Fundamentals of Quantitative Design and Analysis - Computer Architecture: A Quantitative Approach - page 24

FIGURE 1.11 Growth in clock rate of microprocessors in Figure 1.1 . Between 1978 and
1986, the clock rate improved less than 15% per year while performance improved by 25%
per year. During the “renaissance period” of 52% performance improvement per year between
1986 and 2003, clock rates shot up almost 40% per year. Since then, the clock rate has been
nearly flat, growing at less than 1% per year, while single processor performance improved at
less than 22% per year.
Distributing the power, removing the heat, and preventing hot spots have become increas-
ingly difficult challenges. Power is now the major constraint to using transistors; in the past, it
was raw silicon area. Hence, modern microprocessors offer many techniques to try to improve
energy efficiency despite flat clock rates and constant supply voltages:
1. Do nothing well . Most microprocessors today turn of the clock of inactive modules to save
energy and dynamic power. For example, if no floating-point instructions are executing,
the clock of the floating-point unit is disabled. If some cores are idle, their clocks are
stopped.
2. Dynamic Voltage-Frequency Scaling (DVFS) . The second technique comes directly from the
formulas above. Personal mobile devices, laptops, and even servers have periods of low
activity where there is no need to operate at the highest clock frequency and voltages.
Modern microprocessors typically offer a few clock frequencies and voltages in which to
operate that use lower power and energy. Figure 1.12 plots the potential power savings via
DVFS for a server as the workload shrinks for three different clock rates: 2.4 GHz, 1.8 GHz,
and 1 GHz. The overall server power savings is about 10% to 15% for each of the two steps.