Introduction
As mentioned in my last blog entry, for IIS-hosted WCF services, WCF holds the worker thread coming from ASP.NET until the whole request is completed to avoid a Denial of Service (DOS) attack. I also mentioned that on Windows 2008 Server, IIS7 has introduced the following registry setting to provide request throttling for all incoming requests:
[HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\ASP.NET\2.0.50727.0]
“MaxConcurrentRequestsPerCpu”=dword:0000000c
Based on this, in Orcas SP1 (.NET 3.5 SP1), WCF has implemented the asynchronous HTTP Module/Handler to allow better server scalability for high latency requests.
Asynchronous WCF HTTP Module/Handler
Besides the existing synchronous WCF HTTP Module/Handler types, WCF introduced the asynchronous versions. Together, WCF has the following four types implemented:
· Synchronous Module:
System.ServiceModel.Activation.HttpModule, System.ServiceModel, Version=3.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089
· Asynchronous Module (new):
System.ServiceModel.Activation.ServiceHttpModule, System.ServiceModel, Version=3.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089
· Synchronous Handler:
System.ServiceModel.Activation.HttpHandler, System.ServiceModel, Version=3.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089
· Asynchronous Handler (new):
System.ServiceModel.Activation.ServiceHttpHandlerFactory, System.ServiceModel, Version=3.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089
Asynchronous HTTP Module
The new type ServiceHttpModule implements the System.Web.IHttpModule and it registers the async event HttpApplication.AddOnPostAuthenticateRequestAsync with a pair of async handlers:
static public IAsyncResult BeginProcessRequest(object sender, EventArgs e, AsyncCallback cb, object extraData)
static public void EndProcessRequest(IAsyncResult ar)
When a request comes in, WCF does some validation and returns from BeginProcessRequest immediately after passing the request up to internal processing logic. In this way, the worker process is immediately released.
If the service operation is asynchronous and it is waiting for slow I/O operations, the whole stack does not hold any thread. The only performance concern would be the memory usage to hold objects used to process the request. This allows many concurrent requests to be served by WCF without using a lot of threads and thus greatly improves the scalability of the service. This is very helpful when completing each request takes significant amount of time and many clients are served. In some private testing, WCF can easily support more than 10K concurrent slow requests, which is not possible with the synchronous HttpModule.
Asynchronous HTTP Handler Factory
You might have already noticed that WCF implements the Handler Factory instead of the asynchronous Handler directly. This allows more flexibility in the future to support different types of HTTP Handlers. The ServiceHttpHandlerFactory creates a stateless asynchronous HTTP handler of the following type which implements System.Web.IHttpAsyncHandler:
System.ServiceModel.Activation.ServiceHttpHandler
It implements the pair of asynchronous request handlers:
public IAsyncResult BeginProcessRequest(HttpContext context, AsyncCallback cb, object extraData)
public void EndProcessRequest(IAsyncResult result)
Default Installation – Synchronous Version
Though the asynchronous version of HTTP Module/Handler is added in this release, the default installation is still the synchronous version. Why is this?
First of all, you should not use asynchronous version of HTTP Module/Handler on Windows 2003 Server (with IIS6) without knowing the risk. If you have many clients, the service would be easily DOS attacked by too much memory usage due to huge amount of pending requests queued up in WCF transport layer without the throttling inside IIS/ASP.NET.
Because of the limitation on IIS6, WCF did not change the setup to switch to purely asynchronous due to the ramifications between IIS6 and IIS7 for this quick release. This means that in order to use the asynchronous version, you have to perform some manual registration.
Don’t worry, I have provided a private tool to do this registration for you.
WCF Module/Handler Registration Tool
You can find the simple tool called WcfAsyncWebUtil.exe attached in this blog. Here is the usage:
Usage: WcfAsyncWebUtil <options>
Options:
/is
Install WCF Synchronous HTTP module and handler into IIS7 configuration
in Integrated Mode.
