breadcrumbs: X-Subresources > page_name: x-subresources-1 title: ‘X-Subresources: python server experiment’

Experiment #1

Andrew Oates


The purpose of this experiment was to determine if significant gains could be had from adding the X-subresource headers to pages. If that was the case, further experimentation would be done.

Specifically, I tested the effect of the prefetching on a page with several external Javascript resources, which would need to be loaded and run serially by the browser.


The experiment consisted of loading a single test page from a server, and timing the page load times under various conditions. There were three components: the server, the test files, and the client.


The server was a small HTTP server written in Python designed to serve static content and optionally scan the pages for tags and generate X-subresource for any external resources referenced in the document. It can also add an artificial latency to simulate adverse network conditions --- in this case, it simply waits a certain amount of time after receiving a GET request before sending back the response.

TODO(aoates): publish the modified python server.


The client was a standard build of Firefox 3.0.10 with a custom extension installed.

The extension listens for incoming requests, grabs any X-subresource headers and initiates prefetch requests for all the referenced resources.

TODO(aoates): publish the firefox extension.

Test Pages

The test pages consist of two variations on one page. In each case, the page loads and executes 10 external Javascript files, and collects timing information on how long the page load as well as each external JS load takes. The individual Javascript files print out a message to the page, and stall for a small amount of time (around 100ms).

In the first variation, test5.html, the external Javascript files are hosted locally, alongside the page. They are therefore served by the Python server, which cannot handle multiple parallel connections. In the second variation, test5_corp.html, the Javascript files are hosted on a server which can handle parallel connections.

TODO(aoates): publish the actual web pages.

Experimental Groups

There were four experimental groups, testing two variables:

  • Presence of X-subresource headers in test page headers, and
  • Location of external resources; either locally, served by the small Python HTTP server, or on remotely, on and served by Apache.

In each case, the test page itself was served locally by the Python webserver (which generated the headers). The server was run with the following command:

./ --latency 300 [--no-xsubresource]with the last parameter varying. This introduced a “latency” of 300ms to all content served by the Python web server; the server waited 300ms after receiving the request to send back the response. The number 300 was chosen to approximate the latency of downloading one of the javascript files from a typical server.

Trials and Results

Each group was run several times, and a representative set of results is presented in the table below. For each group, an overall page load time is presented, as is the load time of each Javascript script element (the time taken to load and execute the Javascript file). Results of Experiment #1 External Resource (local)Apache/2.0.55 (remote)X-subresource HeadersYesPage Load: 3065 ms JS #1 Load: 306 ms JS #2 Load: 301 ms JS #3 Load: 309 ms JS #4 Load: 306 ms JS #5 Load: 301 ms JS #6 Load: 312 ms JS #7 Load: 299 ms JS #8 Load: 303 ms JS #9 Load: 296 ms JS #10 Load: 317 msPage Load: 1366 ms JS #1 Load: 379 ms JS #2 Load: 108 ms JS #3 Load: 116 ms JS #4 Load: 106 ms JS #5 Load: 107 ms JS #6 Load: 106 ms JS #7 Load: 107 ms JS #8 Load: 107 ms JS #9 Load: 107 ms JS #10 Load: 106 msNoPage Load: 4178 ms JS #1 Load: 422 ms JS #2 Load: 415 ms JS #3 Load: 411 ms JS #4 Load: 415 ms JS #5 Load: 415 ms JS #6 Load: 415 ms JS #7 Load: 414 ms JS #8 Load: 417 ms JS #9 Load: 422 ms JS #10 Load: 416 msPage Load: 3777 ms JS #1 Load: 487 ms JS #2 Load: 353 ms JS #3 Load: 342 ms JS #4 Load: 342 ms JS #5 Load: 352 ms JS #6 Load: 356 ms JS #7 Load: 352 ms JS #8 Load: 347 ms JS #9 Load: 484 ms JS #10 Load: 351 ms


Let's examine each set of results.

Python/local without prefetching

In this trial, there was an overall page load time (from execution of the first script tag to sending the body's onLoad event) of 4178 ms, with each individual javascript element taking a little over 400 ms to load and execute. This is exactly as expected --- with a latency of 300 ms, and an execution duration of 100 ms, each javascript element should take around 400ms to execute. Multiply that by 10 elements, and we get a ~4000 ms page load time. This is our worst-case scenario.

Apache/remote without prefetching

Assuming we modeled the latency accurately with our local Python webserver (which we didn't), these should be about the same as the local results. Each Javascript load is 50-60ms faster, however. This can be chalked up to Apache being just plain better than our local server.

Python/local with prefetching

In this case, we shave about 1 second off our overall page load time. By examining the server output, it is clear that the page is sending off its prefetch requests all at the start of the page load, and the files are downloaded in the network thread. However, since the server cannot handle parallel connections/requests, the files must be read in serial. In this case, it looks like the prefetches are around 100ms ahead of the execution in the UI thread. Since we can load and execute at the same time, that 100ms stacks to give us a savings of around a second. Pretty good!

Apache/remote with prefetching

In this case, since Apache can handle multiple concurrent connections, the browser can shoot off all its requests at once. Once again, the loading is a little behind the execution of the scripts, but since we are downloading them all concurrently, we've finished fetching the scripts by the time the second one starts executing. Each subsequent script therefore only takes execution time (no downloading needed), so we get excellent savings, very close to our optimal page load time of ~1200-1300 ms.


Adding the X-subresource headers can be costly. In this case, it increased the size of the headers on test5.html from 166 bytes to 747 bytes. The appended headers were:X-subresource: test5_1.js; type=application/x-javascript X-subresource: test5_2.js; type=application/x-javascript X-subresource: test5_3.js; type=application/x-javascript X-subresource: test5_4.js; type=application/x-javascript X-subresource: test5_5.js; type=application/x-javascript X-subresource: test5_6.js; type=application/x-javascript X-subresource: test5_7.js; type=application/x-javascript X-subresource: test5_8.js; type=application/x-javascript X-subresource: test5_9.js; type=application/x-javascript X-subresource: test5_10.js; type=application/x-javascript Note that the current implementation of the extension doesn‘t use any of the metadata included in the header (it just sends off a blind request), so that could be eliminated. Additionally, these could be concatenated into one large X-subresource header:X-subresource: test5_1.js, test5_2.js, test5_3.js, test5_4.js, test5_5.js, test5_6.js, test5_7.js, test5_8.js, test5_9.js, test5_10.js This would bring the size of the header down to 302 bytes. While much more reasonable, this is not insignificant. However, as plaintext, it’s a good candidate for an optimization like gzip'd headers.


When prefetching is enabled (as in the 1st row of results), the browser doesn‘t have to block network loads on executing javascript. It can read the prefetch headers, send off requests, and forget about them --- they’ll continue in the network thread while work is being done in the UI thread.

The most fruitful type of sub-resource to be prefetched is probably Javascript; when the browser encounters an external JS file, it has to block the main/UI thread until the file is downloaded and executed. If, however, the file has been prefetched, the time taken to download is instant savings on the overall load time of the page. In a case like this, with multiple consecutive scripts, the difference can be dramatic.

Future Work

Some areas of future work include:

  • Testing with a dummynet for varying network bandwidth, RTT, and loss rate.
  • Writing an Apache module that generates X-subresource headers
  • Re-writing the extension in C++
  • Getting it to work with images and style sheets
  • Making it more robust in the face of strange cache settings (or, more generally, knowing when prefetching will make the user experience worse)