计算机组成与设计RISC-V版导读--Chapter1

Chapter1 Computer Abstractions and Technology

写过论文的人对abstraction一定不会陌生；不错，本章就是一些综述的内容，对于热衷技术的人而言肯定乏味的很，但作为一个行业管理者而言，可谓字字珠玑啊；

1.1 introduction

首先介绍了计算机科学在广阔领域的发展，主要是如下领域：

automobiles
cell phones
Human genome project
world wide web
search engines

然后总结传统计算机应用及其特点，主要分为3个领域：

PCs
Servers
Embedded computers

最后欢迎大家来到后PC时代，介绍后PC时代的特点：

最大特点就是PMD替换PC
PMD：Personal Mobile Device

那么读者可以从这本书学习到什么呢？答案如下：

高级软件语言（C,JAVA）如何转为硬件能理解的语言，硬件如何执行程序；
软硬件接口是什么，软件如何让硬件执行特定功能；
什么决定了程序的性能，怎么提高性能；
硬件工程师可利用什么技术提高性能；
硬件工程师可利用什么技术提高energy efficiency；
Parallelism的原因及其后续演进；
现代计算机架构中个great ideas；

1.2 Eight Great Ideas in Computer Architecture.

过去60年，计算机架构出现了8个great ideas

Design for Moore's Law
Use Abstraction to Simplify Design
Make the Common Case Fast
Performance via parallelism
Performance via Pipelining
Performance via Prediction
Hierarchy of Memories
Dependability via Redundancy

1.3 Below Your Program

Application software在system software之上，而system software又在hardware之上；

有两种system software：

operating system
compiler
将高级语言编写的程序翻译成机器能执行的指令；

From a High-Level Language to the Language of Hardware
机器所能理解的语言叫做
instruction：通常为binary形式的，如1001010100101110，此指令为两个数相加；

1.4 Under the Covers

以ipad2为例，从LCD到CPU的硬件简介。

1.5 Technologies for Buildig Processors and Memory

简单介绍半导体工艺
the cost of an integrated circuit can be expressed in 3 simple equations:
Cost per die=Cost per water/(Dies per wafer x yield)
Dies per wafer=wafer area /Die area
Yield=1/(1+(Defects per area x Die area/2))^2

1.6 Performance

Defining Performance
Measuring Performance
- time is the measure of computer performance，时间有不同的定义方式：
  - wall clock time 经过时间
  - response time 响应时间
  - elapsed time 运行时间
- CPU execution time/CPU time
  - user CPU time:单纯执行程序的时间
  - system CPU time：为了执行程序而调用操作系统的时间；
  - 二者很难精确区分；
- 一般我们喜欢用elapsed time来评价一个程序的性能，其实elapsed time和CPU time还是有区别的
  - we will use the term system performance to refer to elapsed time on an unloaded system and CPU performance to refer to user CPU time;
- 如一些server对IO很依赖，从而性能需要评价软硬件综合性能，而一些application则可能只关注throughput或者response time，或者两者的组合。所以为提高performance，你必须要知道哪些方面构成了performance matric matters，从而方便找到performance的瓶颈。
CPU Performance and its Factors
- 减少应用程序的时钟个数或者提高时钟频率
Instruction Performance
- 时钟周期数=指令个数指令平均时钟个数
- 指令平均时钟个数：clock cycles per instrunction,简称CPI
  - 每个指令执行的时钟个数不同，CPI为所有指令执行的时钟个数的平均数；
  - CPI provides one way of comparing two different implementations of the identical instruction set architecture，since the number of instructions executed for a program will, of course, be the same.
The Clasic CPU Performance Equation
- CPU time=Instrunction count CPIClock cycle time(or 1/Clock rate)
- 注意IPC=1/CPI，IPC: instructions per clock
- 注意现在很多CPU有变频特点，如Intel i7可以将频率提升10%直到CPU太热在降回来，这种技术被称为turbo技术；

1.7 The Power Wall

首先介绍了8代Intel CPU的频率和功耗走势图；需要注意的是在Pentium4（2001）时，频率达到3.6GHz,功耗达到103M，起后频率与功耗都有降低；

其次介绍单个晶体管的动态功耗(焦耳和瓦特角度）：
Energy1/2Capacitive load Voltages²
Power1/2Capacitive load Voltages² Frequency switched

Frequency switched和时钟频率相关；
with regard to the Figure, how could clock rates grow by a factor of 1000 while power increased by only a factor of 30?(频率有1000倍增长而功耗只有30倍增长)
Energy and thus power can be reduced by lowering the voltage，每代CPU都采用这种技术，即每代电压减少15%（0.0225）

In 20 years, voltages have gone from 5V to 1V, which is why the increase in power is only 30 times.(0.0225x1000=22.530)

现代的问题是，继续降低电压会使晶体管too leaky（服务器芯片40%功耗来源于leakage）

to try to adress the power problem, designers have already attached large devices to increase cooling. and they turn off parts of the chip that are not used in a given clock cycle.
尽管需要高昂的降温设备（300W功耗），这种方案还是应用于PC和server，而PMD则不需要；

Power is a challenge for integrated circuits for 2 reasons:

power must be brought in and distributed around the chip;现代芯片可能需要数百个ground和power的引脚
power is dissipated as heat and must be removed.(功耗消耗为热量需要更贵的散热设备)

1.8 The Sea Change: The Switch from Uniprocessors to Multiprocessors

为了解决power问题，引入了multiprocessor的概念，这就要求现代程序员必须要考虑重新编写已有的程序以适应multiprocers；

目前看程序员的转型还很少，期待未来的改变；
For parallel programming, the challenges include scheduling,load balancing,time for synchronization and overhead for communication between the parties .

后面介绍各个章节为parallel revolution而引入的内容；

1.9 Real Stuff: Benchmarking the Intel Core i7

每个章节的结束会举个实例来复习本章内容，第一章结束介绍Benchmark程序如何对不同的CPU进行评价；

如今的Benchmark大多出自SPEC：System Performance Evaluation Cooperative
感兴趣可以看看，具体不介绍了；

1.10 Fallacies and Pitfalls

每章的结束也会有谬论与陷阱这一小节；

1.11 Concluding Remarks

总结本章所有内容。

1.12 Historical Perspective and Further Reading

online reading