Task.Wait and “Inlining”

“What does Task.Wait do?”

Simple question, right?  At a high-level, yes, the method achieves what its name implies, preventing the current thread from making forward progress past the call to Wait until the target Task has completed, one way or another.  If the Task ran to completion, Wait will return successfully.  If the Task completed due to faulting or being canceled, it will throw an exception indicating the relevant details (since the waiting code likely expects that all work to be completed by the task ran successfully if Wait returns successfully).

The details are a bit more interesting.  Wait could simply block on some synchronization primitive until the target Task completed, and in some cases that’s exactly what it does.  But blocking threads is an expensive venture, in that a thread ties up a good chunk of system resources, and a blocked thread is dead weight until it’s able to continue executing useful work.  Instead, Wait prefers to execute useful work rather than blocking, and it has useful work at its finger tips: the Task being waited on.  If the Task being Wait’d on has already started execution, Wait has to block.  However, if it hasn’t started executing, Wait may be able to pull the target task out of the scheduler to which it was queued and execute it inline on the current thread.

To do that, Wait asks for assistance from the target scheduler, by making a call to the TaskScheduler’s TryExecuteTaskInline method.  TryExecuteTaskInline can do whatever logic the scheduler needs in order to validate that the current context is acceptable for inlining the Task.  If it is, the scheduler tries to execute the task then and there as part of the call to TryExecuteTaskInline (using the base TaskScheduler’s TryExecuteTask method) and returns whether the Task could be executed.  If it couldn’t be executed in TryExecuteTaskInline, the scheduler returns false, and Wait will need to block until the Task completes. A Task may not be able to be executed if it’s already been or is currently being executed elsewhere. As an example of this, the default scheduler (based on the ThreadPool) is aggressive about inlining, but only if the task can be efficiently removed from the data structures that hold tasks internally in the ThreadPool (such as if the task is living in the local queue associated with the thread attempting to inline it).

This is also very similar to what happens when Task.RunSynchronously is used to execute a Task (RunSynchronously may be used instead of Task.Start to run a Task currently in the TaskStatus.Created state).  RunSynchronously results in the Framework calling the target scheduler’s TryExecuteTaskInline, allowing the scheduler to decide whether to inline or not.  If the scheduler chooses not to inline, the Task will instead be queued to the scheduler, and the system will block until the Task is completed.

Of course, schedulers may want to behave differently depending on whether the code is explicitly choosing to inline (e.g. RunSynchronously) or whether the inlining is happening implicitly (e.g. Wait).  To support this difference, in addition to accepting as a parameter the Task to be inlined, TryExecuteTaskInline also accepts as a parameter a Boolean taskWasPreviouslyQueued.  This value will be false if it is known that the specified task has not yet been submitted to the scheduler, and true otherwise. If RunSynchronously is being used, taskWasPreviouslyQueued will be false; if Wait, it will be true.  The default scheduler does, in fact, differentiate between these two cases, in that it always supports explicit inlining through RunSynchronously.