Thursday, August 25, 2011

Tuning IIS 6.0 to Improve ASP.NET Performance

In the Patterns and Practices Group's "Improving .NET Application Performance and Scalability", which is available in full text online and as a PDF download from the above link, as well as in softcover through MSPress and major booksellers, there are over 1000 pages and appendixes of detailed information about how to improve .NET application performance and scalability, written by the top experts in the business. One area that is both little understood and potentially confusing is the tuning of Internet Information Services 6.0.

Formula for Reducing Contention

The formula for reducing contention can give you a good empirical start for tuning the ASP.NET thread pool. Consider using the Microsoft product group-recommended settings that are shown in Table 6.1 if the following conditions are true:
  • You have available CPU.
  • Your application performs I/O bound operations such as calling a Web method or accessing the file system.
  • The ASP.NET Applications/Requests In Application Queue performance counter indicates that you have queued requests.
Table 6.1: Recommended Threading Settings for Reducing Contention
Configuration setting Default value (.NET Framework 1.1) Recommended value
maxconnection 2 12 * #CPUs
maxIoThreads 20 100
maxWorkerThreads 20 100
minFreeThreads 8 88 * #CPUs
minLocalRequestFreeThreads 4 76 * #CPUs
To address this issue, you need to configure the following items in the Machine.config file. Apply the recommended changes that are described in the following section, across the settings and not in isolation. For a detailed description of each of these settings, see "Thread Pool Attributes" in Chapter 17, "Tuning .NET Application Performance."
  • Set maxconnection to 12 * # of CPUs . This setting controls the maximum number of outgoing HTTP connections that you can initiate from a client. In this case, ASP.NET is the client. Set maxconnection to 12 * # of CPUs.
  • Set maxIoThreads to 100 . This setting controls the maximum number of I/O threads in the .NET thread pool. This number is automatically multiplied by the number of available CPUs. Set maxloThreads to 100.
  • Set maxWorkerThreads to 100 . This setting controls the maximum number of worker threads in the thread pool. This number is then automatically multiplied by the number of available CPUs. Set maxWorkerThreads to 100.
  • Set minFreeThreads to 88 * # of CPUs . This setting is used by the worker process to queue all the incoming requests if the number of available threads in the thread pool falls below the value for this setting. This setting effectively limits the number of requests that can run concurrently to maxWorkerThreads minFreeThreads . Set minFreeThreads to 88 * # of CPUs. This limits the number of concurrent requests to 12 (assuming maxWorkerThreads is 100).
  • Set minLocalRequestFreeThreads to 76 * # of CPUs . This setting is used by the worker process to queue requests from localhost (where a Web application sends requests to a local Web service) if the number of available threads in the thread pool falls below this number. This setting is similar to minFreeThreads but it only applies to localhost requests from the local computer. Set minLocalRequestFreeThreads to 76 * # of CPUs.
Discussion: The proviso above indicates that these settings should be used when your application has I/O bound operations and the Applications/Requests In Application Queue perfcounter indicates you have queued requests. However, I have found that settings approaching those indicated can improve performance on ASP.NET apps that do not exhibit these conditions. I recommend using the "Homer" web stress tool from at least one remote machine (and preferably more than one machine, with the supplied ASP controller page), or the .NET ACT Application Center Test application, to throw a good solid load at your app and carefully measure the performance statistics with each set of both the default and the above settings. In particular, pay close attention to the Requests per second and the time to last byte readings. This baseline testing scenario should provide the basis for further tuning if it is necessary, and it doesn't take long at all. You can only improve something if you have metrics, and the way you get the metrics is to take the time to get them! You can easily script all kinds of "user paths" through your ASP.NET application with testing software such as is mentioned here, and get the important baseline metrics you need. One more thing-- rule number 1 of software testing and debugging:
"When you are going to change something, ONLY CHANGE ONE THING AT A TIME!" Test it, get the metrics, and only then, proceed.

