For this test, I’ve set up a server (1 intel processor/512MB ram/12MB video memory) with Ubuntu Server 10.10 (32 bit).
I could test some static resources but nowadays, particularly all websites are dynamic ones so I’ve installed Mysql/PHP and of course Apache2, all with the default settings, on my server.
Next, I’ve downloaded WordPress and installed it on the server, changed the appearance to ‘pixel theme’ and added a few posts with images in.

Time to test

I’m not going to do fancy stuff, the module is installed with the default options enabled and I’ll just use Firebug to do the tests.

First the tests without the module installed:

The results with the module installed:

Mod_pagespeed_statistics:

Conclusion:

• The amount of requests have been reduced in this case with 2 (-15%)
• The pagesize has been reduced with 5.2KB (-1.5%)
• The loadtime is shorter when mod_pagespeed is enabled (-13%)

The 50% profit claim is a little bit optimistic because the bottleneck in the pageload is the dynamic content and that’s a part where this module doesn’t do anything. But in general, the module does a reasonably good job. I get a 13% profit with mostly default values, a good configured server should propably even do better.
I will definitely test this further and maybe put it one day on a production server.

The config used in this test looks like this:

<IfModule pagespeed_module>
    SetOutputFilter MOD_PAGESPEED_OUTPUT_FILTER
     ModPagespeed on
     ModPagespeedUrlPrefix "http://localhost/"
     ModPagespeedFileCachePath "/var/mod_pagespeed/cache/"
     ModPagespeedGeneratedFilePrefix "/var/mod_pagespeed/files/"
     ModPagespeedRewriteLevel CoreFilters
     ModPagespeedDomain localhost
     ModPagespeedEnableFilters add_instrumentation
<IfModule>

My full configuration file can be downloaded here.

$ telnet localhost 11211
Trying 127.0.0.1…
Connected to localhost.
Escape character is ‘^]’.
flush_all
OK
quit
Connection to localhost closed by foreign host.
$

You have to log in into the appropriate memcached host and port with telnet. After logged in, you have to use the flush_all command. If this command responses with OK, everything went fine and you can log out of your memcached server using the command quit.

The above command works fine but is a pain in the ass if you have to flush a lot and wants to use your console for other things.
If this is the case, you can run a flush_all in 1 command

echo 'flush_all' | nc localhost 11211

By default, nc (or netcat) creates a TCP socket either in listening mode (server socket) or a socket that is used in order to connect to a server (client mode). Actually, netcat does not care whether the socket is meant to be a server or a client. All it does is to take the data from stdin and transfer it to the other end across the network.