MSDN Magazine - November 2008 - (Page 73)

When you consider a server disk seek time for each Files number of HTTP1.1 ports to two per domain. request is usually less than 10ms, versus roundSo, for example, msn.com gets only two ports. If trips of hundreds of milliseconds repeated many you host all of your static JavaScript, CSS, and imtimes over, you’ll see that distance and the numages on a single domain, as in Figure 4, then the ber of round-trips are easily the most significant older browsers would still open only two ports. contributing factor in long page-load times. Spreading the content over multiple domains alTo combat the impact of long round-trips lows the browser to open more ports. Alternathere are three main solutions you can try: you tively, browsers can be modified to open more can reduce the number of serial round-trips by ports. This is currently under consideration by Time doing more in parallel and getting rid of excess the Internet Explorer team. weight, you can reduce the round-trip-time by Figure 1 HTTP1.0 Waterfall Pages often bottleneck on the download of a moving your location closer to the user, or you Illustration single file or worse on a series of single files, one can reduce server time. after the other. Ideally, a page is loading six to eight Visualizing the page download accurately, as it really flows, is es- files in parallel. Take a look at the bit rate chart in Figure 4. There sential to performance analysis. For example, it’s difficult to know which parts of the page are serial and which are downloading in Ports parallel. File downloads are typically depicted as a waterfall (as you Files see in Figure 1) with one file represented per row in the report. But such a depiction does not account for the fact that since HTTP1.1, TCP connections are reused. Keeping connections open and calling additional files on the existing connection reduces the number of round-trips, but it’s hard to see this in the traditional waterfall illustration. For a more accurate picture of how a page really loads you need to add the TCP port as a dimension of the drawing. Figure 2 shows files loading on seven TCP ports, represented by the grey horizontal lines, and the number of milliseconds that Time have elapsed. The colored bars are the files; red is HTML text, gold 1 2 3 4 5 6 7 8 represents style sheets, tan represents JavaScript, and blue lines are images. Here a single port is opened first, and then two more Figure 2 HTTP1.1 Keep-Alives ports are opened. After the first files (CSS) are loaded, more files First File (JavaScript and images) are loaded on the same ports. Packet Loaded VRTA sits on top of the Microsoft Network Monitor (known as NetMon) 3.2 packet analyzer, though you don’t need to know how to use NetMon. In the latest version of this tool, now to be released publicly, the Global Performance team added visual details for files and packets. Figure 3 shows the details. Once a TCP conResponse nection is created, the pink shaded line shows the TCP response TCP TTFB Packets packet coming from the server. The grey line is the time-to-firstbyte from the server and the stairs indicate the response packets Figure 3 Details for Files and Packets ramping up using TCP slow-start. 12 Easy Steps Client Ports 1) 2) 3) 4) 5) 6) 7) 1655 1656 1657 1658 1659 1660 1661 Domain It’s easy to assume a Web page is always downloading at full speed and that more bytes will only slow down the page. The reality, though, is that unless the user is near the datacenter, it’s very likely the browser will not 4 8 Kbps make use of all the available bandwidth. Here 316 are 12 different steps you can take to allevi253 190 ate the problem. 126 1. open Enough ports Both the browser 63 and the application can limit the number of 4 8 ports opened in parallel. Internet Explorer 7.0, as well as earlier versions, limited the Figure 4 Too Few Ports and Many Small Files msdnmagazine.com gamesdomain.com videogames.yahoo.com l.yimg.com l.yimg.com ads.yimg.com us.1.yimg.com us.bc.yahoo.com 12 Seconds 95 Kbps 12 Seconds November 2008 73 http://www.msn.com http://www.msdnmagazine.com

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

Contents MSDN Magazine - November 2008 Toolbox CLR Inside Out Data Points Cutting Edge Security Briefs Test Your Security IQ Agile SDL Access Control Utility Spotlight RIA Test Run Wicked Code Foundations Team System End Bracket

MSDN Magazine - November 2008

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