优化网页性能：Even Faster Web Sites

Even.Faster.Web.Sites

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"Even Faster Web Sites" 《Even Faster Web Sites》是由Steve Souders撰写的一本关于优化Web性能的专业书籍。这本书深入探讨了如何通过各种技术和策略来提高网站的速度和效率，从而提供更好的用户体验。Steve Souders是网页性能优化领域的权威人物，他在书中分享了他的研究成果和实践经验。该书的核心目标是帮助开发者理解并解决Web性能问题，通过减少页面加载时间，提高用户满意度和网站的搜索引擎排名。内容涵盖了多个关键主题，包括： 1. **减少HTTP请求**：每个HTTP请求都会增加页面加载的时间，因此减少请求次数是提升速度的关键。这可以通过合并CSS和JavaScript文件、使用CSS精灵图（Sprite）以及内联小的CSS和JavaScript来实现。 2. **压缩内容**：通过Gzip压缩可以显著减小HTML、CSS和JavaScript文件的大小，从而加快传输速度。 3. **优化图片**：使用正确的图片格式（如JPEG、PNG、SVG等），进行适当的压缩，并利用CSS3的背景裁剪和渐进式加载技术来优化图片展示。 4. **缓存利用**：利用浏览器缓存和HTTP头信息，使用户在二次访问时能更快地加载页面。 5. **减少DNS查找**：减少DNS查找次数可以加快页面元素的加载，这可以通过合并域名或使用DNS预加载来实现。 6. **异步加载**：使用异步加载JavaScript和CSS，防止阻塞页面渲染。 7. **前端性能最佳实践**：遵循最佳编码和设计实践，如避免重定向、减少DOM元素数量、优化CSS选择器等。 8. **服务器和网络层面的优化**：调整服务器配置，如启用HTTP/2，使用CDN（内容分发网络）来减少延迟和带宽消耗。 9. **性能监测与分析**：学习如何使用工具（如PageSpeed Insights、YSlow等）来评估和改进网站性能。 10. **移动优先**：考虑到移动设备的网络限制，优先考虑移动设备的优化策略。通过这些方法，开发者可以实现更高效、响应更快的Web站点，满足用户对快速加载和流畅体验的需求。这本书不仅提供了理论知识，还包含了许多实际操作的技巧和案例研究，对于任何希望提升Web性能的专业人士来说，都是一份宝贵的资源。

• Rule 5: Put Stylesheets at the Top

• Rule 6: Put Scripts at the Bottom

• Rule 7: Avoid CSS Expressions

• Rule 8: Make JavaScript and CSS External

• Rule 9: Reduce DNS Lookups

• Rule 10: Minify JavaScript

• Rule 11: Avoid Redirects

• Rule 12: Remove Duplicate Scripts

• Rule 13: Configure ETags

• Rule 14: Make Ajax Cacheable

I call them “rules” because there is little ambiguity about their adoption. Consider these

statistics for the top 10 U.S. web sites

for March 2007:

• Two sites used CSS sprites.

• 26% of resources had a future Expires header.

• Five sites compressed their HTML, JavaScript, and CSS.

• Four sites minified their JavaScript.

The same statistics for April 2009 show that these rules are gaining traction:

• Nine sites use CSS sprites.

• 93% of resources have a future Expires header.

• Ten sites compress their HTML, JavaScript, and CSS.

• Nine sites minify their JavaScript.

The rules from High Performance Web Sites still apply and are where most web com-

panies should start. Progress is being made, but there’s still more work to be done on

this initial set of rules.

But the Web isn’t standing still, waiting for us to catch up. Although the 14 rules from

High Performance Web Sites still apply, the growth in web page content and Web 2.0

applications introduces a new set of performance challenges. Even Faster Web Sites

provides the best practices needed by developers to make these next-generation web

sites faster.

The chapters in this book are organized into three areas: JavaScript performance

(Chapters 1–7), network performance (Chapters 8–12), and browser performance

(Chapters 13 and 14). A roundup of the best tools for analyzing performance comes in

the Appendix.

AOL, eBay, Facebook, Google Search, Live Search, MSN.com, MySpace, Wikipedia, Yahoo!, and YouTube,

according to Alexa.

xiv | Preface

Six of the chapters were written by contributing authors:

• Chapter 1, Understanding Ajax Performance, by Douglas Crockford

• Chapter 2, Creating Responsive Web Applications, by Ben Galbraith and Dion

Almaer

• Chapter 7, Writing Efficient JavaScript, by Nicholas C. Zakas

• Chapter 8, Scaling with Comet, by Dylan Schiemann

• Chapter 9, Going Beyond Gzipping, by Tony Gentilcore

• Chapter 10, Optimizing Images, by Stoyan Stefanov and Nicole Sullivan

These authors are experts in each of these areas. I wanted you to hear from them

directly, in their own voices. To help identify these chapters, the name(s) of the con-

tributing author(s) are on the chapter’s opening page.

