Monitoring IIS Performance

The Performance tool that ships with Windows 2000 is a good means for monitoring Microsoft IIS's traffic, stress, and workload. The Performance tool (known as the Performance Monitor in Windows NT) is an open interface that Microsoft and third-party services and products use to report statistical and trend information about their status. Many vendors have added counters to the Performance tool that don't necessarily relate well to one another. Consequently, the information that Performance tool counters collect about IIS can seem inconsistent. To make matters worse, precise definitions for the counters aren't well documented. I attempt to clarify which performance counters are important for monitoring Web services and what those counters really tell you. The information I provide here is based on the Win2K Performance tool; the NT version differs slightly.

The Performance Tool
As I've already mentioned, the Performance tool is a Windows application that provides a GUI for tracking application performance data and statistical information. Each service or application object can expose one or more counters for one or more instances of the object. For example, you can track the Current Connections counter for the www.myserver.com instance of the Web Services object.

The Performance tool uses a standard API call to a service or application to get the counter information. Because the Performance tool requests information only for counters that the user has selected, it doesn't place much load on the application that's reporting information. However, if the application doesn't reply in a timely manner, the Performance tool shows a timeout error. You might be able to decrease such errors by increasing the interval between samples.

To open the Performance tool in Win2K, select Start, Settings, Control Panel, Administrative Tools, Performance. The Performance tool provides three views of the information it gathers: Chart (line graph), Histogram (bar graph), and Report. Icons on the toolbar let you move from one view to another. Most of the time, I use the Chart mode, which Figure 1 shows.

By default, the Performance tool has no counters running. To add counters, click the plus-sign icon on the toolbar to open the Add Counters dialog box , which Figure 2 shows. After you add one or more counters, you can select a counter from the list at the bottom of the screen in the Chart or Histogram views to see the Last (i.e., most recent), Minimum, Maximum, and Average samples and the Duration (i.e., length of the sampling period). By default, the Performance tool samples every second; however, you can set the monitor's properties so that it samples less frequently or you can click the camera icon on the toolbar to force an immediate sample. To highlight a counter's line or bar in the Chart or Histogram view, click the lightbulb icon on the toolbar. Now that you're familiar with some Performance tool basics, all you have to know is which 10 or so counters to add to monitor IIS.

Memory Usage
If you have a Web server with multiple sites that call third-party and custom COM objects, you need to keep track of how much of your system resources those Web sites and objects are using so that you'll know when you need a new box or whether you have to make your current box work harder. Watching memory and CPU usage is a great way to tell whether your machine's essential resources are getting tapped out.

If you think that one of your custom objects—a third-party COM object, an Internet Server API (ISAPI) filter, or an ISAPI extension—is leaking memory on your Web server, add the Memory object's Available MBytes counter. Then, click the Properties icon on the toolbar to open the System Monitor Properties dialog box. On the General tab, select the Graph view and set the monitor to update every 60 seconds. Monitor the graph for a 24-hour period when the production server is handling typical traffic or in a test environment that's running a stress tool such as Microsoft Application Center Test, which comes with Visual Studio .NET Enterprise Developer. If the counter decreases steadily, the machine is leaking memory. Make sure that you watch the graph over a period of time at a low refresh rate because memory use fluctuates a lot.

Note that IIS performance slows down when available memory drops below 60 percent of the RAM on the machine, not including the system swap file. Thus, you can encounter performance problems well before you run out of RAM.

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