CODEWITHSUNDEEP
c#multithreadingasynchronousazurethreadpool
flytzen
flytzen

Reputation: 7438

How to implement an IIS-like threadpool on a worker-server

EDIT I realised my question was not stated clearly enough and have edited it heavily.
This is a bit of an open ended question so apologies in advance.

In a nutshell, I want to implement IIS-style asynchronous request processing in an Azure worker role.

It may be very simple or it may be insanely hard - I am looking for pointers to where to research.

While my implementation will use Azure Workers and Service Bus Queues, the general principle is applicable to any scenario where a worker process is listening for incoming requests and then servicing them.

What IIS does

In IIS there is a fixed-size threadpool. If you deal with all request synchronously then the maximum number of requests you can deal with in parallel == maxthreads. However, if you have to do slow external I/O to serve requests then this is highly inefficient because you can end up with the server being idle, yet have all threads tied up waiting for external I/O to complete.

From MSDN:

On the Web server, the .NET Framework maintains a pool of threads that are used to service ASP.NET requests. When a request arrives, a thread from the pool is dispatched to process that request. If the request is processed synchronously, the thread that processes the request is blocked while the request is being processed, and that thread cannot service another request.

This might not be a problem, because the thread pool can be made large enough to accommodate many blocked threads. However, the number of threads in the thread pool is limited. In large applications that process multiple simultaneous long-running requests, all available threads might be blocked. This condition is known as thread starvation. When this condition is reached, the Web server queues requests. If the request queue becomes full, the Web server rejects requests with an HTTP 503 status (Server Too Busy).

In order to overcome this issue, IIS has some clever logic that allows you to deal with requests asynchronously:

When an asynchronous action is invoked, the following steps occur:

  1. The Web server gets a thread from the thread pool (the worker thread) and schedules it to handle an incoming request. This worker thread initiates an asynchronous operation.

  2. The worker thread is returned to the thread pool to service another Web request.

  3. When the asynchronous operation is complete, it notifies ASP.NET.

  4. The Web server gets a worker thread from the thread pool (which might be a different thread from the thread that started the asynchronous operation) to process the remainder of the request, including rendering the response.

The important point here is when the asynchronous request returns, the return action is scheduled to run on one of the same pool of threads that serves the initial incoming requests. This means that the system is limiting how much work it is doing concurrently and this is what I would like to replicate.

What I want to do

I want to create a Worker role which will listen for incoming work requests on Azure Service Bus Queues and also potentially on TCP sockets. Like IIS I want to have a maxium threadpool size and I want to limit how much actual work the worker is doing in parallel; If the worker is busy serving existing requests - whether new incoming ones or the callbacks from previous async calls - I don't want to pick up any new incoming requests until some threads have been freed up.

It is not a problem to limit how many jobs I start concurrently - that is easy to control; It is limiting how many I am actually working on concurrently.

Let's assume a threadpool of 100 threads.

Somehow, what IIS does is make sure that when an async call returns it uses the same fixed-size thread pool. This means that if I fire off a lot of async calls and they then return and start using my threads, IIS will not accept new requests until those returns have finished. And that is perfect because it ensures a sensible load on the server, especially when I have multiple load-balanced servers and a queue system that the servers pick work from.

I have this sneaky suspicion that this might be very simple to do, there is just something basic I am missing. Or maybe it is insanely hard.

Upvotes: 1

Views: 2355

Answers (6)

flytzen
flytzen

Reputation: 7438

I have been digging around in this and found that it is indeed relatively easy. http://www.albahari.com/threading/ has got some good information and I actually ended up buying the book which that website is essentially promoting.

What I found out is that;

  • Your application has a ThreadPool available to it by default
  • You can limit the number of threads available in the ThreadPool
  • When you use QueueUserWorkItem or Task.Factory.StartNew the job you start run on a Thread in the ThreadPool
  • When you use one of the asynchronous IO calls in the framework (Begin... methods or WebcClient.DownloadStringAsync etc) the the callbacks will also run on a Thread from the ThreadPool (what happens with the IO request itself is outside the scope of this discussion).

So far, so good. The problem is that I can keep calling Task.Factory.StartNew as much as I like and the ThreadPool will simply queue up the work until there are free threads to service them. So, in the case of an Azure Worker, I could easily empty the Queue even though my worker is busy servicing existing requests (and callbacks from existing requests). That is the core of my problem. What I want is to not take anything out of the queue until I actually have some free threads to service the request.

