Best Practices for Speeding up Your Web Site
Minimize HTTP Requests
tag: content
80% of the end-user response time is spent on the front-end. Most of this
time is tied up in downloading all the components in the page: images,
stylesheets, scripts, Flash, etc. Reducing the number of components in turn
reduces the number of HTTP requests required to render the page. This is the
key to faster pages.
One way to reduce the number of components in the page is to simplify the
page's design. But is there a way to build pages with richer content while also
achieving fast response times? Here are some techniques for reducing the number
of HTTP requests, while still supporting rich page designs.
Combined files are a way to
reduce the number of HTTP requests by combining all scripts into a single
script, and similarly combining all CSS into a single stylesheet. Combining
files is more challenging when the scripts and stylesheets vary from page to
page, but making this part of your release process improves response times.
are the preferred method for
reducing the number of image requests. Combine your background images into a
single image and use the CSS background-image and background-position properties to display the desired image segment.
combine multiple images into a
single image. The overall size is about the same, but reducing the number of
HTTP requests speeds up the page. Image maps only work if the images are
contiguous in the page, such as a navigation bar. Defining the coordinates of
image maps can be tedious and error prone. Using image maps for navigation is
not accessible too, so it's not recommended.
Inline images use the to
embed the image data in the actual page. This can increase the size of your
HTML document. Combining inline images into your (cached) stylesheets is a way
to reduce HTTP requests and avoid increasing the size of your pages. Inline
images are not yet supported across all major browsers.
Reducing the number of HTTP requests in your page is the place to start.
This is the most important guideline for improving performance for first time
visitors. As described in Tenni Theurer's blog post Browser Cache Usage - Exposed!, 40-60% of daily
visitors to your site come in with an empty cache. Making your page fast for
these first time visitors is key to a better user experience.
Use a Content Delivery Network
tag: server
The user's proximity to your web server has an impact on response times.
Deploying your content across multiple, geographically dispersed servers will
make your pages load faster from the user's perspective. But where should you
start?
As a first step to implementing geographically dispersed content, don't
attempt to redesign your web application to work in a distributed architecture.
Depending on the application, changing the architecture could include daunting
tasks such as synchronizing session state and replicating database transactions
across server locations. Attempts to reduce the distance between users and your
content could be delayed by, or never pass, this application architecture step.
Remember that 80-90% of the end-user response time is spent downloading
all the components in the page: images, stylesheets, scripts, Flash, etc. This
is the Performance Golden Rule. Rather than starting with the difficult
task of redesigning your application architecture, it's better to first
disperse your static content. This not only achieves a bigger reduction in
response times, but it's easier thanks to content delivery networks.
A content delivery network (CDN) is a collection of web servers
distributed across multiple locations to deliver content more efficiently to
users. The server selected for delivering content to a specific user is
typically based on a measure of network proximity. For example, the server with
the fewest network hops or the server with the quickest response time is
chosen.
Some large Internet companies own their own CDN, but it's cost-effective
to use a CDN service provider, such as , Mirror Image
Internet, or . For start-up companies and
private web sites, the cost of a CDN service can be prohibitive, but as your
target audience grows larger and becomes more global, a CDN is necessary to
achieve fast response times. At Yahoo!, properties that moved static content
off their application web servers to a CDN improved end-user response times by
20% or more. Switching to a CDN is a relatively easy code change that will
dramatically improve the speed of your web site.
Add an Expires or a Cache-Control Header
tag: server
There are two things in this rule:
- For static
components: implement "Never expire" policy by setting far
future Expires header
- For dynamic
components: use an appropriate Cache-Control header to help the browser with conditional requests
Web page designs are getting richer and richer, which means more scripts,
stylesheets, images, and Flash in the page. A first-time visitor to your page
may have to make several HTTP requests, but by using the Expires header you
make those components cacheable. This avoids unnecessary HTTP requests on
subsequent page views. Expires headers are most often used with images, but
they should be used on all components including scripts, stylesheets,
and Flash components.
Browsers (and proxies) use a cache to reduce the number and size of HTTP
requests, making web pages load faster. A web server uses the Expires header in
the HTTP response to tell the client how long a component can be cached. This
is a far future Expires header, telling the browser that this response won't be
stale until April 15, 2010.
Expires: Thu,
15 Apr 2010 20:00:00 GMT
If your server is Apache, use the ExpiresDefault directive to set an
expiration date relative to the current date. This example of the
ExpiresDefault directive sets the Expires date 10 years out from the time of
the request.
ExpiresDefault "access plus 10 years"
Keep in mind, if you use a far future Expires header you have to change
the component's filename whenever the component changes. At Yahoo! we often
make this step part of the build process: a version number is embedded in the
component's filename, for example, yahoo_2.0.6.js.
