Fundamentals of Quantitative Design and Analysis - Computer Architecture: A Quantitative Approach - page 17

are now 300 to 500 times cheaper per bit than DRAM. This technology is central to server
and warehouse scale storage, and we discuss the trends in detail in Appendix D.
■ Network technology —Network performance depends both on the performance of switches
and on the performance of the transmission system. We discuss the trends in networking
in Appendix F.
These rapidly changing technologies shape the design of a computer that, with speed and
technology enhancements, may have a lifetime of three to five years. Key technologies such
as DRAM, Flash, and disk change sufficiently that the designer must plan for these changes.
Indeed, designers often design for the next technology, knowing that when a product begins
shipping in volume that the next technology may be the most cost-effective or may have per-
formance advantages. Traditionally, cost has decreased at about the rate at which density in-
creases.
Although technology improves continuously, the impact of these improvements can be in
discrete leaps, as a threshold that allows a new capability is reached. For example, when MOS
technology reached a point in the early 1980s where between 25,000 and 50,000 transistors
could it on a single chip, it became possible to build a single-chip, 32-bit microprocessor. By
the late 1980s, first-level caches could go on a chip. By eliminating chip crossings within the
processor and between the processor and the cache, a dramatic improvement in cost-perform-
ance and energy-performance was possible. This design was simply infeasible until the tech-
nology reached a certain point. With multicore microprocessors and increasing numbers of
cores each generation, even server computers are increasingly headed toward a single chip for
all processors. Such technology thresholds are not rare and have a significant impact on a wide
variety of design decisions.
Performance Trends: Bandwidth Over Latency
As we shall see in Section 1.8 , bandwidth or throughput is the total amount of work done in a
given time, such as megabytes per second for a disk transfer. In contrast, latency or response
time is the time between the start and the completion of an event, such as milliseconds for a
disk access. Figure 1.9 plots the relative improvement in bandwidth and latency for techno-
logy milestones for microprocessors, memory, networks, and disks. Figure 1.10 describes the
examples and milestones in more detail.