IS CS-2019S2-04

题目来源：Problem 4 日期：2024-08-02 题目主题：CS-计算机体系结构-流水线与指令集

解题思路

本题涉及微处理器指令执行的基本原理及其性能优化方法，包括流水线处理、结构性冒险、数据冒险、控制冒险等概念。以下解答将包括逐条指令执行时间的计算、流水线方法的说明及其理论性能、管道冒险的解释，以及循环展开法如何避免这些冒险。

Solution

Question 1: Execution Time and Throughput without Pipelining

To determine the execution time per instruction, we need to sum up the time taken by each stage since instructions are executed sequentially.

• Fetch: $5\times 10^{-10}$ seconds
• Decode: $5\times 10^{-10}$ seconds
• Execute: $5\times 10^{-10}$ seconds
• Write: $5\times 10^{-10}$ seconds

The total execution time per instruction is:

$$\text{Execution Time}=4\times (5\times 10^{-10})=2\times 10^{-9}\text{ seconds/instruction}$$

The throughput, which is the number of instructions processed per second, is the inverse of the execution time:

$$\text{Throughput}=\frac{1}{2\times 10^{-9}}=5\times 10^8\text{ instructions/second}$$

Question 2: Execution with Pipelining

In the pipelining method, the execution of different stages for different instructions overlaps. For example, while the first instruction is in the Execute stage, the second instruction can be in the Decode stage, and the third instruction can be in the Fetch stage.

Assuming an ideal pipelining scenario (without any stalls or hazards), the time to process each instruction after the pipeline is filled is determined by the longest stage time, which is $5\times 10^{-10}$ seconds.

Thus, the ideal throughput with pipelining is:

$$\text{Ideal Throughput}=\frac{1}{5\times 10^{-10}}=2\times 10^9\text{ instructions/second}$$

Question 3: Pipeline Hazards

Pipeline hazards are situations that prevent the next instruction in the pipeline from executing in its designated clock cycle. There are three main types of hazards:

• Structural Hazard: Occurs when hardware resources are insufficient to support all active instructions. For example, if a pipeline requires two ALU operations simultaneously but there is only one ALU available, a structural hazard occurs.

• Data Hazard: Occurs when an instruction depends on the result of a previous instruction still in the pipeline. For instance, if an instruction needs the result of a previous ALU operation, but the result is not yet available, it has to wait.

• Control Hazard: Happens when the pipeline makes wrong decisions based on the next instruction (typically involving branches). For example, if the pipeline predicts a branch and loads the wrong instructions, it needs to discard them and fetch the correct ones, causing a delay.

Question 4: Loop Unrolling Method

Loop Unrolling is an optimization technique that aims to reduce the overhead of loop control instructions and improve the pipeline performance by increasing the loop body size. This technique duplicates the loop body multiple times, decreasing the number of iterations and thus reducing the frequency of control hazards (e.g., branch mispredictions).

For example, instead of looping four times to execute a set of instructions, the loop body can be expanded so that it is executed only twice, but each time performing twice the number of operations.

By reducing the number of iterations, loop unrolling can also help in reducing the number of data hazards, as it allows for better scheduling of instructions that use the same data.

知识点

微处理器流水线结构冒险数据冒险控制冒险

解题技巧和信息

• 流水线效率: 流水线技术提高了处理器的指令吞吐量，但受限于冒险等因素的影响。理想情况下，流水线的吞吐量等于处理器时钟频率与每个阶段的时间间隔的倒数。
• 冒险管理: 了解和识别不同类型的冒险可以帮助制定有效的冒险管理策略，例如数据冒险的前向传递和控制冒险的分支预测。
• 循环展开: 是一种常用的代码优化技术，有助于减少控制开销并提高指令并行性。

重点词汇

• Microprocessor (微处理器)
• Pipeline (流水线)
• Structural hazard (结构性冒险)
• Data hazard (数据冒险)
• Control hazard (控制冒险)
• Loop unrolling (循环展开)

参考资料

1. “Computer Architecture: A Quantitative Approach” by John L. Hennessy and David A. Patterson, Chapter 4 - Pipelining
2. “Computer Organization and Design” by David A. Patterson and John L. Hennessy, Chapter 6 - Pipelining and Hazards