Using a far future Expires header affects page views only after a user has
already visited your site. It has no effect on the number of HTTP requests when
a user visits your site for the first time and the browser's cache is empty.
Therefore the impact of this performance improvement depends on how often users
hit your pages with a primed cache. (A "primed cache" already
contains all of the components in the page.) We measured this at Yahoo! and found the number of
page views with a primed cache is 75-85%. By using a far future Expires header,
you increase the number of components that are cached by the browser and
re-used on subsequent page views without sending a single byte over the user's
Internet connection.
Gzip Components
tag: server
The time it takes to transfer an HTTP request and response across the
network can be significantly reduced by decisions made by front-end engineers.
It's true that the end-user's bandwidth speed, Internet service provider,
proximity to peering exchange points, etc. are beyond the control of the
development team. But there are other variables that affect response times.
Compression reduces response times by reducing the size of the HTTP response.
Starting with HTTP/1.1, web clients indicate support for compression with
the Accept-Encoding header in the HTTP request.
Accept-Encoding: gzip, deflate
If the web server sees this header in the request, it may compress the
response using one of the methods listed by the client. The web server notifies
the web client of this via the Content-Encoding header in the response.
Content-Encoding: gzip
Gzip is the most popular and effective compression method at this time. It
was developed by the GNU project and standardized by .
The only other compression format you're likely to see is deflate, but it's
less effective and less popular.
Gzipping generally reduces the response size by about 70%. Approximately
90% of today's Internet traffic travels through browsers that claim to support
gzip. If you use Apache, the module configuring gzip depends on your version:
Apache 1.3 uses while Apache 2.x uses .
There are known issues with browsers and proxies that may cause a mismatch
in what the browser expects and what it receives with regard to compressed
content. Fortunately, these edge cases are dwindling as the use of older
browsers drops off. The Apache modules help out by adding appropriate Vary
response headers automatically.
Servers choose what to gzip based on file type, but are typically too
limited in what they decide to compress. Most web sites gzip their HTML
documents. It's also worthwhile to gzip your scripts and stylesheets, but many
web sites miss this opportunity. In fact, it's worthwhile to compress any text
response including XML and JSON. Image and PDF files should not be gzipped
because they are already compressed. Trying to gzip them not only wastes CPU
but can potentially increase file sizes.
Gzipping as many file types as possible is an easy way to reduce page
weight and accelerate the user experience.
Put Stylesheets at the Top
tag: css
While researching performance at Yahoo!, we discovered that moving
stylesheets to the document HEAD makes pages appear to be loading
faster. This is because putting stylesheets in the HEAD allows the page to
render progressively.
Front-end engineers that care about performance want a page to load
progressively; that is, we want the browser to display whatever content it has
as soon as possible. This is especially important for pages with a lot of
content and for users on slower Internet connections. The importance of giving
users visual feedback, such as progress indicators, has been well researched
and documented. In our case the HTML page is the
progress indicator! When the browser loads the page progressively the header,
the navigation bar, the logo at the top, etc. all serve as visual feedback for
the user who is waiting for the page. This improves the overall user
experience.
The problem with putting stylesheets near the bottom of the document is
that it prohibits progressive rendering in many browsers, including Internet
Explorer. These browsers block rendering to avoid having to redraw elements of
the page if their styles change. The user is stuck viewing a blank white page.
The clearly states that
stylesheets are to be included in the HEAD of the page: "Unlike A, [LINK]
may only appear in the HEAD section of a document, although it may appear any
number of times." Neither of the alternatives, the blank white screen or
flash of unstyled content, are worth the risk. The optimal solution is to
follow the HTML specification and load your stylesheets in the document HEAD.
Put Scripts at the Bottom
tag: javascript
The problem caused by scripts is that they block parallel downloads. The suggests that browsers
download no more than two components in parallel per hostname. If you serve
your images from multiple hostnames, you can get more than two downloads to
occur in parallel. While a script is downloading, however, the browser won't
start any other downloads, even on different hostnames.
In some situations it's not easy to move scripts to the bottom. If, for
example, the script uses document.write to insert part of the page's
content, it can't be moved lower in the page. There might also be scoping
issues. In many cases, there are ways to workaround these situations.
An alternative suggestion that often comes up is to use deferred scripts.
The DEFER attribute indicates that the script does not contain document.write,
and is a clue to browsers that they can continue rendering. Unfortunately,
Firefox doesn't support the DEFER attribute. In Internet Explorer,
the script may be deferred, but not as much as desired. If a script can be
deferred, it can also be moved to the bottom of the page. That will make your
web pages load faster.
