MSDN Magazine - April 2008 - (Page 65)

Figure 1 The Original Performance Equation services/analyze is a great option), as can Bandwidth (see speedtest.net) and roundtrip time (using Ping). Tools like websiteoptimization.com/ services/analyze will also report the size of a Web page’s HTML, CSS, Figure 2 The Web Version of the Performance Equation JavaScript, images, and so forth. Concurrent Requests is essentially a constantly (Internet Explorer® defaults to 2). That leaves Cs and Cc, which need some additional development effort. It’s relatively straightforward to write code in an ASP.NET page that notes the exact second execution of the page begins and subtracts that time from the current time when execution com pletes. The same is true on the client side; a bit of JavaScript can execute right at the top of the HTML page to note the time and then subtract the time at the point that the OnLoad event fires when the page is completed. In fact, all these elements can be coded for if you want to build in a debug mode of your Web site that utilizes the performance equa tion. And there’s good reason to do so: if you can routinely render the performance equation elements on the browser, then you can easily detect where your performance problems lie. For example, suppose you have an ASP.NET application whose users are on another continent and have low bandwidth. With high ping times (> 200ms) and low bandwidth (< 500kbps), your users would be highly sensitive to the total payload and the number of roundtrips in your application. Looking at your application in the context of those users is vital since their experience will be very different from yours. While more traditional strategies for decomposing server tiers might have you considering separate servers for data access, complex computations, and so on, independent of the actual generation of the Web pages, I’d rather have five Web servers that do everything than three Web servers and two business object servers. All those outofprocess calls between the Web servers and business object servers create a lot of overhead. Specialization should only be done for a known and expected benefit. And the fastest solution is not always the best. The goal of scalability is consistency of performance. You want to narrow the performance range as the load increases; whether there’s one user or one thousand, you want a given page rendered for all users in the same amount of time. Eventually you will need to optimize your server code to scale more effectively. Virtually every aspect of the performance equa tion scales linearly, except for compute time on the server; you can always add more bandwidth (and it’s fairly easy to know when), and Figure 3 Elements of the Performance Equation Variable R Definition Response time. The total time from the user requesting a page (by clicking a link, and so on) to when the full page is rendered on the user’s computer. Typically measured in seconds. Total bytes sent to the browser, including markup and all resources (such as CSS, JS, and image files). Rate of transfer to and from the browser. This may be asymmetrical and might represent multiple speeds if a given page is generated from multiple sources. Usually, it is averaged together to create a single bandwidth expressed in bytes per second. The number of resource files a given page needs. These resource files will include CSS, JS, images, and any other files retrieved by the browser in the process of rendering the page. In the equation, the HTML page is accounted for separately by adding in round-trip time (RTT) before the AppTurns expression. The time it takes to round-trip, regardless of bytes transferred. Every request pays a minimum of one RTT for the page itself. Typically measured in milliseconds. Number of simultaneous requests a browser will make for resource files. By default, Internet Explorer performs two concurrent requests. This setting can be adjusted but rarely is. Compute time on the server. This is the time it takes for code to run, retrieve data from the database, and compose the response to be sent to the browser. Measured in milliseconds. Compute time on the client. This is the time it takes for a browser to actually render the HTML on the screen, execute JavaScript, implement CSS rules, and so on. Payload Bandwidth Scaling Problems As consultants, we know we’re probably dealing with a scaling issue when the application performs well in the test environment but poorly in the real world. Usually, the only difference between the two is the number of simultaneous users. If the application per formed poorly all the time, you would have a performance prob lem rather than a scaling problem. There are three strategies available that you can employ in order to improve scaling: specialization, optimization, and distribution. How you apply them will vary, but the actual strategies are straight forward and consistent. The goal of specialization is to break your application into smaller pieces in order to isolate the problem. For example, you might want to consider moving static resource files such as images, CSS, and JS files off of the ASP.NET servers. A server welltuned for ASP.NET is not especially wellsuited to serving those sorts of files. For this reason, a separate group of IIS servers tuned to serve resource files can make a substantial difference in the scalability of the applica tion you are running. If you perform a lot of compression or encryption (for SSL), set ting up servers dedicated to SSL can help. You should be aware that there are even specialized hardware devices available for compres sion and SSL termination. AppTurns RTT Concurrent Requests Cs Cc april2008 65 http://speedtest.net http://websiteoptimization.com/

Table of Contents Feed for the Digital Edition of MSDN Magazine - April 2008

MSDN Magazine - April 2008 Contents Toolbox CLR Inside Out Basic Instincts Cutting Edge Foundations Test Run Service Station Windows with C++ Going Places { End Bracket }

MSDN Magazine - April 2008

For optimal viewing of this digital publication, please enable JavaScript and then refresh the page. If you would like to try to load the digital publication without using Flash Player detection, please click here.