Computer Architecture: A Quantitative Approach

• Authors: John L. Hennessy;David A. Patterson
• Venue: Computer Architecture: A Quantitative Approach
• Year: 2003

From the Book:I am very lucky to have studied computer architecture under Prof. David Patterson at U.C. Berkeley more than 20 years ago. I enjoyed the courses I took from him, in the early days of RISC architecture. Since leaving Berkeley to help found Sun Microsystems, I have used the ideas from his courses and many more that are described in this important book. The good news today is that this book covers incredibly important and contemporary material. The further good news is that much exciting and challenging work remains to be done, and that working from Computer Architecture: A Quantitative Approach is a great way to start. The most successful architectural projects that I have been involved in have always started from simple ideas, with advantages explainable using simple numerical models derived from hunches and rules of thumb. The continuing rapid advances in computing technology and new applications ensure that we will need new similarly simple models to understand what is possible in the future, and that new classes of applications will stress systems in different and interesting ways. The quantitative approach introduced in Chapter 1 is essential to understanding these issues. In particular, we expect to see, in the near future, much more emphasis on minimizing power to meet the demands of a given application, across all sizes of systems; much remains to be learned in this area. I have worked with many different instruction sets in my career. I first programmed a PDP-8, whose instruction set was so simple that a friend easily learned to disassemble programs just by glancing at the hole punches in paper tape! I wrote a lot of code in PDP-11 assembler, including an interpreter for the Pascal programming language and for the VAX (which was used as an example in the first edition of this book); the success of the VAX led to the widespread use of UNIX on the early Internet. The PDP-11 and VAX were very conventional complex instruction set (CISC) computer architectures, with relatively compact instruction sets that proved nearly impossible to pipeline. For a number of years in public talks I used the performance of the VAX 11/780 as the baseline; its speed was extremely well known because faster implementations of the architecture were so long delayed. VAX performance stalled out just as the x86 and 680x0 CISC architectures were appearing in microprocessors; the strong economic advantages of microprocessors led to their overwhelming dominance. Then the simpler reduced instruction set (RISC) computer architectures聴pioneered by John Cocke at IBM; promoted and named by Patterson and Hennessy; and commercialized in POWER PC, MIPS, and SPARC聴were implemented as microprocessors and permitted highperformance pipeline implementations through the use of their simple registeroriented instruction sets. A downside of RISC was the larger code size of programs and resulting greater instruction fetch bandwidth, a cost that could be seen to be acceptable using the techniques of Chapter 1 and by believing in the future CMOS technology trends promoted in the now-classic views of Carver Mead. The kind of clear-thinking approach to the present problems and to the shape of future computing advances that led to RISC architecture is the focus of this book. Chapter 2 (and various appendices) presents interesting examples of contemporary and important historical instruction set architecture. RISC architecture聴the focus of so much work in the last twenty years聴is by no means the final word here. I worked on the design of the SPARC architecture and several implementations for a decade, but more recently have worked on two different styles of processor: picoJava, which implemented most of the Java Virtual Machine instructions聴a compact, high-level, bytecoded instruction set聴and MAJC, a very simple and multithreaded VLIW for Java and media-intensive applications. These two architectures addressed different and new market needs: for lowpower chips to run embedded devices where space and power are at a premium, and for high performance for a given amount of power and cost where parallel applications are possible. While neither has achieved widespread commercial success, I expect that the future will see many opportunities for different ISAs, and an in-depth knowledge of history here often gives great guidance聴the relationships between key factors, such as the program size, execution speed, and power consumption, returning to previous balances that led to great designs in the past. Chapters 3 and 4 describe instruction-level parallelism (ILP): the ability to execute more than one instruction at a time. This has been aided greatly, in the last 20 years, by techniques such as RISC and VLIW (very long instruction word) computing. But as later chapters here point out, both RISC and especially VLIW as practiced in the Intel itanium architecture are very power intensive. In our attempts to extract more instruction-level parallelism, we are running up against the fact that the complexity of a design that attempts to execute N instructions simultaneously grows like N2: the number of transistors and number of watts to produce each result increases dramatically as we attempt to execute many instructions of arbitrary programs simultaneously. There is thus a clear countertrend emerging: using simpler pipelines with more realistic levels of ILP while exploiting other kinds of parallelism by running both multiple threads of execution per processor and, often, multiple processors on a single chip. The challenge for designers of high-performance systems of the future is to understand when simultaneous execution is possible, but then to use these techniques judiciously in combination with other, less granular techniques that are less power intensive and complex. In graduate school I would often joke that cache memories were the only great idea in computer science. But truly, where you put things affects profoundly the design of computer systems. Chapter 5 describes the classical design of cache and main