Avoid CSS Expressions
tag: css
CSS expressions are a powerful (and dangerous) way to set CSS properties
dynamically. They're supported in Internet Explorer, starting with . As an example, the background color
could be set to alternate every hour using CSS expressions.
background-color: expression( (new Date()).getHours()%2 ?
"#B8D4FF" : "#F08A00" );
As shown here, the expression method accepts a JavaScript
expression. The CSS property is set to the result of evaluating the JavaScript
expression. The expression method is ignored by other browsers, so it is useful for
setting properties in Internet Explorer needed to create a consistent experience
across browsers.
The problem with expressions is that they are evaluated more frequently
than most people expect. Not only are they evaluated when the page is rendered
and resized, but also when the page is scrolled and even when the user moves
the mouse over the page. Adding a counter to the CSS expression allows us to
keep track of when and how often a CSS expression is evaluated. Moving the
mouse around the page can easily generate more than 10,000 evaluations.
One way to reduce the number of times your CSS expression is evaluated is
to use one-time expressions, where the first time the expression is evaluated
it sets the style property to an explicit value, which replaces the CSS
expression. If the style property must be set dynamically throughout the life
of the page, using event handlers instead of CSS expressions is an alternative
approach. If you must use CSS expressions, remember that they may be evaluated
thousands of times and could affect the performance of your page.
Make JavaScript and CSS External
tag: javascript, css
Many of these performance rules deal with how external components are
managed. However, before these considerations arise you should ask a more basic
question: Should JavaScript and CSS be contained in external files, or inlined
in the page itself?
Using external files in the real world generally produces faster pages
because the JavaScript and CSS files are cached by the browser. JavaScript and
CSS that are inlined in HTML documents get downloaded every time the HTML
document is requested. This reduces the number of HTTP requests that are
needed, but increases the size of the HTML document. On the other hand, if the
JavaScript and CSS are in external files cached by the browser, the size of the
HTML document is reduced without increasing the number of HTTP requests.
The key factor, then, is the frequency with which external JavaScript and
CSS components are cached relative to the number of HTML documents requested.
This factor, although difficult to quantify, can be gauged using various
metrics. If users on your site have multiple page views per session and many of
your pages re-use the same scripts and stylesheets, there is a greater
potential benefit from cached external files.
Many web sites fall in the middle of these metrics. For these sites, the
best solution generally is to deploy the JavaScript and CSS as external files.
The only exception where inlining is preferable is with home pages, such as
and .
Home pages that have few (perhaps only one) page view per session may find that
inlining JavaScript and CSS results in faster end-user response times.
For front pages that are typically the first of many page views, there are
techniques that leverage the reduction of HTTP requests that inlining provides,
as well as the caching benefits achieved through using external files. One such
technique is to inline JavaScript and CSS in the front page, but dynamically
download the external files after the page has finished loading. Subsequent
pages would reference the external files that should already be in the
browser's cache.
Reduce DNS Lookups
tag: content
The Domain Name System (DNS) maps hostnames to IP addresses, just as
phonebooks map people's names to their phone numbers. When you type
into your browser, a DNS resolver contacted by the browser
returns that server's IP address. DNS has a cost. It typically takes 20-120
milliseconds for DNS to lookup the IP address for a given hostname. The browser
can't download anything from this hostname until the DNS lookup is completed.
DNS lookups are cached for better performance. This caching can occur on a
special caching server, maintained by the user's ISP or local area network, but
there is also caching that occurs on the individual user's computer. The DNS
information remains in the operating system's DNS cache (the "DNS Client
service" on Microsoft Windows). Most browsers have their own caches,
separate from the operating system's cache. As long as the browser keeps a DNS
record in its own cache, it doesn't bother the operating system with a request
for the record.
Internet Explorer caches DNS lookups for 30 minutes by default, as
specified by the DnsCacheTimeout registry setting. Firefox caches DNS lookups for 1
minute, controlled by the network.dnsCacheExpiration configuration
setting. (Fasterfox changes this to 1 hour.)
When the client's DNS cache is empty (for both the browser and the
operating system), the number of DNS lookups is equal to the number of unique
hostnames in the web page. This includes the hostnames used in the page's URL,
images, script files, stylesheets, Flash objects, etc. Reducing the number of
unique hostnames reduces the number of DNS lookups.
Reducing the number of unique hostnames has the potential to reduce the
amount of parallel downloading that takes place in the page. Avoiding DNS
lookups cuts response times, but reducing parallel downloads may increase
response times. My guideline is to split these components across at least two
but no more than four hostnames. This results in a good compromise between
reducing DNS lookups and allowing a high degree of parallel downloads.
Minify JavaScript and CSS
tag: javascript, css
Minification is the practice of removing unnecessary characters from code
to reduce its size thereby improving load times. When code is minified all
comments are removed, as well as unneeded white space characters (space,
newline, and tab). In the case of JavaScript, this improves response time
performance because the size of the downloaded file is reduced. Two popular
tools for minifying JavaScript code are
and YUI Compressor. The YUI compressor can also
minify CSS.
