The async and await keywords don't cause additional threads to be created?

Question

I\'m confused. How can one or many Task run in parallel on a single thread? My understanding of parallelism is obviously wrong.

Bits of MSDN I

Answer 1

They don't run in parallel, they take turns. When progress is blocked for the running Task, it stores its state and yields control to a ready Task. It's cooperative multitasking, not true parallelism.

Threads operate on the sample principle. However there are several key differences I'd like to highlight.

First, simply because async/await aren't OS threads:

• Tasks won't see different Thread IDs
• Thread-local storage is not automatically context-switched when a Task yields.

Secondly, differences in behavior:

• async/await use cooperative multitasking, Win32 threads use pre-emption. So it's necessary that all blocking operations explicitly yield control using the async/await model. So you can end up blocking an entire thread and all its Tasks by making a blocking call to a function not written to yield.
• Tasks won't be executed in parallel on a multiprocessing system. Since re-entrancy is controlled, this makes keeping data structures consistent a whole lot easier.

As Stephen points out in a comment, you can get simultaneous execution in multiple OS threads (along with all the complexity and potential race conditions) if you use a multithreaded synchronization context. But the MSDN quotes were about the single-threaded context case.

Finally, other places this same design paradigm is used, you can learn a lot about good practices for async/await by studying these:

• Win32 Fibers (uses the same call style as threads, but cooperative)
• Win32 Overlapped I/O operations, Linux aio
• Coroutines