Instruction-Level Parallelism and Its Exploitation - Computer Architecture: A Quantitative Approach - page 242

FIGURE 3.45 The relative performance and energy efficiency for a set of single-
threaded benchmarks shows the i7 920 is 4 to over 10 times faster than the Atom 230
but that it is about 2 times less power efficient on average! Performance is shown in the
columns as i7 relative to Atom, which is execution time (i7)/execution time (Atom). Energy is
shown with the line as Energy (Atom)/Energy (i7). The i7 never beats the Atom in energy effi-
ciency, although it is essentially as good on four benchmarks, three of which are floating point.
The data shown here were collected by Esmaeilzadeh et al. [2011] . The SPEC benchmarks
were compiled with optimization on using the standard Intel compiler, while the Java bench-
marks use the Sun (Oracle) Hotspot Java VM. Only one core is active on the i7, and the rest
are in deep power saving mode. Turbo Boost is used on the i7, which increases its perform-
ance advantage but slightly decreases its relative energy efficiency.
But, the average power consumption for the i7 is just under 43 W, while the average power
consumption of the Atom is 4.2 W, or about one-tenth of the power! Combining the perform-
ance and power leads to a energy efficiency advantage for the Atom that is typically more than
1.5 times beter and often 2 times beter! This comparison of two processors using the same
underlying technology makes it clear that the performance advantages of an aggressive su-
perscalar with dynamic scheduling and speculation come with a significant disadvantage in
energy efficiency.