New mod_pagespeed: cache advances, progressive JPEGs
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| Bharath |
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| Jan-Willem |
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| Joshua |
By Joshua Marantz, Jan-Willem Maessen, and Bharath
Bhushan, PageSpeed Team
When mod_pagespeed launched in November 2010, one of its benefits was to help websites better
exploit
browser caching by signing URLs with the resource content hash. This improves the
user experience coming back to the same site, and navigating within a site.
In mod_pagespeed 1.2 we have released two new features that improve the caching experience for
users coming to a site for the first time:
canonicalize_javascript_libraries
and
insert_dns_prefetch.
For additional speedups, converting jpegs to progressive format has been added to the Core
Filter Set, and the scope of optimization has been extended to include resources served by
external servers, even if they are not running mod_pagespeed.
Your web page loads faster when JQuery is preloaded in users' browser
Numerous web sites use common JavaScript libraries such as jQuery and jQuery UI. But when one
library is stored on many sites, browsers end up re-downloading that library for each new site
– a waste of time and bandwidth. The new
canonicalize_javascript_libraries
filter in mod_pagespeed finds such libraries on your site and replaces them with links to the
equivalent libraries on
ajax.googleapis.com.
With the optimization, a browser will notice that your site is requesting the library from the
same shared library provider as a previous site it visited, and will use the copy in its
cache.
It’s possible to do this by hand, but there are a number of reasons why you might prefer to
automate the process. Most important is that you may be using third-party code on your web
sites that includes some of these libraries. Using
canonicalize_javascript_libraries
lets you replace these with hosted versions without having to touch third-party code. It also
lets you use local, un-minified JavaScript source code for these libraries while you are
debugging your site, and then transition automatically to using minified hosted code when you
deploy. The filter spots external libraries using a hash signature; we’ve added a new
configuration file, pagespeed_libraries.conf, that stores these signatures, so that you can
upgrade the signature configuration without disrupting the rest of your apache
installation.
Resolving DNS entries early for critical assets saves hundreds of
milliseconds
DNS resolution time varies from <1ms for locally cached results, to hundreds of
milliseconds due to the cascading nature of DNS. This can contribute significantly to total
page load time. Below is a WebPagetest waterfall showing how DNS lookup time can affect page
load time.
The new
insert_dns_prefetch
filter inserts
<link rel="dns-prefetch"> tags to allow
the browser to pre-resolve DNS for resources on the page. The waterfall below shows the
improvement after inserting the hints.
<link rel="dns-prefetch"> is supported on
Chrome,
Firefox
and
Internet
Explorer.
Improved performance by optimizing external resources and progressive
JPEG
In addition to these new capabilities, mod_pagespeed 1.2 can
proxy
and optimize resources from trusted domains. This feature enables you to optimize
resources even from servers that don't run mod_pagespeed. Beyond compressing and
cache-extending such resources, this can improve performance of sites running SPDY where the
best practices for performance are to serve all resources from the same domain (see
mod_spdy).
Further,
convert_jpeg_to_progressive
is now a ‘core’ filter. Large JPEG images are now transcoded to progressive. This both
improves the browser experience and makes such files smaller.
To see more details about the release, check out the
release
notes and
mod_pagespeed
download page.
Joshua Marantz runs
Google’s PageSpeed team in Cambridge, MA, which is dedicated to making the web faster for
everyone. Josh has been working on making software run fast for several decades, at Google and
before that on accelerated chip simulation.
Jan Maessen wrote the earliest version of the image and JavaScript filters in mod_pagespeed
and has been with the team ever since. Before joining Google, he was a co-designer and library
implementer for the Fortress programming language.
Bharath Bhushan works on making website performance better. He has a Masters in CS from IIT
Madras, India.
Posted by Scott Knaster,
Editor
