make TryAsyncContinuation async

[caleblloyd]

• Helps increase efficiency for Async calls by removing Synchronous calls in Async Code
• Helps increase efficiency for Async and Sync calls by reducing need for context switching

I used a method that retrieved 10 blog posts from the Performance Test database. I logged every call to GetAwaiter().GetResult() to see what Managed Thread ID the calls were running on.

Async calls to GetAwaiter().GetResult() Before Change:

Thread ID: 4 Kicking off task
Thread ID: 5 Kicking off task
Thread ID: 5 Kicking off task
Thread ID: 3 Kicking off task
Thread ID: 3 Kicking off task
Thread ID: 3 Kicking off task
Thread ID: 3 Kicking off task
Thread ID: 3 Kicking off task

Async calls to GetAwaiter().GetResult() After Change:

Thread ID: 5 Kicking off task (this happens in MysqlDataReader.Dispose when it calls NextResult)

Sync calls to GetAwaiter().GetResult() Before Change:

Thread ID: 5 Kicking off task
Thread ID: 4 Kicking off task
Thread ID: 10 Kicking off task
Thread ID: 5 Kicking off task
Thread ID: 10 Kicking off task
Thread ID: 5 Kicking off task
Thread ID: 5 Kicking off task
Thread ID: 5 Kicking off task
Thread ID: 4 Kicking off task
Thread ID: 5 Kicking off task
Thread ID: 5 Kicking off task
Thread ID: 10 Kicking off task
Thread ID: 5 Kicking off task
Thread ID: 5 Kicking off task
Thread ID: 4 Kicking off task
Thread ID: 5 Kicking off task
Thread ID: 5 Kicking off task
Thread ID: 10 Kicking off task
Thread ID: 5 Kicking off task
Thread ID: 5 Kicking off task
Thread ID: 5 Kicking off task

Sync calls to call to GetAwaiter().GetResult() After Change:

Thread ID: 5 Kicking off task
Thread ID: 5 Kicking off task
Thread ID: 5 Kicking off task
Thread ID: 5 Kicking off task
Thread ID: 5 Kicking off task
Thread ID: 5 Kicking off task
Thread ID: 5 Kicking off task
Thread ID: 5 Kicking off task
Thread ID: 5 Kicking off task
Thread ID: 5 Kicking off task
Thread ID: 5 Kicking off task
Thread ID: 5 Kicking off task
Thread ID: 5 Kicking off task
Thread ID: 5 Kicking off task

[bgrainger]

This PR is replacing a hand-written continuation with a compiler-generated one that is less efficient due to the async state machine. It also allocates extra (unnecessary) Tasks.

task.ContinueWith(TryAsyncContinuation) schedules TryAsyncContinuation to run when the task is completed. There is no "blocking" because the antecedent task has finished, so GetAwaiter().GetResult() returns immediately (or throws).

Adding await task.ConfigureAwait(false) to the body of TryAsyncContinuation sets up an almost identical continuation, except it requires extra compiled-generated code and temporary objects. Behind the scenes, the compiler-generated code calls GetResult() on the task's awaiter.

Your change does suggest an approach that I haven't tested yet, though: using GetAwaiter().OnCompleted() to schedule the continuation instead of ContinueWith. However, the documentation explicitly says "This API supports the product infrastructure and is not intended to be used directly from your code." so it seems unwise. I haven't checked the implementation, but I'd be very surprised if OnCompleted wasn't implemented in terms of ContinueWith, though.

[bgrainger]

but I'd be very surprised if OnCompleted wasn't implemented in terms of ContinueWith, though

OnCompleted calls SetContinuationForAwait which calls AddTaskContinuation; ContinueWith calls ContinueWithCore which also calls AddTaskContinuation. So the two should be roughly equivalent in terms of overhead.

[caleblloyd]

Thanks for the information and references.

There is no "blocking" because the antecedent task has finished

This makes sense.

Behind the scenes, the compiler-generated code calls GetResult() on the task's awaiter

Good to know. So it's running on a different thread anyways, I just can't log the result because it's compiler generated. This makes total sense. In both of the Sync Before/After examples there are 14 calls on Thread ID: 5. The only difference is I've masked logging.

I'll read up more on these articles, it's great to know what's actually going on in the compiler!