As mentioned in Exercise 1.4, cell phones run a wide variety of applications. We’ll make the same assumptions for this exercise as the previous one, that it is

Exercise 1.4

A cell phone performs very different tasks, including streaming music, streaming video, and reading email. These tasks perform very different computing tasks. Battery life and overheating are two common problems for cell phones, so reducing power and energy consumption are critical. In this problem,

we consider what to do when the user is not using the phone to its full computing capacity. For these problems, we will evaluate an unrealistic scenario in which the cell phone has no specialized processing units. Instead, it has a quad-core, general purpose processing unit. Each core uses 0.5 W at full use. For email-related tasks,

the quad-core is 8 as fast as necessary.

a. How much dynamic energy and power are required compared to running at full power? First, suppose that the quad-core operates for 1/8 of the time and is idle for the rest of the time. That is, the clock is disabled for 7/8 of the time, with no leakage occurring during that time. Compare total dynamic energy as well as dynamic power while the core is running.

b. How much dynamic energy and power are required using frequency and voltage scaling? Assume frequency and voltage are both reduced to 1/8 the entire time.

c. Now assume the voltage may not decrease below 50% of the original voltage. This voltage is referred to as the voltage floor, and any voltage lower than that will lose the state. Therefore, while the frequency can keep decreasing, the voltage cannot. What are the dynamic energy and power savings in this case?

d. How much energy is used with a dark silicon approach? This involves creating specialized ASIC hardware for each major task and power gating those elements when not in use. Only one general-purpose core would be provided, and the rest of the chip would be filled with specialized units.

For email, the one core would operate for 25% the time and be turned completely off with power gating for the other 75% of the time. During the other 75% of the time, a specialized ASIC unit that requires 20% of the energy of a core would be running. 0.5 W per core and that a quad core runs email 3 as fast.

a. Imagine that 80% of the code is parallelizable. By how much would the frequency and voltage on a single core need to be increased in order to execute at the same speed as the four-way parallelized code?

b. What is the reduction in dynamic energy from using frequency and voltage scaling in part a?

c. How much energy is used with a dark silicon approach? In this approach, all hardware units are power gated, allowing them to turn off entirely

(causing no leakage). Specialized ASICs are provided that perform the same computation for 20% of the power as the general-purpose processor. Imagine that each core is power gated. The video game requires two ASICS and two cores. How much dynamic energy does it require compared to the baseline of parallelized on four cores?