JavaScript Performance

In my work analyzing today’s web sites, I consistently see that JavaScript is the key to

better-performing web applications, so I’ve started the book with these chapters.

Douglas Crockford wrote Chapter 1, Understanding Ajax Performance. Doug describes

how Ajax changes the way browsers and servers interact, and how web developers need

to understand this new relationship to properly identify opportunities for improving

performance.

Chapter 2, Creating Responsive Web Applications, by Ben Galbraith and Dion Almaer,

ties JavaScript performance back to what really matters: the user experience. Today’s

web applications invoke complex functions at the click of a button and must be eval-

uated on the basis of what they’re forcing the browser to do. The web applications that

succeed will be written by developers who understand the effects of their code on

response time.

I wrote the next four chapters. They focus on the mechanics of JavaScript—the best

way to package it and load it, and where to insert it in your pages. Chapter 3, Splitting

the Initial Payload, describes the situation facing many web applications today: a huge

JavaScript download at the beginning of the page that blocks rendering as well as further

downloads. The key is to break apart this monolithic JavaScript for more efficient

Chapters 4 and 5 go together. In today’s most popular browsers, external scripts block

everything else in the page. Chapter 4, Loading Scripts Without Blocking, explains how

to avoid these pitfalls when loading external scripts. Loading scripts asynchronously

presents a challenge when inlined code depends on them. Luckily, there are several

techniques for coupling inlined code with the asynchronous scripts on which they de-

pend. These techniques are presented in Chapter 5, Coupling Asynchronous Scripts.

Preface | xv

Chapter 6, Positioning Inline Scripts, presents performance best practices that apply to

inline scripts, especially the impact they have on blocking parallel downloads.

I think of Chapter 7, Writing Efficient JavaScript, written by Nicholas C. Zakas, as the

complement to Doug’s chapter (Chapter 1). Whereas Doug describes the Ajax land-

scape, Nicholas zooms in on several specific techniques for speeding up JavaScript.

Network Performance

Web applications aren’t desktop applications—they have to be downloaded over the

Internet each time they are used. The adoption of Ajax has resulted in a new style of

data communication between servers and clients. Some of the biggest opportunities for

growth in the web industry are in emerging markets where Internet connectivity is a

challenge, to put it mildly. All of these factors highlight the need for improved network

performance.

In Chapter 8, Scaling with Comet, Dylan Schiemann describes an architecture that goes

beyond Ajax to provide high-volume, low-latency communication for real-time appli-

cations such as chat and document collaboration.

Chapter 9, Going Beyond Gzipping, describes how turning on compression isn’t enough

to guarantee optimal delivery of your web site’s content. Tony Gentilcore reveals a

little-known phenomenon that severely hinders the network performance of 15% of

the world’s Internet users.

Stoyan Stefanov and Nicole Sullivan team up to contribute Chapter 10, Optimizing

Images. This is a thorough treatment of the topic. This chapter reviews all popular

image formats, presents numerous image optimization techniques, and describes the

image compression tools of choice.

The remaining chapters were written by me. Chapter 11, Sharding Dominant Do-

mains, reminds us of the connection limits in the popular browsers of today, as well as

the next generation of browsers. It includes techniques for successfully splitting

resources across multiple domains.

Chapter 12, Flushing the Document Early, walks through the benefits and many gotchas

of using chunked encoding to start rendering the page even before the full HTML

document has arrived.

Browser Performance

Iframes are an easy and frequently used technique for embedding third-party content

in a web page. But they come with a cost. Chapter 13, Using Iframes Sparingly, explains

the downsides of iframes and offers a few alternatives.

Chapter 14, Simplifying CSS Selectors, presents the theories about how complex selec-

tors can impact performance, and then does an objective analysis to pinpoint the

situations that are of most concern.

xvi | Preface

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We appreciate, but do not require, attribution. An attribution usually includes the title,

author, publisher, and ISBN. For example: “Even Faster Web Sites, by Steve Souders.

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Acknowledgments

I first want to thank the contributing authors: Dion Almaer, Doug Crockford, Ben

Galbraith, Tony Gentilcore, Dylan Schiemann, Stoyan Stefanov, Nicole Sullivan, and

Nicholas Zakas. They’ve made this a special book. Each of them is an expert in his or

her own right. Most of them have written their own books. By sharing their expertise,

they’ve helped create something unique.

xviii | Preface

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优化网页性能：Even Faster Web Sites

[高性能网站建设进阶指南].(Even.Faster.Web.Sites).Steve.Souders.文字版

Even Faster Web Sites: Performance Best Practices for Web Developers (PDF 电子书)

fasterrcnn的特点

pytorch.fasterrcnn 训练自己的数据集

model = models.detection.fasterrcnn_resnet50_fpn(pretrained=True)

FINAL_FEATURES = self.fasterRCNN._head_to_tail(FINAL_FEATURES)这句代码的作用

faster rcnn

加载预训练的Faster R-CNN模型

faster-RCNN

Faster R-CNN

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