Computer Abstractions and Techno

1.7 The Power Wall

首先上一张图，是时钟频率和功耗的图：

这个图的背后，是一个公式在起作用：

也就是电容的功率计算。背后的原因在于：

The dominant technology for integrated circuits is called CMOS (complementary metal oxide semiconductor). For CMOS, the primary source of energy consumption is so-called dynamic energy—that is, energy that is consumed when transistors switch states from 0 to 1 and vice versa.

那么，从能耗方面就是：

其中：

Frequency switched is a function of the clock rate. The capacitive load per transistor is a function of both the number of transistors connected to an output (called the fanout) and the technology, which determines the capacitance of both wires and transistors.

这也就是为什么，时钟频率增长了1000倍，而功耗只增长了30倍？

Energy and thus power can be reduced by lowering the voltage, which occurred with each new generation of technology, and power is a function of the voltage squared. Typically, the voltage was reduced about 15% per generation. In 20 years, voltages have gone from 5V to 1V, which is why the increase in power is only 30 times.

现在的问题是，电压无法继续下降：

The modern problem is that further lowering of the voltage appears to make the transistors too leaky, like water faucets that cannot be completely shut off. Even today about 40% of the power consumption in server chips is due to leakage. If transistors started leaking more, the whole process could become unwieldy.

而冷却技术在经济上又不可持续、或者不可行：

Although there are many more expensive ways to cool chips and thereby raise their power to, say, 300 watts, these techniques are generally too costly for personal computers and even servers, not to mention personal mobile devices.

所以就产生了所谓的功耗墙，需要找到新的处理器设计方法。

两点展开：

Although dynamic energy is the primary source of energy consumption in CMOS, static energy consumption occurs because of leakage current that flows even when a transistor is off. In servers, leakage is typically responsible for 40% of the energy consumption. Thus, increasing the number of transistors increases power dissipation, even if the transistors are always off. A variety of design techniques and technology innovations are being deployed to control leakage, but it’s hard to lower voltage further.

Power is a challenge for integrated circuits for two reasons. First, power must be brought in and distributed around the chip; modern microprocessors use hundreds of pins just for power and ground! Similarly, multiple levels of chip interconnect are used solely for power and ground distribution to portions of the chip. Second, power is dissipated as heat and must be removed. Server chips can burn more than 100 watts, and cooling the chip and the surrounding system is a major expense in warehouse scale computers (see Chapter 6).