/ia
Install WCF Asynchronous HTTP module and handler into IIS7 configuration
in Integrated Mode.
/l
List WCF HTTP module and handler registered into IIS7 configuration in
Integrated Mode.
/t [ <throttle> ]
Configure the throttling registry MaxConcurrentRequestsPerCpu for the ASP.NET
integrated pipeline to the value <throttle>. Default is 100. This argument is
used together with /ia only
For example, if you want to register asynchronous HTTP Module/Handler with the ASP.NET throttling to be 1000, you can run the following command:
WcfAsyncWebUtil.exe /ia /t 1000
To list the installed WCF HTTP Module/Handler, you can run:
WcfAsyncWebUtil.exe /l
Two Threads per Request
In .NET 3.0 and 3.5, there is a special behavior that you would observe for IIS-hosted WCF services. Whenever a request comes in, the system would use two threads to process the request:
· One thread is the CLR ThreadPool thread which is the worker thread that comes from ASP.NET.
· Another thread is an I/O thread that is managed by the WCF IOThreadScheduler (actually created by ThreadPool.UnsafeQueueNativeOverlapped).
When you have a high latency request, you would see the following callstack in the debugger before the request is completed (with .NET 3.5):
0ee6ee8c 5094dad5 mscorlib_ni!System.Threading.WaitHandle.WaitOne()+0xa
0ee6ee8c 50951e3b System_ServiceModel_ni!System.ServiceModel.Activation.HostedHttpRequestAsyncResult.ExecuteSynchronous(System.Web.HttpApplication, Boolean)+0x8d
0ee6eeb4 65fe626d System_ServiceModel_ni!System.ServiceModel.Activation.HttpModule.ProcessRequest(System.Object, System.EventArgs)+0x143
0214e4f4 65fe3fd1 System_Web_ni!System.Web.HttpApplication+SyncEventExecutionStep.System.Web.HttpApplication.IExecutionStep.Execute()+0x5d
0ee6ef08 65fe804f System_Web_ni!System.Web.HttpApplication.ExecuteStep(IExecutionStep, Boolean ByRef)+0x41
0ee6efb8 65fe4df2 System_Web_ni!System.Web.HttpApplication+PipelineStepManager.ResumeSteps(System.Exception)+0x63b
060f05dc 66003a92 System_Web_ni!System.Web.HttpApplication.BeginProcessRequestNotification(System.Web.HttpContext, System.AsyncCallback)+0x56
0ee6f040 65fd8022 System_Web_ni!System.Web.HttpRuntime.ProcessRequestNotificationPrivate(System.Web.Hosting.IIS7WorkerRequest, System.Web.HttpContext)+0x352
0ee6f0c4 65fd7e07 System_Web_ni!System.Web.Hosting.PipelineRuntime.ProcessRequestNotificationHelper(IntPtr, IntPtr, IntPtr, Int32)+0x1f2
0ee6f0f4 01782374 System_Web_ni!System.Web.Hosting.PipelineRuntime.ProcessRequestNotification(IntPtr, IntPtr, IntPtr, Int32)+0x1b
0ee6f138 6a2abf3c webengine!MgdGetCurrentNotification+0x236
0ee6f19c 74a22ea0 webengine!MgdGetPreloadedSize+0x4d
0ee6f1b0 74a23696 iiscore+0x2ea0
0ee6f560 6a2ac222 webengine!MgdCanDisposeManagedContext+0xb5
0ee6f570 015f1311 webengine!MgdIndicateCompletion+0x22
System_Web_ni!System.Web.Hosting.PipelineRuntime.ProcessRequestNotificationHelper(IntPtr, IntPtr, IntPtr, Int32)+0x289
This is the worker thread that comes from ASP.NET. Another thread is still processing the request in WCF:
01fbf0ac 0e9001c7 App_Web_rsfhd6l1!HelloWorld.SimpleService.Hello(System.String)+0x14
01fbf0ac 50b8d90b System_ServiceModel_ni!DynamicClass.SyncInvokeHello(System.Object, System.Object[], System.Object[])+0x3f
