Performance testing: considerations and best practices

Your test environment should be capable of simulating production environment conditions. To do this, keep the following considerations in mind during the test cycle:

  • Do not place too much stress on the client.
  • Create baselines for your test setup.
  • Allow for think time in your test script.
  • Consider test duration.
  • Minimize redundant requests during testing.
  • Consider simultaneous versus concurrent users.
  • Set an appropriate warm up time.

The next sections describe each of these considerations.


Do Not Place Too Much Stress on the Client
Do not overly stress the client computer used to generate application load. The processor and memory usage on your client computers should be well below the threshold limit (CPU: 75 percent). Otherwise, the client computer may end up as a bottleneck during your testing. To avoid this, consider distributing the load on multiple client computers. Also, monitor the network interface bandwidth utilization to ensure that the network is not getting congested.
Create Baselines for Your Test Setup
Create baselines for your test setup for all types of tests you need to perform. The setup should be representative of the real life environment. This has two advantages. First, the results from various categories of tests do not reflect the type of hardware you are using. This means that your application modifications are the only variable. Second, you can depend on the test results because the test setup closely mirrors the real life environment.
Allow for Think Time in Your Test Script
Think time reflects the amount of time that a typical user is likely to pause for thought while interacting with your application. During this time, the user views the information displayed on the page or enters details such as credit card numbers or addresses. You should average the think time across various operations.
If you do not include think time in your script, there is no time gap between two subsequent requests on a per-user basis. This directly translates to all users firing requests to the server concurrently. So, for a concurrent load of 200 users, there will be 200 requests fired at a given instance to the server. This is not a true representation of how your typical users use your application.

Note: Omitting think time can be helpful when you want to generate excessive load on the server.

Make sure that the think time in your test script is based on real life conditions. This varies according to the operations performed by the user, and it varies depending on the page the user is interacting with.
For your test scripts, you can program for either a fixed think time between consecutive requests or a random think time ranging between minimum and maximum values.
Consider Test Duration
You should base the time duration for your tests on the end goal. If the goal is to load test and monitor the application behavior for your workload pattern, the test duration might range from 20 minutes to as long as one week. If the site is expected to experience users of similar profile, and the average user is expected to perform the same set of operations during the intraday activity, a test of 20 minutes to one hour is sufficient for generating data for load testing. You may want to run load tests for an extended period — four to five days — to see how your application performs on the peak operating capacity for a longer duration of time.
However, to generate test data for your site if your site expects users of different profiles during various hours of operation, you may need to test for at least eight to 10 hours to simulate various user profiles in the workload pattern.
For stress testing purposes, the end goal is to run tests to identify potential resource leaks and the corresponding degradation in application performance. This may require a longer duration, ranging from a couple of hours to a week, depending on the nature of the bottleneck.
For tests used to measure the transaction cost of an operation using transaction cost analysis, you might need to run test for only approximately 20 minutes.
More Information
For more information about capacity planning and transaction cost analysis, see ”How To: Perform Capacity Planning for .NET Applications” in the “How To” section of this guide.
Minimize Redundant Requests During Testing
Make sure that your test load script simulates an appropriate load for your particular scenario and does not generate additional redundant requests. For example, consider a logon scenario. The complete operation typically consists of two requests:
* A GET request used to retrieve the initial page where the user supplies a logon name and password.
* A POST request when the user clicks the Logon button to verify the credentials.

The GET request is the same for all users and can easily be cached and served to all users. However, it is the POST request that is critical from a performance perspective. In this example, your test script should avoid sending a GET request before submitting a POST request. By avoiding the GET request within your test script, the client threads of the load generator are able to iterate faster through the loop for a given set of users, thereby generating more POST requests. This results in a more effective stress test for the actual POST operation. However, there may be conditions when even the response to a GET request is customized for a user; therefore, you should consider including both the GET and POST requests in the stress tests.
Consider Simultaneous vs. Concurrent Users
Simultaneous users have active connections to the same Web site, whereas concurrent users connect to the site at exactly the same moment. Concurrent access is likely to occur at infrequent intervals. Your site may have 100 to 150 concurrent users but 1,000 to 1,500 simultaneous users.
To stress test your application by using tools such as ACT, use a think time of zero in your test scripts. This allows you to stress your application without any time lag, with all users concurrently accessing the site. By using an appropriate think time, however, you can more accurately mirror a real life situation in which a user takes some time before submitting a new request to the server.
High numbers of simultaneous users tend to produce spikes in your resource usage and can often cause interaction that is beyond the concurrent load a server can handle. This results in occasional “server busy” errors. Tests with simultaneous users are very useful because they help you identify the actual load your system can handle without sending back too many server busy errors.
Set an Appropriate Warm-up Time
ACT supports warm-up times. The warm-up time is used in a test script to ensure your application reaches a steady state before the test tool starts to record results. The warm-up time causes ACT to ignore the data from the first few seconds of a test run. This is particularly important for ASP.NET applications because the first few requests trigger just-in-time (JIT) compilation and caching.
The warm up time is particularly relevant for short duration tests, so that the initial startup time does not skew the test results.
To determine an appropriate warm-up time for your application, use the ASP.NET Applications\Compilations Total counter to measure the effects of JIT compilation. This counter should increase every time a user action triggers the JIT complier.
In some cases you may want to know how long it takes to compile and cache. It should be a separate test; it should not be averaged into your steady state measurements.
Best Practices for Performance Testing
When you test, consider the following best practices.
Do
When performing performance testing, make sure you do the following:

  • Clear the application and database logs after each performance test run. Excessively large log files may artificially skew the performance results.
  • Identify the correct server software and hardware to mirror your production environment.
  • Use a single graphical user interface (GUI) client to capture end-user response time while a load is generated on the system. You may need to generate load by using different client computers, but to make sense of client-side data, such as response time or requests per second, you should consolidate data at a single client and generate results based on the average values.
  • Include a buffer time between the incremental increases of users during a load test.
  • Use different data parameters for each simulated user to create a more realistic load simulation.
  • Monitor all computers involved in the test, including the client that generates the load. This is important because you should not overly stress the client.
  • Prioritize your scenarios according to critical functionality and high-volume transactions.
  • Use a zero think time if you need to fire concurrent requests,. This can help you identify bottleneck issues.
  • Stress test critical components of the system to assess their independent thresholds.

Do Not

  • Do not allow the test system resources to cross resource threshold limits by a significant margin during load testing, because this distorts the data in your results.
  • Do not run tests in live production environments that have other network traffic. Use an isolated test environment that is representative of the actual production environment.
  • Do not try to break the system during a load test. The intent of the load test is not to break the system. The intent is to observe performance under expected usage conditions. You can stress test to determine the most likely modes of failure so they can be addressed or mitigated.
  • Do not place too much stress on the client test computers.

Related content: https://feelsgood11.wordpress.com/2008/10/27/some-of-best-practices-in-performance-testing/

Source: http://channel9.msdn.com/Wiki/PerformanceWiki/PerformanceTestingGuidelines/

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