MSDN Magazine - November 2008 - (Page 76)

independently of the other. The TCP layer is running at the kernel level or in the NIC card. It’s very fast. When the request packet comes in, it forwards the data up to the HTTP layer, which may need to do a disk seek. The TCP layer can’t send off an acknowledgment until the HTTP layer can respond (see Figure 8). 8. Check for Unnecessary Round-trips Not everything that happens during a page download actually needs to happen. Some actions are performed or decisions made simply because the browser is not given adequate instructions, like how long can a file be cached. 9. Set Expiration Dates Static files such as JavaScript, CSS, images, and XMLs are cached by the browser for when the user loads the page next. The browser will look for an expiration date in the file’s HTTP header to see if the file is still good. Unfortunately, many sites are not setting these dates. So the browser pulls the files from Quiz: how Do you improve page loading? Answer these Web performance questions before you read the article and again after you’ve finished it to see how much your own performance has improved! 1. How many ports should you have running concurrently? a. As few as possible b. Many on broadband and fewer on narrowband 2. Which is faster to download across a WAN? a. Many small files b. A few large files of the same aggregate weight 3. JavaScript only blocks other JavaScript files. a. False b. True 4. Are Keep-Alives on by default? a. Yes b. No 5. The default expiration date a. Is blank b. Is 30 days from the download date 6. How do you stop Etags from reloading the file? a. Set an expiration date b. Ask the user to wait for the file to reload 7. Compression of HTML, JavaScript, and CSS files often achieves what reduction ratio? a. 1.5 to 1 b. 3 or more to 1 8. Image files are always compressed. a. True b. False Quiz answers: 1=b, 2=b, 3=a, 4=a, 5=a, 6=a, 7=b, 8=b 76 msdn magazine the cache, doesn’t find a date, and sends a Get-if-Modified to the server. In most cases, the server responds with 304 Not-Modified, and the file is used from the cache. Performant sites set their dates to three years in the future. Changing the file name or path, or adding an argument string, causes the browser to reload the file. Pause to consider how many new servers and how much bandwidth was acquired to handle Get requests for files the users already had. Another benefit of expiration dates is how they affect Entity tags (Etags). Etags tend to cause performance problems. They are meant to uniquely identity files using a number generated by the server. In clusters of servers, each server will create a different number. For example, let’s say a browser is sending a Get-If-Modified for a file it found in its cache. If there is only one server, the file will most likely match and a 304 Not-Modified error will be sent. But, if the user is accessing a larger server farm, it’s most likely to hit a different server with a different entity tag, causing that server to reload the entire file to the browser. Using VRTA, if you notice more than one JavaScript file getting a 200-OK from the server then the server is not recognizing the entity tag and will resend the entire file. Not only is the file reloaded, but often the browser will recognize that the file is the same as the one in the cache and will attempt to stop the server from completely resending it. The browser does this by sending a Reset flag to close the TCP port. That, in turn, increases the number of round-trips because of the fact that a new port is needed to complete the download of the remaining files. A simple solution is to set the expiration date for the file so the browser can reuse the cached file without making a call to the server. 10. think before you Redirect Site addresses change over time and redirects are often used to point users to new locations. However, these redirects chew up time and should be replaced. Consider whether the site could respond from the old URL instead. 11. Use Compression Most JavaScript, CSS, and HTML files can be compressed by as much as 4-to-1. XML compresses even more. The neat thing about these file types is that the server can compress them one time and then store the compressed version. This reduces CPU load on the server and volume of bits on the networks. Dynamic files such as ASPX need to be compressed for every user on the fly, so they are usually compressed by external devices like load balancers. I’m often asked whether compression consumes a lot of time on the browser and whether all browsers handle compression. First, consider the time it takes to load a large file across the network. If VRTA ran every file into a compression utility and then showed the ratio of the new file to the old in the compressibility column, ratios of 1 or less would represent files that are already compressed. In that instance, JavaScript files on this site are taking one to five seconds to load. Uncompressing these files would take milliseconds for most modern PCs. Browsers must send a “will accept gzip, deflate” statement in the Get request to tell the server it can handle compression. Full-size images should always be reformatted when used as thumbnails. Also, it’s a good idea to check out different formats Visual Round Trip Analyzer

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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