01fbf0ac 50b6d245 System_ServiceModel_ni!System.ServiceModel.Dispatcher.SyncMethodInvoker.Invoke(System.Object, System.Object[], System.Object[] ByRef)+0x1fb
01fbf108 509137ad System_ServiceModel_ni!System.ServiceModel.Dispatcher.DispatchOperationRuntime.InvokeBegin(System.ServiceModel.Dispatcher.MessageRpc ByRef)+0xd5
01fbf148 509136a6 System_ServiceModel_ni!System.ServiceModel.Dispatcher.ImmutableDispatchRuntime.ProcessMessage5(System.ServiceModel.Dispatcher.MessageRpc ByRef)+0xad
01fbf174 50913613 System_ServiceModel_ni!System.ServiceModel.Dispatcher.ImmutableDispatchRuntime.ProcessMessage4(System.ServiceModel.Dispatcher.MessageRpc ByRef)+0x76
062b75a8 50913459 System_ServiceModel_ni!System.ServiceModel.Dispatcher.ImmutableDispatchRuntime.ProcessMessage3(System.ServiceModel.Dispatcher.MessageRpc ByRef)+0x33
062b75a8 50912257 System_ServiceModel_ni!System.ServiceModel.Dispatcher.ImmutableDispatchRuntime.ProcessMessage2(System.ServiceModel.Dispatcher.MessageRpc ByRef)+0x59
062b75a8 50911f8f System_ServiceModel_ni!System.ServiceModel.Dispatcher.ImmutableDispatchRuntime.ProcessMessage1(System.ServiceModel.Dispatcher.MessageRpc ByRef)+0x127
01fbf1f0 509115ff System_ServiceModel_ni!System.ServiceModel.Dispatcher.MessageRpc.Process(Boolean)+0xdf
01fbf394 5090f8c9 System_ServiceModel_ni!System.ServiceModel.Dispatcher.ChannelHandler.DispatchAndReleasePump(System.ServiceModel.Channels.RequestContext, Boolean, System.ServiceModel.OperationContext)+0x1ff
01fbf3dc 5090f35e System_ServiceModel_ni!System.ServiceModel.Dispatcher.ChannelHandler.HandleRequest(System.ServiceModel.Channels.RequestContext, System.ServiceModel.OperationContext)+0x169
01fbf42c 5090f2f1 System_ServiceModel_ni!System.ServiceModel.Dispatcher.ChannelHandler.AsyncMessagePump(System.IAsyncResult)+0x5e
01fbf42c 50232d68 System_ServiceModel_ni!System.ServiceModel.Dispatcher.ChannelHandler.OnAsyncReceiveComplete(System.IAsyncResult)+0x41
01fbf42c 50904501 SMDiagnostics_ni!System.ServiceModel.Diagnostics.Utility+AsyncThunk.UnhandledExceptionFrame(System.IAsyncResult)+0x28
01fbf468 50992b36 System_ServiceModel_ni!System.ServiceModel.AsyncResult.Complete(Boolean)+0xb1
01fbf4d8 50992215 System_ServiceModel_ni!System.ServiceModel.Channels.InputQueue`1+AsyncQueueReader[[System.__Canon, mscorlib]].Set(Item<System.__Canon>)+0x46
01fbf4d8 50991ffb System_ServiceModel_ni!System.ServiceModel.Channels.InputQueue`1[[System.__Canon, mscorlib]].EnqueueAndDispatch(Item<System.__Canon>, Boolean)+0x1f5
00000000 5091d7e5 System_ServiceModel_ni!System.ServiceModel.Channels.InputQueue`1[[System.__Canon, mscorlib]].EnqueueAndDispatch(System.__Canon, System.ServiceModel.Channels.ItemDequeuedCallback, Boolean)+0x6b
00000001 50977b7e System_ServiceModel_ni!System.ServiceModel.Channels.SingletonChannelAcceptor`3[[System.__Canon, mscorlib],[System.__Canon, mscorlib],[System.__Canon, mscorlib]].Enqueue(System.__Canon, System.ServiceModel.Channels.ItemDequeuedCallback, Boolean)+0x75
01fbf570 5094f396 System_ServiceModel_ni!System.ServiceModel.Channels.HttpChannelListener.HttpContextReceived(System.ServiceModel.Channels.HttpRequestContext, System.ServiceModel.Channels.ItemDequeuedCallback)+0x1ce