This is a very simple example of how this could be achieved. In essence, I am using an AutoResetEvent to make sure that I don't start another task from the queue until the previous task has actually started. Granted, I do actually take stuff out of the queue before there is a free thread, but on balance this should avoid crazy overloads of the worker and allow me to spin up more workers to share the load.

ThreadPool.SetMaxThreads(5, 1000); // Limit to 5 concurrent threads
ThreadPool.SetMinThreads(5, 10); // Ensure we spin up all threads

var jobStart = new AutoResetEvent(true);

// The "listen" loop
while (true) 
{   
    var job = this.jobQueue.Dequeue();
    jobStart.WaitOne(); // Wait until the previous job has actually been started
    Task.Factory.StartNew(
        () =>
            {
                jobStart.Set(); // Will happen when the threadpool allocates this job to a thread
                this.Download(job);
            });

}

This can - and probably should - be made a lot more sophisticated, including having timeouts, putting the work item back in the queue if a thread can't be allocated within a reasonable time and so on. An alternative would be to use ThreadPool.GetAvailableThreads to check if there are free threads before starting to listen to the queue but that feels rather more error prone.

Upvotes: 1

silijon
silijon

Reputation: 942

I've used the SmartThreadPool project in the past as a per-instance pool and, if I'm reading you correctly, it should have all the callback and worker-limiting functionality you need. My company actually has it running currently on Azure for the exact purpose you describe of reading message bus requests asynchronously.

Upvotes: 1

makerofthings7
makerofthings7

Reputation: 61433

Have you considered having a dedicated WCF instance (not WAS or IIS hosted) to buffer the long running requests? It will have its own dedicated app pool, with a separate Max value setting from IIS that won't contend with your ASP.NET HTTP requests. (HTTP requests are served by

Then use IIS Async methods to call WCF with the constrained app pool.

Upvotes: 1

hocho
hocho

Reputation: 1749

From what I understand you want to constrain the number of threads used for processing a certain type of message at the same time.

One approach would be to simply wrap the message processor, invoked on a new thread with something like

try
{
   Interlocked.Increment(ref count)

   Process(message);
}
finally 
{
    Interlocked.Decrement(ref count)
}

Before invoking the wrapper, simply check if the ‘count’ is less than your threshold count; and stop polling/handling more messages till the count is sufficiently lower.

EDIT Added more information based on comment

Frans, not sure why you see the infrastructure and business code being coupled. Once you place your business process to be serviced as a task on a new thread to run asynchronously, you need not worry about performing additional IO bound calls asynchronously. This is a simpler model to program in.

Here is what I am thinking.

// semi - pseudo-code

// Infrastructure – reads messages from the queue 
//    (independent thread, could be a triggered by a timer)
while(count < maxCount && (message = Queue.GetMessage()) != null)
{
    Interlocked.Increment(ref count);

  // process message asynchronously on a new thread
  Task.Factory.StartNew(() => ProcessWrapper(message));     
}

// glue / semi-infrastructure - deals with message deletion and exceptions 
void ProcessWrapper(Message message)
{
   try
   {
      Process(message);
      Queue.DeleteMessage(message);
   }
   catch(Exception ex)
   {
      // Handle exception here.
      // Log, write to poison message queue etc ...
   }
   finally 
   {
      Interlocked.Decrement(ref count)
   }
}

// business process
void Process(Message message)
{
  // actual work done here
  ;
}

Upvotes: 0

usr
usr

Reputation: 171168

Somehow, what IIS does is make sure that when an async call returns it uses the same fixed-size thread pool.

This is not true: When your code runs in response to an HTTP-Request you decide on what threads the continuation function executes. Usually, this is the thread pool. And the thread pool is an appdomain-wide resource that is shared among all requests.

I think IIS does less "magic" than you think it does. All it does is to limit the number of parallel HTTP-requests and the backlog size. You decide what happens once you have been given control by ASP.NET.

If your code is not protected against overloading the server, you will overload the server even on IIS.

Upvotes: 0

David Makogon
David Makogon

Reputation: 71031

Creating a threadpool should be considered as independent of Windows Azure. Since a Worker Role instance is effectively Windows 2008 Server R2 (or SP2), there's nothing really different. You'd just need to set things up from your OnStart() or Run().

One thing you wanted to do was use queue length as a determining factor when scaling to more/less worker instances. Note that Service Bus Queues don't advertise queue length, where Windows Azure Queues (based on Storage, vs. Service Bus) do. With Windows Azure Queues, you'll need to poll synchronously for messages (whereas Service Bus Queues have long-polling operations). Probably a good idea to review the differences between Service Bus Queues and Windows Azure Queues, here.

Upvotes: 1

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