For the benchmark, we do not want to access Internet directly. We want to compare the results from one run to the other, so we don't want the pages to change arbitrarily. Using the Web for benchmarking would also create an important load on the servers.
To use real pages without going online, we create databases of web pages with the mirror application:
Those databases are snapshots of webpages at a given point in time, and they are used as input of the benchmarks.
Using the benchmarks
There are two ways to exploit the databases with the benchmark: the online and offline modes. The difference lies in the way we provide the database's content to the benchmarks:
In the "online mode", we use a basic web server to serve the database over HTTP. The benchmarks use the complete stack to load pages, as they would if we were loading the page from Internet.
In the "offline mode", the database is loaded directly by the benchmarks and is used as the source of data. In that case, the network is not involved. This mode is mostly useful for the benchmarks that do not involve the network (like measuring the rendering speed).
What is measured
The benchmark suite is still a work in progress. Currently, there are benchmarks for:
the page loading performance (with or without rendering)
the rendering performance
the scrolling performance
How you can use that?
If you use WebKit, and are interested in great performance, you can use the performance suite to profile the use case you are interested in, and optimize those cases.
If you evaluate the use of WebKit for embedded, you can use the benchmark to evaluate how good WebKit performs on the hardware.
If you make patches for WebKit's performance, have a look on how to contribute. You can also join us on IRC in #qtwebkit on freenode.