01fbf5b8 5094e4cf System_ServiceModel_ni!System.ServiceModel.Activation.HostedHttpTransportManager.HttpContextReceived(System.ServiceModel.Activation.HostedHttpRequestAsyncResult)+0xc6
0216803c 5094defd System_ServiceModel_ni!System.ServiceModel.Activation.HostedHttpRequestAsyncResult.HandleRequest()+0xdb
01fbf60c 5094dea5 System_ServiceModel_ni!System.ServiceModel.Activation.HostedHttpRequestAsyncResult.BeginRequest()+0x19
01fbf638 50903be6 System_ServiceModel_ni!System.ServiceModel.Activation.HostedHttpRequestAsyncResult.OnBeginRequest(System.Object)+0x29
01fbf674 50903b26 System_ServiceModel_ni!System.ServiceModel.Channels.IOThreadScheduler+CriticalHelper+WorkItem.Invoke2()+0x36
01fbf688 50903ab5 System_ServiceModel_ni!System.ServiceModel.Channels.IOThreadScheduler+CriticalHelper+WorkItem.Invoke()+0x4a
01fbf6bc 5090390f System_ServiceModel_ni!System.ServiceModel.Channels.IOThreadScheduler+CriticalHelper.ProcessCallbacks()+0x185
01fbf6e8 5090388b System_ServiceModel_ni!System.ServiceModel.Channels.IOThreadScheduler+CriticalHelper.CompletionCallback(System.Object)+0x6f
01fbf720 50232e1f System_ServiceModel_ni!System.ServiceModel.Channels.IOThreadScheduler+CriticalHelper+ScheduledOverlapped.IOCallback(UInt32, UInt32, System.Threading.NativeOverlapped*)+0xf
01fbf720 79405534 SMDiagnostics_ni!System.ServiceModel.Diagnostics.Utility+IOCompletionThunk.UnhandledExceptionFrame(UInt32, UInt32, System.Threading.NativeOverlapped*)+0x2f
01fbf744 79e7c74b mscorlib_ni!System.Threading._IOCompletionCallback.PerformIOCompletionCallback(UInt32, UInt32, System.Threading.NativeOverlapped*)+0x60
01fbf758 79e7c6cc mscorwks!LogHelp_TerminateOnAssert+0x3433
01fbf7d8 79f00eca mscorwks!LogHelp_TerminateOnAssert+0x33b4
01fbf7f8 79f00e75 mscorwks!GetCLRFunction+0x1de16
Why is this? Is this a scalability issue for WCF? I will explain this below.
Asynchronous Request Processing
One of the most significant pieces for WCF is that it is highly asynchronous from bottom to top for the whole channel stack and the programming model. The advantage is apparent: applications have high flexibility by issuing operations asynchronously, doing some other stuff in parallel, and coming back to handle the completed operations once they are completed. This is also scalable when the asynchronous operations are I/O bound so that you do not need to hold extra threads to wait for operation completions.
ASP.NET also supports asynchronous request processing mechanisms through IHttpModule and IHttpAsyncHandler. With these interfaces, ASP.NET can push many concurrent requests up to the layers which have such asynchronous implementations. In this way, the whole stack need a small number of threads to process a large number of concurrent requests given appropriate queuing for the requests.
However, this also exposes a memory problem when this is on IIS 6.0. When there is unlimited number of incoming requests, all of the requests would be pushed up by ASP.NET to upper layers and it would cause significant memory consumption. Thus it would cause the server to be unavailable to do its work correctly.
