A cell phone performs very different tasks, including streamingmusic, streaming video, and reading email. These tasks perform verydifferent computing tasks. Battery life and overheating are twocommon problems for cell phones, so reducing power and energyconsumption are critical. In this problem, we consider what to dowhen the user is not using the phone to its full computingcapacity. For these problems, we will evaluate an unrealisticscenario in which the cell phone has no specialized processingunits. Instead, it has a quad-core general purpose processing unit.Each core uses 0.5W at full use. For email-related tasks, thequad-core is 8x as fast as necessary.
1 How much dynamic energy and power are required compared torunning at full power? First, suppose that the quad-core operatesfor 1 / 8 of the time and is idle for the rest of the time. Thatis, the clock is disabled for 7 / 8 of the time, with no leakageoccurring during that time. Compare total dynamic energy as well asdynamic power while the core is running.
2 How much dynamic energy and power are required using frequencyand voltage scaling? Assume frequency and voltage are both reducedto 1 / 8 the entire time.
3 Now assume the voltage may not decrease below 50% of theoriginal voltage; otherwise, changes in logic state may occur (thisvoltage is often referred to as the voltage floor). Power can stillbe reduced to 1 / 8. What are the dynamic energy and power savingsin this case compared to the original design at full power?
4 How much energy would be used with a dark silicon approach(compared to the original quad-core design at power full)? Thisinvolves creating specialized ASIC hardware for each major task andpower gating those elements (to zero power) when not in use. Onlyone general-purpose core (instead of 4) would be provided, and therest of the chip would be filled with specialized units. For email,the one core would operate for 25% of the time and be turnedcompletely off with power gating for the other 75% of the time.During the other 75% of the time, a specialized ASIC unit thatrequires 20% of the energy of a core would be running.
