data is written to disk—but most of the time is spent by the processor decompressing the input video and converting that into compressed output video to be burned to disk.

On a single-core system, it might be possible to have two movies being converted at the same time while ignoring any issues that there might be with disk or memory requirements. The two tasks could be set off at the same time, and the processor in the computer would spend some time converting one video and then some time converting the other. Because the processor can execute only a single task at a time, only one video is actually being compressed at any one time. If the two videos show progress meters, the two meters will both head toward 100% completed, but it will take (roughly) twice as long to convert two videos as it would to convert a single video.

On a multicore system, there are two or more available cores that can perform the video conversion. Each core can work on one task. So, having the system work on two films at the same time will utilize two cores, and the conversion will take the same time as converting a single film. Twice as much work will have been achieved in the same time.

Multicore systems have the capability to do more work per unit time than single-core systems—two films can be converted in the same time that one can be converted on a single-core system. However, it’s possible to split the work in a different way. Perhaps the multiple cores can work together to convert the same film. In this way, a system with two cores could convert a single film twice as fast as a system with only one core.

This book is about using and developing for multicore systems. This is a topic that is often described as complex or hard to understand. In some way, this reputation is justified. Like any programming technique, multicore programming can be hard to do both correctly and with high performance. On the other hand, there are many ways that multicore systems can be used to significantly improve the performance of an application or the amount of work performed per unit time; some of these approaches will be more difficult than others.

Perhaps saying “multicore programming is easy” is too optimistic, but a realistic way of thinking about it is that multicore programming is perhaps no more complex or no more difficult than the step from procedural to object-oriented programming. This book will help you understand the challenges involved in writing applications that fully utilize multicore systems, and it will enable you to produce applications that are functionally correct, that are high performance, and that scale well to many cores.

Who Is This Book For?

If you have read this far, then this book is likely to be for you. The book is a practical guide to writing applications that are able to exploit multicore systems to their full advantage. It is not a book about a particular approach to parallelization. Instead, it covers various approaches. It is also not a book wedded to a particular platform. Instead, it pulls examples from various operating systems and various processor types. Although the book does cover advanced topics, these are covered in a context that will enable all readers to become familiar with them.

The book has been written for a reader who is familiar with the C programming language and has a fair ability at programming. The objective of the book is not to teach programming languages, but it deals with the higher-level considerations of writing code that is correct, has good performance, and scales to many cores.

The book includes a few examples that use SPARC or x86 assembly language. Readers are not expected to be familiar with assembly language, and the examples are straightforward, are clearly commented, and illustrate particular points.

Objectives of the Book

By the end of the book, the reader will understand the options available for writing programs that use multiple cores on UNIX-like operating systems (Linux, Oracle Solaris, OS X) and Windows. They will have an understanding of how the hardware implementation of multiple cores will affect the performance of the application running on the system (both in good and bad ways). The reader will also know the potential problems to avoid when writing parallel applications. Finally, they will understand how to write applications that scale up to large numbers of parallel threads.

Structure of This Book

This book is divided into the following chapters.

Chapter 1 introduces the hardware and software concepts that will be encountered in the rest of the book. The chapter gives an overview of the internals of processors. It is not necessarily critical for the reader to understand how hardware works before they can write programs that utilize multicore systems. However, an understanding of the basics of processor architecture will enable the reader to better understand some of the concepts relating to application correctness, performance, and scaling that are presented later in the book. The chapter also discusses the concepts of threads and processes.

Chapter 2 discusses profiling and optimizing applications. One of the book’s premises is that it is vital to understand where the application currently spends its time before work is spent on modifying the application to use multiple cores. The chapter covers all the leading contributors to performance over the application development cycle and discusses how performance can be improved.

Chapter 3 describes ways that multicore systems can be used to perform more work per unit time or reduce the amount of time it takes to complete a single unit of work. It starts with a discussion of virtualization where one new system can be used to replace multiple older systems. This consolidation can be achieved with no change in the software. It is important to realize that multicore systems represent an opportunity to change the way an application works; they do not require that the application be changed. The chapter continues with describing various patterns that can be used to write parallel applications and discusses the situations when these patterns might be useful.

Chapter 4 describes sharing data safely between multiple threads. The chapter leads with a discussion of data races, the most common type of correctness problem encountered in multithreaded codes. This chapter covers how to safely share data and synchronize threads at an abstract level of detail. The subsequent chapters describe the operating system–specific details.

Chapter 5 describes writing parallel applications using POSIX threads. This is the standard implemented by UNIX-like operating systems, such as Linux, Apple’s OS X, and Oracle’s Solaris. The POSIX threading library provides a number of useful building blocks for writing parallel applications. It offers great flexibility and ease of development.

Chapter 6 describes writing parallel applications for Microsoft Windows using Windows native threading. Windows provides similar synchronization and data sharing primitives to those provided by POSIX. The differences are in the interfaces and requirements of these functions.

Chapter 7 describes opportunities and limitations of automatic parallelization provided by compilers. The chapter also covers the OpenMP specification, which makes it relatively straightforward to write applications that take advantage of multicore processors.

Chapter 8 discusses how to write parallel applications without using the functionality in libraries provided by the operating system or compiler. There are some good reasons for writing custom code for synchronization or sharing of data. These might be for finer control or potentially better performance. However, there are a number of pitfalls that need to be avoided in producing code that functions correctly.

Chapter 9 discusses how applications can be improved to scale in such a way as to maximize the work performed by a multicore system. The chapter describes the common areas where scaling might be limited and also describes ways that these scaling limitations can be identified. It is in the scaling that developing for a multicore system is differentiated from developing for a multiprocessor system; this chapter discusses the areas where the implementation of the hardware will make a difference.

Chapter 10 covers a number of alternative approaches to writing parallel applications. As multicore processors become mainstream, other approaches are being tried to overcome some of the hurdles of writing correct, fast, and scalable parallel code.

Chapter 11 concludes the book.

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