So in the release of .Net 3.0 and 3.5, WCF implemented synchronous versions of HTTP module and handler instead of asynchronous ones. See the following for more details.
ASP.NET Threading on IIS 6.0
When ASP.NET is hosted in IIS 6.0, requests are handed over to ASP.NET on IIS I/O threads. ASP.NET uses CLR ThreadPool worker threads to handle requests. The CLR ThreadPool automatically adjusts the number of threads based on the server workload. In order to prevent from using too many threads or using too much memory, ASP.NET has a limit on the number of threads concurrently executing requests. This is controlled by the httpRuntime/minFreeThreads and httpRuntime/minLocalRequestFreeThreads settings. If the limit is exceeded, the request is queued in the application-level queue, and executed later when the concurrency falls back down below the limit. Thomas Marquardt has written an excellent blog on this:
http://blogs.msdn.com/tmarq/archive/2007/07/21/asp-net-thread-usage-on-iis-7-0-and-6-0.aspx (TM0707)
In .NET 2.0, the magic config “processModel/autoConfig” in machine.config automatically configures the ASP.NET threading settings for most applications. For those which require special settings need to be configured differently. The following KB article was provided as the general guidance:
http://support.microsoft.com/kb/821268 (KB821268)
However, there is no throttling for concurrent requests in ASP.NET on IIS 6.0.
WCF Request Throttling and Tuning
Because of lacking the throttling for concurrent requests in ASP.NET, WCF would get unlimited concurrent requests pushed up from ASP.NET if WCF had implemented asynchronous HTTP module/handler on IIS 6.0. It would cause severe memory usage problem. Because of this concern, WCF implemented the synchronous HTTP module/handler in 3.0 and 3.5 instead.
Here I only describe how the HTTP module works for WCF. The logic for the HTTP handler is similar. When a request comes in from ASP.NET, WCF grabs the request if it is a WCF request (by checking the registered BuildProvider of the extension to see whether it is WCF service BuildProvider). After that, it immediately switches to a new I/O thread and hold the ASP.NET worker thread until the request is completed. You may ask, why doesn’t WCF reuse the ASP.NET worker thread to handle the request until it is completed? Here are the reasons:
· As long as the ASP.NET worker thread is not returned back to ASP.NET, the request is not able to be completed. The client would be blocked.
· When the WCF operation is one-way, the request has to be completed before the service operation is invoked. So the ASP.NET worker thread has to return earlier.
· WCF supports people to complete a two-way operation earlier in a service operation when you call RequestContext.Close.
All of these require that WCF should not hold the ASP.NET worker thread once the request is claimed to be “completed”. At the same time, WCF has to hold the ASP.NET worker thread in a waited state before the request is completed due to the requirement of synchronous request processing logic. WCF relies on this ASP.NET thread throttling logic to limit concurrent requests.
Intuitively this seems to have scalability issue. Fortunately for most WCF services, especially high-throughput ones, this has quite low impact as long as the server is busy processing requests. This is demonstrated as the CPU usage of the server.
On multi-proc/multi-core servers, the default limit of concurrent requests may not be enough for the server to achieve best throughput. This would require larger limits for ASP.NET worker threads to be used to handle requests.