Kernel Mode Caching

If you deploy your application on Windows Server 2003, ASP.NET pages automatically benefit from the IIS 6.0 kernel cache. The kernel cache is managed by the HTTP.sys kernel-mode device driver. This driver handles all HTTP requests. Kernel mode caching may produce significant performance gains because requests for cached responses are served without switching to user mode.
The following default setting in the Machine.config file ensures that dynamically generated ASP.NET pages can use kernel mode caching, subject to the requirements listed below.
Dynamically generated ASP.NET pages are automatically cached subject to the following restrictions:
  • Pages must be retrieved by using HTTP GET requests. Responses to HTTP POST requests are not cached in the kernel.
  • Query strings are ignored when responses are cached. If you want a request for to be cached in the kernel, all requests for are served from the cache, regardless of the query string.
  • Pages must have an expiration policy. In other words, the pages must have an Expires header.
  • Pages must not have VaryByParams .
  • Pages must not have VaryByHeaders .
  • The page must not have security restrictions. In other words, the request must be anonymous and not require authentication. The HTTP.sys driver only caches anonymous responses.
  • There must be no filters configured for the W3wp.exe file instance that are unaware of the kernel cache.
Discussion: The "enableKernelOutputCache = "true" setting IS NOT present in the default machine.config "httpRunTime" element. Since it is not present, we should be able to expect that the default setting of "true" is automatic. Personally, I feel better explicitly putting the attribute in there, and setting it to "true". As an aside, I have found that it is ALWAYS a good idea to KEEP A BACKUP COPY of your machine.config stored somewhere safe.

Tuning the Thread Pool for Burst Load Scenarios

If your application experiences unusually high loads of users in small bursts (for example, 1000 clients all logging in at 9 A.M. in the morning), your system may be unable to handle the burst load. Consider setting minWorkerThreads and minIOThreads as specified in Knowledge Base article 810259, "FIX: SetMinThreads and GetMinThreads API Added to Common Language Runtime ThreadPool Class," at;en-us;810259 .
Discussion: The .NET Threadpool is somewhat limited in its flexibility and is specifically limited in terms of how many instances you may have per process, since it is static. If you have ASP.NET applications that specifically need to run background thread processing, you may wish to investigate using a custom threadpool class. I have used Ami Bar's SmartThreadPool with great success, and have even modified it to provide a ThreadPriority overload. You can have more than one instance of this pool, and each can be custom configured. This type of approach provides maximum flexibility while simultaneously permitting individual threadpool tuning of critical resources.

Tuning the Thread Pool When Calling COM Objects

ASP.NET Web pages that call single-threaded apartment (STA) COM objects should use the ASPCOMPAT attribute. The use of this attribute ensures that the call is executed using a thread from the STA thread pool. However, all calls to an individual COM object must be executed on the same thread. As a result, the thread count for the process can increases during periods of high load. You can monitor the number of active threads used in the ASP.NET worker process by viewing the Process:Thread Count (aspnet_wp instance) performance counter.
The thread count value is higher for an application when you are using ASPCOMPAT attribute compared to when you are not using it. When tuning the thread pool for scenarios where your application extensively uses STA COM components and the ASPCOMPAT attribute, you should ensure that the total thread count for the worker process does not exceed the following value.
75 + ((maxWorkerThread + maxIoThreads) * #CPUs * 2)

Evaluating the Change

To determine whether the formula for reducing contention has worked, look for improved throughput. Specifically, look for the following improvements:
  • CPU utilization increases.
  • Throughput increases according to the ASP.NET Applications\Requests/Sec performance counter.
  • Requests in the application queue decrease according to the ASP.NET Applications\Requests In Application Queue performance counter.
If this change does not improve your scenario, you may have a CPU-bound scenario. In a CPU-bound scenario, adding more threads may increase thread context switching, further degrading performance.
When tuning the thread pool, monitor the Process\Thread Count (aspnet_wp) performance counter. This value should not be more than the following.
75 + ((maxWorkerThread + maxIoThreads) * #CPUs) If you are using AspCompat, then this value should not be more than the following.
75 + ((maxWorkerThread + maxIoThreads) * #CPUs * 2) Values beyond this maximum tend to increase processor context switching.
Discussion: There is a long list of attention items that revolve around and are tightly woven into the IIS tuning issue for ASP.NET application tuning and scalability. These include, but are not limted to the following:
  • Improving page response times.
  • Designing scalable Web applications.
  • Using server controls efficiently.
  • Using efficient caching strategies.
  • Analyzing and applying appropriate state management techniques.
  • Minimizing view state impact.
  • Improving performance without impacting security.
  • Minimizing COM interop scalability issues.
  • Optimizing threading.
  • Optimizing resource management.
  • Avoiding common data binding mistakes.
  • Using security settings to reduce server load.
  • Avoiding common deployment mistakes.
Author: Peter A. Bromberg