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計算機結構與作業系統

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國立臺灣大學 2026 · 考試 · 第 12 題

You are evaluating the energy efficiency of two RISC-V dual-core processors running a specific workload. When only one core is active, the second core is power-gated (i.e., no power consumption). Once the workload completes, both cores are power-gated.

Workload: $10^9$ instructions, of which 75% are parallelizable (i.e., can be split across two cores), the remaining 25% are strictly serial.
Processor A: Two cores, Frequency: 3.0 GHz, Voltage: 1.1V, Avg. CPI (per core) = 1.2, Capacitance = 1.2 nF, Leakage Power = 500mW (total for 2 cores)
Processor B: Two cores, Frequency: 2.0 GHz, Voltage: 0.9V, Avg. CPI (per core) = 1, Capacitance = 1.0 nF, Leakage Power = 400mW (total for 2 cores)
Dynamic power = $C \times V^2 \times F$

Select correct description(s).

(A)
In both Processor A and Processor B, 25% of the total execution time is executed in serial mode.
(B)
Processor A has the same leakage energy as Processor B.
(C)
$2 < \text{Energy(A)} / \text{Energy(B)} < 2.3$ .
(D)
If the metric of energy efficiency is defined as Energy x Delay, Processor A is more energy efficient than Processor B.
(E)
Although Processor A operates at a higher frequency than Processor B, the target workload runs faster on Processor B due to its lower average CPI.

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