Throttling Tuning
In order to achieve high throughput on high-end servers, you would need to make the following throttling tuning:
· Increase ASP.NET thread throttling to allow more concurrent worker threads to handle requests
· Increase WCF service throttling to allow more concurrent requests
The latter reflects the change of the known WCF throttling settings such as MaxConcurrentCalls, MaxConcurrentInstances, and MaxConcurrentSessions. The former would require fine tuning of ASP.NET thread throttling as documented in the KB article KB821268 mentioned above. Basically ASP.NET does not execute more than the following number of concurrent requests:
(maxWorkerThreads*number of CPUs)-minFreeThreads
This is 12 by default on a single CPU machine. In order to get more threads to process requests, you need to increase maxWorkerThreads. Once you can get CPU fully loaded with the settings, setting “minWorkerThreads” to 2 or 3 would allow WCF to have best throughput. Here is sample setting in machine.config:
<processModel autoConfig="false" maxWorkerThreads="500" maxIoThreads="500" minWorkerThreads=”2”/>
<httpRuntime minFreeThreads="250" minLocalRequestFreeThreads="250"/>
The above settings work for most WCF applications. However, when the service has very slow operations which are I/O bound (for example, they further connects to backend database or network or file system to perform high-latency operations), you would need to use many outstanding ASP.NET worker threads to achieve parallel requests. This is bad when the number of requests is huge, for example, several thousands or more. Each thread would hold a fair amount of system resource, i.e., memory. It would cause the service to be very slow to respond. This really requires a real asynchronous design to solve the problem. Because of this, ASP.NET introduced a new throttling logic for concurrent requests in IIS 7.0 to support this scenario for WCF.
ASP.NET Throttling on IIS 7.0 in Integrated Mode
As Thomas mentioned in his blog entry (TM0707) mentioned above, the following registry value is introduced to throttle concurrent requests:
[HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\ASP.NET\2.0.50727.0]
“MaxConcurrentRequestsPerCpu”=dword:0000000c
The default value is 12 per CPU which means is similar as the default setting for ASP.NET worker threads. This was introduced in ASP.NET 2.0 SP1 (shipped with .NET 3.5) and thus it is available in Windows 2008. It only applies to the IIS 7.0 Integrated mode, which is exactly where WCF is registered on Windows 2008.
Scalable Solutions for High Latency Services
Though the above throttling support was added to Windows 2008, WCF still uses synchronous module/handler instead of asynchronous ones in the released versions due to timing constraint. However, the internal WCF design does allow asynchronous request processing and it is just one step away from the real asynchronous logic. Here I provide a simple wrapper for that.
Custom HttpModule Using Reflection
The internal WCF types support asynchronous HTTP module. Since they are internal, we have to rely on reflection to create a real one.
First of all, you would need to register the async events for your HttpModule as following:
context.AddOnPostAuthenticateRequestAsync(
MyHttpModule.beginEventHandler,
MyHttpModule.endEventHandler);
This is registered to the PostAuthenticateRequest for the same reasons as the default WCF HttpModule.
Secondly, you would need to use reflection to create the instance of the internal WCF type System.ServiceModel.Activation.HostedHttpRequestAsyncResult:
Type hostedHttpRequestAsyncResultType = typeof(ServiceHostingEnvironment).Assembly.GetType("System.ServiceModel.Activation.HostedHttpRequestAsyncResult");
ConstructorInfo hostedHttpRequestAsyncResult = hostedHttpRequestAsyncResultType.GetConstructor(BindingFlags.NonPublic | BindingFlags.Instance | BindingFlags.Public,
null, new Type[] { typeof(HttpApplication), typeof(bool), typeof(AsyncCallback), typeof(object) }, null);
return (IAsyncResult)hostedHttpRequestAsyncResult.Invoke(new object[] { application, false, cb, extraData });
Once you have the custom HttpModule, you can replace it with the default one that is registered in the WAS configuration %windir%\system32\inetsrv\config\applicationhost.config:
<system.webServer>
<modules>
<add name="ServiceModel" type="System.ServiceModel.Activation.HttpModule, System.ServiceModel, Version=3.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089" preCondition="managedHandler" />
<modules>
<system.webServer>
To do that, you can also define local settings in web.config:
<system.webServer>
<modules>
<remove name="ServiceModel"/>
<add name="MyAsyncWCFHttpModule" type="AsyncWcfModule.MyHttpModule, AsyncWcfModule" preCondition="managedHandler" />
</modules>
<validation validateIntegratedModeConfiguration="false" />
</system.webServer>
The sample code is attached.
Improvement in .NET 3.5 SP1
In .NET 3.5 SP1, we will provide the real asynchronous HTTP module/handler for WCF to better solve this problem.