Obfuscation is an alternative optimization that can be applied to source
code. It's more complex than minification and thus more likely to generate bugs
as a result of the obfuscation step itself. In a survey of ten top U.S. web
sites, minification achieved a 21% size reduction versus 25% for obfuscation.
Although obfuscation has a higher size reduction, minifying JavaScript is less
risky.
In addition to minifying external scripts and styles, inlined and blocks can and should also be
minified. Even if you gzip your scripts and styles, minifying them will still
reduce the size by 5% or more. As the use and size of JavaScript and CSS
increases, so will the savings gained by minifying your code.
Avoid Redirects
tag: content
Redirects are accomplished using the 301 and 302 status codes. Here's an
example of the HTTP headers in a 301 response:
HTTP/1.1 301
Moved Permanently
Location:
Content-Type:
text/html
The browser automatically takes the user to the URL specified in the Location field. All the information necessary for a redirect is in the headers.
The body of the response is typically empty. Despite their names, neither a 301
nor a 302 response is cached in practice unless additional headers, such as Expires or Cache-Control, indicate it should be. The meta refresh tag and
JavaScript are other ways to direct users to a different URL, but if you must
do a redirect, the preferred technique is to use the standard 3xx HTTP status
codes, primarily to ensure the back button works correctly.
The main thing to remember is that redirects slow down the user
experience. Inserting a redirect between the user and the HTML document delays
everything in the page since nothing in the page can be rendered and no
components can start being downloaded until the HTML document has arrived.
One of the most wasteful redirects happens frequently and web developers
are generally not aware of it. It occurs when a trailing slash (/) is missing
from a URL that should otherwise have one. For example, going to
results in a 301 response containing a redirect to
(notice the added trailing slash). This is fixed in Apache by using Alias or mod_rewrite, or the DirectorySlash directive if you're using
Apache handlers.
Connecting an old web site to a new one is another common use for
redirects. Others include connecting different parts of a website and directing
the user based on certain conditions (type of browser, type of user account, etc.).
Using a redirect to connect two web sites is simple and requires little
additional coding. Although using redirects in these situations reduces the
complexity for developers, it degrades the user experience. Alternatives for
this use of redirects include using Alias and mod_rewrite if the two code paths are hosted on the same server. If
a domain name change is the cause of using redirects, an alternative is to
create a CNAME (a DNS record that creates an alias pointing from one domain
name to another) in combination with Alias or mod_rewrite.
Remove Duplicate Scripts
tag: javascript
It hurts performance to include the same JavaScript file twice in one
page. This isn't as unusual as you might think. A review of the ten top U.S.
web sites shows that two of them contain a duplicated script. Two main factors
increase the odds of a script being duplicated in a single web page: team size
and number of scripts. When it does happen, duplicate scripts hurt performance
by creating unnecessary HTTP requests and wasted JavaScript execution.
Unnecessary HTTP requests happen in Internet Explorer, but not in Firefox.
In Internet Explorer, if an external script is included twice and is not
cacheable, it generates two HTTP requests during page loading. Even if the
script is cacheable, extra HTTP requests occur when the user reloads the page.
In addition to generating wasteful HTTP requests, time is wasted
evaluating the script multiple times. This redundant JavaScript execution
happens in both Firefox and Internet Explorer, regardless of whether the script
is cacheable.
One way to avoid accidentally including the same script twice is to
implement a script management module in your templating system. The typical way
to include a script is to use the SCRIPT tag in your HTML page.
An alternative in PHP would be to create a function called insertScript.
In addition to preventing the same script from being inserted multiple
times, this function could handle other issues with scripts, such as dependency
checking and adding version numbers to script filenames to support far future
Expires headers.
Configure ETags
tag: server
Entity tags (ETags) are a mechanism that web servers and browsers use to
determine whether the component in the browser's cache matches the one on the
origin server. (An "entity" is another word a "component":
images, scripts, stylesheets, etc.) ETags were added to provide a mechanism for
validating entities that is more flexible than the last-modified date. An ETag
is a string that uniquely identifies a specific version of a component. The
only format constraints are that the string be quoted. The origin server
specifies the component's ETag using the ETag response header.
HTTP/1.1 200
OK
Last-Modified: Tue, 12 Dec 2006 03:03:59 GMT
ETag:
"10c24bc-4ab-457e1c1f"
Content-Length: 12195
來自 “ ITPUB部落格 ” ,連結:http://blog.itpub.net/200919/viewspace-1030331/,如需轉載,請註明出處,否則將追究法律責任。
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