memory hierarchies and virtual memory. And now, new, higher-level programming languages like Java support much more reliable software because they insist on the use of garbage collection and array bounds checking, so that security breaches from "buffer overflow" and insidious bugs from false sharing of memory do not creep into large programs. It is only languages, such as Java, that insist on the use of automatic storage management that can implement true software components. But garbage collectors are notoriously hard on memory hierarchies, and the design of systems and language implementations to work well for such areas is an active area of research, where much good work has been done but much exciting work remains. Java also strongly supports thread-level parallelism聴a key to simple, powerefficient, and high-performance system implementations that avoids the N2 problem discussed earlier but brings challenges of its own. A good foundational understanding of these issues can be had in Chapter 6. Traditionally, each processor was a separate chip, and keeping the various processors synchronized was expensive, both because of its impact on the memory hierarchy and because the synchronization operations themselves were very expensive. The Java language is also trying to address these issues: we tried, in the Java Language Specification, which I coauthored, to write a description of the memory model implied by the language. While this description turned out to have (fixable) technical problems, it is increasingly clear that we need to think about the memory hierarchy in the design of languages that are intended to work well on the newer system platforms. We view the Java specification as a first step in much good work to be done in the future. As Chapter 7 describes, storage has evolved from being connected to individual computers to being a separate network resource. This is reminiscent of computer graphics, where graphics processing that was previously done in a host processor often became a separate function as the importance of graphics increased. All this is likely to change radically in the coming years聴massively parallel host processors are likely to be able to do graphics better than dedicated outboard graphics units, and new breakthroughs in storage technologies, such as memories made from molecular electronics and other atomic-level nanotechnologies, should greatly reduce both the cost of storage and the access time. The resulting dramatic decreases in storage cost and access time will strongly encourage the use of multiple copies of data stored on individual computing nodes, rather than shared over a network. The "wheel of reincarnation," familiar from graphics, will appear in storage. Chapter 8 provides a great foundational description of computer interconnects and networks. My model of these comes from Andy Bechtolsheim, another of the cofounders of Sun, who famously said, "Ethernet always wins."More modestly stated: given the need for a new networking interconnect, and despite its shortcomings, adapted versions of the Ethernet protocols seem to have met with overwhelming success in the marketplace. Why? Factors such as the simplicity and familiarity of the protocols are obvious, but quite possibly the most likely reason is that the people who are adapting Ethernet can get on with the job at hand rather than arguing about details that, in the end, aren聮t dispositive. This lesson can be generalized to apply to all the areas of computer architecture discussed in this book. One of the things I remember Dave Patterson saying many years ago is that for each new project you only get so many "cleverness beans." That is, you can be very clever in a few areas of your design, but if you try to be clever in all of them, the design will probably fail to achieve its goals聴or even fail to work or to be finished at all. The overriding lesson that I have learned in 20 plus years of working on these kinds of designs is that you must choose what is important and focus on that; true wisdom is to know what to leave out. A deep knowledge of what has gone before is key to this ability. And you must also choose your assumptions carefully. Many years ago I attended a conference in Hawaii (yes, it was a boondoggle, but read on) where Maurice Wilkes, the legendary computer architect, gave a speech. What he said, paraphrased in my memory, is that good research often consists of assuming something that seems untrue or unlikely today will become true and investigating the consequences of that assumption. And if the unlikely assumption indeed then becomes true in the world, you will have done timely and sometimes, then, even great research! So, for example, the research group at Xerox PARC assumed that everyone would have access to a personal computer with a graphics display connected to others by an internetwork and the ability to print inexpensively using Xerography. How true all this became, and how seminally important their work was! In our time, and in the field of computer architecture, I think there are a number of assumptions that will become true. Some are not controversial, such as that Moore聮s Law is likely to continue for another decade or so and that the complexity of large chip designs is reaching practical limits, often beyond the point of positive returns for additional complexity. More controversially, perhaps, molecular electronics is likely to greatly reduce the cost of storage and probably logic elements as well, optical interconnects will greatly increase the bandwidth and reduce the error rates of interconnects, software will continue to be unreliable because it is so difficult, and security will continue to be important because its absence is so debilitating. Taking advantage of the strong positive trends detailed in this book and using them to mitigate the negative ones will challenge the next generation of computer architects, to design a range of systems of many shapes and sizes. Computer architecture design problems are becoming more varied and interesting. Now is an exciting time to be starting out or reacquainting yourself with the latest in this field, and this book is the best place to start. See you in the chips!