IIS 7 & IIS 8
Scalability problem using web requests initiating asynch web service ...
Last post Feb 15, 2013 05:04 PM by Parashuram
Jan 09, 2013 03:53 PM|LINK
We are experiencing some problems in production (IIS 7.5 integrated managed pipeline mode and .NET 4.5 application) when to may of our incoming IIS requests trigger asynchronous web service requests. There is a quite nice MSDN blog post
here by Thomas Marquardt that gives a very good picture of the potential problems that can arise. I believe our problem is due to the ServicePointManager.DefaultConnectionLimit low default value and we are about to deploy this fix (requires code change
as can be read about in the blog.
However, I have quite a few remaining questions from this blog post. I have posted these questions yesterday in the blog post (quite miserably since there was a cap on the word lenght...), but since I am guessing it may take some time before Thomas may response,
I repost the questions here as well in hope that someone in the community knows some of the answers. You may need to read the blog post first to be in tune with what I write below, but I can promise it is good reading. If
someone knows one or several of the answers to the questions, please just refer to the what question it is in the answer (eg 1.c)
Here is my comment(s) in the blog post with nice WYSIWYG markup:
Thanks [Thomas Marquardt] for some great explanations and for taking the time to answer all these questions. It really clarifies things. In our case, we have a problem in production with an .NET 4.5 / IIS 7.5 web server running integrated [managed
pipeline] mode when we get a high load of requests that utilises a high degree of outgoing
asynchronous web service requests (stack trace reveals HttpWebRequest usage beneath the hood of the HttpClient class usage). Based on my reading here, it seems the solution is to set the
ServicePointManager.DefaultConnectionLimit programmatically in production code as you suggest (since we use autoConfig set to true). However before I try this in our production environment (initiating a redeploy) I have some remaining categorised
questions with regards to what I’ve read (sublabeled (a), (b) etc to make your answering easier):
I can of course follow up on this thread with answers from our internal investigation to my questions.
Jan 12, 2013 03:38 PM|LINK
As Thomas has already replied your question, I will post his reply in this thread below.
Tobias, in response to the post you gave on the IIS forum:
1) Yes, appRequestQueueLimit only applies to IIS 6 (also 7 when running in classic mode).
2a) IIS 7 and later have the queues that you mention.
2b) The HTTP.sys kernel queue is not the same as the ASP.NET process-wide queue.
2c) The HTTP.sys kernel queue is essentially a completion port on which user-mode (IIS) receives requests from kernel-mode (HTTP.sys). It has a queue limit, and when that is exceeded you will receive a 503 status code. The HTTPErr log will also indicate
that this happened by logging a 503 status and QueueFull.
2d) I do not know the details of how HttpClient or HttpWebRequest are implemented. You need to ensure that you are closing/disposing all System.Net objects properly. You likely need to increase connectionManagment/maxconnection in the config file or increase
it programmatically via ServicePointManager.DefaultConnectionLimit. You may also need to modify the default registry values for TcpTimedWaitDelay and MaxUserPorts if your connections are sitting in the TIME_WAIT state or you do not have enough ports available.
Be careful with these registry values--you need to know what you're doing, and why you're doing it. Perhaps the System.Net folks have a forum?
2e) "Process(w3wp)\Thread Count" and the ".NET CLR LocksAndThreads" performance counters will help a little, but ultimately you will need to resort to the debugger (windbg) and the sos.dll debugger extension. It has a !ThreadPool command that will tell
you how many threads are active in the pool and what the maximum limits are.
2f) "Web Service\Current Connections" is the number of connections to IIS. This has nothing to do with your outbound System.Net connections.
3) ASP.NET v4.5 has a performance counter in the "ASP.NET" category specifically for the native queue. This is new to v4.5.
4a) Perhaps, but I'm not familiar with the "TCPv4\Connection Established" performance counter.
4b) Yes, I would be careful about changing TcpTimedWaitDelay and/or MaxUserPort. You need to know what you're doing, and why you're doing it.
Jan 14, 2013 08:11 AM|LINK
Yes, thanks. I saw that last week. I tried to post a follow up comment on his blog, but it seems he turned it off now for further comments. Quite understandable if Thomas is not working with the se things anymore. I have some follow up questions at the bottom
of this comment though.
Though Thomas answers I think answered most of my questions. However, we were also able to establish last week that
setting ServicePointManager.DefaultConnectionLimit to int maxValue really does not make a difference since this already is the new default value in .NET 4.5. So essentially, it is not our
problem. We will most likely open a Microsoft support case on this issue later on this week. But as an input to this, I will write a summary of the problem + possible solutions as found o the Internet. I might as well post
it here before I mark anything as an answer. I'll follow up with another dedicated post for this.
Follow up information/questions on previous questsions/answers:
I found the performance counter for the HTTP.sys: HTTP Service Request Queues.CurrentQueueSize.
2 further questions regarding number of queues in the request pipeline:
So it seems that 3 queues have been identified so far then (1) HTTP.sys queue, (2) "native" queue, and the (3) .NET Thread Pool. I've looked a bit at the decompiled source code of WebClient that we are using (we changed from HttpClient
since this was too slow), but it is a bit too much source code to dig into. I am guessing though that at the .NET Framework level (above the CLR ThreadPool queue), there are no other queues that can cause the problem.
f) Is this a wrong assumption that between the .NET Threadpool and the WebClient asyc usage there are no queues? (this may be the wrong forum for that question, I know)
Information: Likewise, at a lower level, I guess there may be other queues in Kernal mode. Eg. network adaptor buffer. However, since this is Kernal execution level, it should
be more likely that the user mode level queues would be become the bottlenecks, not the native level queues. However, on the performance counter category [Network Interface] there is a counter called "Packets Received Discarded" that may rise if there if the
network card (driver) buffer is full (according to the description of the performance counter).
2 (f). Information:
For ayone interested: we found the performance couters for the outgoing connections in the [.NET CLR Networking 188.8.131.52] category. Several useful HttpWebRequests counters there.
2(e) Thread Pool thread counter question
So I don't understand why the "Process(w3wp)\Thread Count" and the ".NET CLR LocksAndThreads" only helps "a bit" as Thomas states. Why are these not accurate enough?
Jan 15, 2013 07:29 AM|LINK
I have posted a follow up summary of the problems and potential solution candidates on
the stacktrace forum since it may be a bit more active with possible responses. But we'll
see. I may post o the ASP.NET forum as well if no ones has any input there.
Feb 15, 2013 05:04 PM|LINK
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