What's the thread context for events in .Net using existing APIs?

Question

When using APIs handling asynchronous events in .Net I find myself unable to predict how the library will scale for large numbers of objects.

For example, using the Microsoft.Office.Interop.UccApi library, when I create an endpoint it gets events when phone events happen. Now let's say I want to create 1000 endpoints. The number of events per endpoint is small, but is what's happening behind the scenes in the API able to keep up with the event flow? I don't know because it never says how it's architected.

Let's say I want to create all 1000 objects in the main thread. Then I want to put the Login method into a large thread pool so all objects login in parallel. Then once all the objects have logged in the next phase will begin.

Are the event callbacks the API raises happening in the original creating thread? A separate threadpool? Or the same threadpool I'm accessing with ThreadPool.QueueUserWorkItem?

Would I be better putting each object in it's own thread? Grouping a few objects in each thread? Or is it fine just creating all 1000 objects in the main thread and through .Net magic it will all be OK?

thanx

Answer 1

It really depends on a lot of things, primarily how powerful your hardware is. The threadpool does have a certain number of threads (which you can increase) that it will make available for your application. So if all of your events are firing at the same time some will most likely be waiting for a few moments while your threadpool waits for threads to become free again. The tradeoff is that you don't have the performance hit of creating new threads all the time either. Probably creating 1000 threads isn't the right answer either.

It may turn out that this is ideal, both because of the performance gains in reusing threads but also because having 1000 threads all running simultaneously might be more memory / CPU usage than it's worth.

Answer 2

The events from interop assemblies are just wrappers around the COM connection points. The thread on which the call from the connection point arrive depends on the threading model of the object that advised on that connection point. COM will ensure the proper thread switching for this.

If your objects are implemented on the main thread, which in .Net is usually an STA, all events should arrive on that same thread. If you want your calls to arrive on a random thread from the COM thread pool (which I think is the same as the CLR thread pool), you need to create your objects on a thread that is configured as an MTA.

I would strongly advise against creating a thread for each object: 1) If you create these threads as STA, each of them will have a message queue, waisting system resource; 2) If you create them as MTA, nothing guarantees you the event call will arrive on your thread; 3) You'll have 1000 idle threads doing nothing and just waiting on an event to shutdown; and 4) Starting up and shutting down all these threads will have terrible perf cost on your application.

Answer 3

I just wanted to note that in .NET 2.0 and greater it's possible to programmatically increase the maximum number of threads in the thread pool using ThreadPool.SetMaxThreads(). Given this you can put a hard cap on the number of threads and so ensure the scheduler won't be brought to it's knees by the overhead.

Even more useful in this sort of case, you can set the minimum number of threads with ThreadPool.SetMinThreads(). With this you can ensure that you only pay the "horrible performance price" Franci is talking about once, at application startup. You could balance this against the expected number peak of users and so ensure you won't be creating tons of new threads.

A single new thread creation won't destroy you. What I would be worried about is the case where a lot of threads need to be created at the same time. If you can say that this will only happen at startup you would be golden.