computer system, we are talking about energy flows: the rate at which energy must be provided to the computer system from a battery or wall outlet, which is the same as the rate at which that energy, now converted to heat, must be extracted from the system. The temperature of the chip or system rises above the ambient temperature and causes heat energy to flow into the environment. To limit the system’s temperature rise, heat must be extracted efficiently. (Transferring heat from the computer system to the environment is the task of the cooling subsystem in a computer.) Thus, referring to a chip’s power requirements is equivalent to talking about power consumed and dissipated.
When we talk about scaling computing performance, we implicitly mean to increase the computing performance that we can buy for each dollar we spend. If we cannot scale down the energy per function as fast as we scale up the performance (functions per second), the power (energy per second) consumed by the system will rise, and the increase in power consumption will increase the cost of the system. More expensive hardware will be needed to supply the extra power and then remove the heat that it generates. The cost of managing the power in and out of the system will rise to dominate the cost of the hardware.
Historically, technology scaling has done a good job of scaling down energy cost per function as the total cost per function dropped, and so the overall power needs of systems were relatively constant as performance (functions per second) dramatically increased. Recently, the cost per function has been dropping faster than the power per function, which means that the overall power of constant-cost chips has been growing. The power problem is getting worse because of the recent difficulty in continuing to scale down power-supply voltages, as is described later in this chapter.
Our ability to supply power and cool chips is not improving rapidly, so for many computers the performance per dollar is now limited by power issues. In addition, computers are increasingly available in a variety of form-factors and many, such as cell phones, have strict power limits because of user constraints. People do not want to hold hot cell phones, and so the total power budget needs to be under a few watts when the phone is active. Designers today must therefore find the best performance that can be achieved within a specified power envelope.
To assist in understanding these issues, this chapter reviews integrated circuit (IC) technology scaling. It assumes general knowledge of electric circuits, and some readers may choose to review the findings listed here and then move directly to Chapter 4. The basic conclusions of this chapter are as follows: