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Monday, July 31, 2006

ASP Tips: Tip 15: More on Fine Tuning

Use Server.Transfer Instead of Response.Redirect Whenever Possible

Response.Redirect tells the browser to request a different page. This function is often used to redirect the user to a log on or error page. Since a redirect forces a new page request, the result is that the browser has to make two round trips to the Web server, and the Web server has to handle an extra request. IIS 5.0 introduces a new function, Server.Transfer, which transfers execution to a different ASP page on the same server. This avoids the extra browser-to-Web-server round trip, resulting in better overall system performance, as well as better response time for the user. Check out New Directions in Redirection, which talks about Server.Transfer and Server.Execute.

Also see Leveraging ASP in IIS 5.0 for a full list of the new features in IIS 5.0 and ASP 3.0.

Use Trailing Slashes in Directory URLs

A related tip is to make sure to use a trailing slash (/) in URLs that point to directories. If you omit the trailing slash, the browser will make a request to the server, only to be told that it's asking for a directory. The browser will then make a second request with the slash appended to the URL, and only then will the server respond with the default document for that directory, or a directory listing if there is no default document and directory browsing has been enabled. Appending the slash cuts out the first, futile round trip. For user-friendliness, you may want to omit the trailing slash in display names.

For example, write:

<a href=""
title="MSDN Web Workshop"></a>

This also applies to URLs pointing to the home page on a Web site: Use the following:
<a href="">, not <a href="">.

Avoid Using Server Variables

Accessing server variables causes your Web site to make a special request to the server and collect all server variables, not just the one that you requested. This is akin to needing to retrieve a specific item in a folder that you have in that musty attic of yours. When you want that one item, you have to go to the attic to get the folder first, before you can access the item. This is the same thing that happens when you request a server variable—the performance hit occurs the first time you request a server variable. Subsequent requests for other server variables do not cause performance hits.

Never access the Request object unqualified (for example, Request("Data")). For items not in Request.Cookies, Request.Form, Request.QueryString, or Request.ClientCertificate, there is an implicit call to Request.ServerVariables. The Request.ServerVariables collection is much slower than the other collections.

Upgrade to the Latest and Greatest

System components are constantly updated and we recommend that you upgrade to the latest and greatest. Best of all would be to upgrade to Windows 2000 (and hence, IIS 5.0, ADO 2.5, MSXML 2.5, Internet Explorer 5.0, VBScript 5.1, and JScript 5.1). IIS 5.0 and ADO 2.5 implement spectacular performance gains on multiprocessor machines. Under Windows 2000, ASP scales nicely to four processors or more, whereas under IIS 4.0, ASP didn't scale well beyond two processors. The more script code and ADO usage in your application, the more performance benefits you'll see after upgrading to Windows 2000.

If you can't upgrade to Windows 2000 just yet, you can upgrade to the latest releases of SQL Server, ADO, VBScript and JScript, MSXML, Internet Explorer, and Windows NT 4 Service Packs. All of them offer improved performance as well as increased reliability.

Tune Your Web Server

There are several IIS tuning parameters that can improve site performance. For example, with IIS 4.0, we've often found that increasing the ASP ProcessorThreadMax parameter (see IIS documentation) can have significant benefits, especially on sites that tend to wait on back-end resources such as databases or other middle-ware products such as screen-scrapers. In IIS 5.0, you may find that turning on ASP Thread Gating is more effective than trying to find an optimal setting for AspProcessorThreadMax, as it is now known.

For good references, see Tuning IIS below.

The optimal configuration settings are going to be determined by (among other factors) your application code, the hardware it runs on, and the client workload. The only way to discover the optimal settings is to run performance tests, which brings us to the next tip.

Do Performance Testing

As we said before, performance is a feature. If you are trying to improve performance on a site, set a performance goal, then make incremental improvements until you reach your goal. Don't save all performance testing for the end of the project. Often, at the end of a project, it's too late to make necessary architectural changes, and you disappoint your customer. Make performance testing a part of your daily testing. Performance testing can be done against individual components, such as ASP pages or COM objects, or on the site as a whole.

Many people test the performance of their Web sites by using a single browser to request pages. This will give you a good feel for the responsiveness of the site, but it will tell you nothing about how well the site performs under load.

Generally, to accurately measure performance, you need a dedicated testing environment. This environment should include hardware that somewhat resembles production hardware in terms of processor speed, number of processors, memory, disk, network configuration, and so on. Next, you need to define your client workload: how many simultaneous users, the frequency of requests they will be making, the types of pages they'll be hitting, and so forth. If you don't have access to realistic usage data from your site, you'll need to guesstimate. Finally, you need a tool that can simulate your anticipated client workloads. Armed with these tools, you can start to answer questions such as "How many servers will I need if I have N simultaneous users?" You can also sniff out bottlenecks and target these for optimization.

Some good Web stress-testing tools are listed below. We highly recommend the Microsoft Web Application Stress (WAS) Toolkit. WAS allows you to record test scripts and then simulate hundreds or thousands of users hitting your Web servers. WAS reports numerous statistics, including requests per second, response time distributions, and error counts. WAS has both a rich-client and a Web-based interface; the Web interface allows you to run tests remotely.

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