Chapter 3. Processors
The increase in microprocessor performance over the last 20 years has been phenomenal. The prediction made by Gordon Moore, the founder of Intel, that transistor counts on microprocessors would double every 18 months (and thus performance would increase proportionately), has certainly held true, if not been exceeded. In the early 1980s, the fastest microprocessor from Intel was the 4004, which was clocked at less than 4 KHz; by comparison, in mid-2001, the latest generations of IA-32 processors from Intel and AMD are running at clock rates of about 2.0 GHz -- and execute more than one instruction per clock cycle (I survey some the key elements in microprocessor design later in this chapter). This jump is so huge that it is hard to comprehend. If automobile performance followed the same growth curve as microprocessors over the last 20 years, the successor to the DeLorean DMC-12 (1981-1983), which had a top speed of perhaps 140 miles per hour , [1] would have doubled 12 times: it would travel at almost 287 thousand miles per hour, which is equivalent to about 410 times the speed of sound, or about one-sixteenth of the speed of light. Even that's pretty difficult to comprehend. If a pack of bubble gum cost a quarter in 1983, and prices increased at the same rate as microprocessor performance, that same pack of gum would cost just over a thousand dollars.
It's not clear whether this rate of increase is sustainable. It has often been predicted that the rate will have to slow. However, enterprising engineers have found ways to improve the technologies used for processor fabrication and design so that this has not yet become the case. The fact that microprocessor performance increases have far outpaced the performance increases of the other components in a computer system is a unifying theme for this book: how do we make the other components go as fast as possible, to try and optimize the efficiency of the processor? Processor performance is without a doubt the most-quoted figure in computer performance. This can easily be verified simply by going to the local computer store and looking at their advertisements; the information in the largest print, often set apart and highlighted, is the clock rate of the microprocessor. It is much more appealing to have the latest 2.0-GHz processor than a few very fast hard drives . However, while it is absolutely true that microprocessor performance is a critical component of overall system performance, it is often overrated. Despite the central importance of the microprocessor, to most system administrators it is little more than a replaceable black ceramic box with a heat sink attached. While it's true that most systems administration tasks don't require any significant knowledge of electrical or computer engineering, performance tuning is at heart about understanding how things work -- it's very difficult to improve something if you don't have any idea how it works. For processor performance, this foundation is microprocessor design. In this chapter, I move from discussing basic microprocessor architecture to a host of supporting areas: caches, which play a vital role in the performance of modern processors; process scheduling, or how the operating system decides what processes should have priority in using the CPU; multiprocessing; the interconnects used to connect processors to other processors and to peripheral devices; and, finally, I discuss some tools that can be used to monitor microprocessor performance. |
