This is a continuation of a series of blog entries on this topic. The series starts here.
The YSlow model of web application performance, depicted back in Equations 3 & 4 in the previous post, leads directly to an optimization strategy to minimize the number of round trips, decrease round trip time, or both. Several of the YSlow performance rules reflect tactics for minimizing the number of round trips to the web server and back that are required to render the page. These include
- designing the Page so there are fewer objects to Request,
- using compression to make objects smaller so they require fewer packets to be transmitted, and
- techniques for packing multiple objects into a single request.
All the text-based files that are used in composing the page – .htm, .css, and .js – tend to compress very well, while the HTTP protocol supports automatic unpacking of gzip-encoded files. There is not a great benefit from compressing files already smaller than the Ethernet MTU, so YSlow recommends packing smaller files into larger ones so that text compression is more effective.
Meanwhile, the performance rules associated with cache effectiveness are designed to minimize RTT, the round trip time. If current copies of the HTML objects requested from the web server can be retrieved from sources physically located considerably closer to the requestor, the average network round trip time for those Requests can be improved.
With its focus on the number and size of the files necessary for the web browser to assemble in order to construct the page’s document object from these component parts, YSlow uses an approach to optimization known in the field of Operations Research (OR) as decomposition. The classic example of decomposition in OR is the time and motion study where a complex task is broken into a set of activities that are performed in sequence to complete a task. The one practical obstacle to using decomposition, however, is that YSlow understands the components that are used to compose the web page, but it lacks measurements of how long the task and its component parts take.
As discussed in the previous section, these measurements would be problematic from the standpoint of a tool like YSlow which analyzes the DOM once it has been completely assembled. YSlow does not attempt to measure the time it took to perform that assembly. Moreover, the way the tool works, YSlow deals with only a single instance of the rendered page. If it did attempt to measure network latency or cache effectiveness or client-side processing compute power, it would be capable of only gathering a single instance of those measurements. There is no guarantee that that single observation would be representative of the range and variation in behavior a public-facing web application would expect to encounter in reality. As we consider the many and varied ways caching technology, for example, is used to speed up page load times, you will start to see just how problematic the use of a single observation of the page load time measurement to represent the range and variation in actual web page load times can be.
Several of the YSlow performance rules reflect the effective use of the caching services that are available for web content. These services include that portion of the local file system that is used for the web client’s cache, a Content Delivery Network, which are caches geographically distributed around the globe, and various server-side caching mechanisms. Effective use of caching improves the round trip time for any static content that can readily be cached. Since network transmission time is roughly a function of distance, naturally, the cache that is physically closest to the web client is the most effective at reducing RTT. Of the caches that are available, the cache maintained by the web browser on the client machine’s file system is physically the closest, and, thus, is usually the best place for caching to occur. The web browser automatically stores a copy of any HTTP objects it has requested that are eligible for caching in a particular folder within the file system. The web browser cache corresponds to the Temporary Internet Files folder in Internet Explorer, for example.
If a file referenced in a GET Request is already resident in the web browser cache – the disk folder where recently accessed cacheable HTTP objects are stored – the browser can add that file to the DOM without having to make a network request. Web servers add an Expiresheader to Response messages to indicate to the web browser that the content is eligible for caching. As the name indicates, the Expires header specifies how long the existing copy of that content remains current. Fetching that content from the browser cache requires a disk operation which is normally significantly faster than a network request. If a valid copy of the content requested is already resident in the browser cache, the round trip time normally improves by an order of magnitude since a block can be fetched from disk in 5-10 milliseconds on average. Note that reading a cached file from disk isn’t always faster than accessing the network to get the same data. Like any other factor, it is important to measure to see which design alternative performs better. In the case of an intranet web application where web browser requests can be fielded very quickly, network access, often involving less than 1 ms of latency, might actually be preferred because it could be much faster to get the Http object requested directly from the IIS kernel-mode cache than for the web client to have to access its local disk folder where Temporary Internet Files are stored.
Note also, that while caching does not help the first time a customer accesses a new web page, it has a substantial impact on subsequent accesses to the page. Web traffic analysis programs will report the number of unique visitors to a web site – each of these is subject to a browser cache that is empty of the any of the content that is requested. This is referred to as a cold start in cache. It is only the repeat visitors that benefit from caching, subject to the repeat visit to the web site occurring prior to the content expiration date and time. In Souders’ book, he reports an encouragingly high number of repeat visits to the Yahoo site as evidence for the YSlow recommendation. When network latency for an external web site is at least 100-200 ms, accessing the local disk-resident browser cache is an order of magnitude faster.
When the web browser is hosted on a mobile phone, which is often configured without a secondary storage device, the capability to cache content is consequently very limited. When Chrome detects it is running on an Android phone, for example, it configures a memory resident cache that will only hold up to 32 files at any one time. If you access any reasonably complex web site landing page with, say, more than 20-30 href= external file references, the effect is to flush the contents of the Chrome mobile phone cache.
Content that is generated dynamically is more problematic to cache. Web 2.0 pages that custom built for a specific customer probably contain some elements that are unique for the user ID, while other web page parts are apt to be shared among many customers. Typically, the web server programs that build dynamic HTML Response messages will simply flag them to expire immediately so that they are ineligible for caching by the web browser. Caching content that is generated dynamically is challenging. Nevertheless, it is appropriate whenever common portions of the pages are reused, especially when it is resource-intensive to re-generate that content on demand. We will discuss strategies and facilities for caching at least some portion of the dynamic content web sites generate in a future Post.
Beyond caching at the local machine, YSlow also recommends the use of a Content Delivery Network (CDN) similar to the Akamai commercial caching engine to reduce the RTT for relatively static Response messages. CDNs replicate your web site content across a set of geographically distributed web servers, something which allows the CDN web server physically closer to the requestor to serve up the requested content. The net result is a reduction in the networking round trip time simply because the CDN server is physically closer to the end user than your corporate site. Note that the benefits of a CDN even extend to first time visitors of your site because they contain up-to-date copies of the most recent static content from your primary web site host. For Microsoft IIS web servers and ASP.NET applications, there are additional server-side caching options for both static and dynamic content that I will explore much later in this discussion.
Extensive use of caching techniques in web technologies to improve page load time is one of the reasons why a performance tool like YSlow does not actually attempt to measure Page Load Time. When YSlow re-loads the page to inventory all the file-based HTTP objects that are assembled to construct the DOM, the web browser is likely to discover many of these objects in its local disk cache, drastically reducing the time it takes to compose and render the web page. Were YSlow to measure the response time, the impact of the local disk cache would bias the results. A tool like the WebPageTest.org site tries to deal with this measurement quandary by accessing your web site a second time, and comparing the results to first-time user access involving a browser cache cold start.
Next: Complications that the simple YSlow model does not fully take into account.