Monday, 25 April 2011

Musings on async

OUT OF DATE: SEE UPDATE


For BookSleeve, I wanted an API that would work with current C#/.NET, but which would also mesh directly into C# 5 with the async/await pattern. The obvious option there was the Task API, which is familiar to anyone who has used the TPL in .NET 4.0, but which gains extension methods with the Async CTP to enable async/await.

This works, but in the process I found a few niggles that made me have a few doubts:

Completion without local scheduling

In my process, I never really want to schedule a task; the task is performed on a separate server, and what I really want to do is signal that something is now complete. There isn’t really an API for that on Task (since it is geared more for operations you are running locally, typically in parallel). The closest you can do is to ask the default scheduler to run your task immediately, but this feels a bit ungainly, not least because task-schedulers are not required to offer synchronous support.

In my use-case, I’m not even remotely interested in scheduling; personally I’d quite like it if Task supported this mode of use in isolation, perhaps via a protected method and a subclass of Task.

(yes, there is RunSynchronously, but that just uses the current scheduler, which in library code you can’t assume is capable of actually running synchronously).

Death by exception

The task API is also pretty fussy about errors – which isn’t unreasonable. If a task fails and you don’t explicitly observe the exception (by asking it for the result, etc), then it intentionally re-surfaces that exception in a finalizer. Having a finalizer is note-worthy in itself, and you get one last chance to convince the API that you’re sane – but if you forget to hook that exception it is a process-killer.

So what would it take to do it ourselves?

So: what is involved in writing our own sync+async friendly API? It turns out it isn’t that hard; in common with things like LINQ (and foreach if you really want), the async API is pattern-based rather than interface-based; this is convenient for retro-fitting the async tools onto existing APIs without changing existing interfaces.

What you need (in the Async CTP Refresh for VS2010 SP1) is:

  • Some GetAwaiter() method (possibly but not necessarily an extension method) that returns something with all of:
  • A boolean IsCompleted property (get)
  • A void OnCompleted(Action callback)
  • A GetResult() method which returns void, or the desired outcome of the awaited operation

So this isn’t a hugely challenging API to implement if you want to write a custom awaitable object. I have a working implementation that I put together with BookSleeve in mind. Highlights:

  • Acts as a set-once value with wait (sync) and continuation (async) support
  • Thread-safe
  • Isolated from the TPL, and scheduling in particular
  • Based on Monitor, but allowing efficient re-use of the object used as the sync-lock (my understanding is that once used/contested in a Monitor, the object instance obtains additional cost; may as well minimise that)
  • Supporting typed (Future<T>) or untyped (Future) usage – compares to Task<T> and Task respectively

My local tests aren’t exhaustive, but (over 500,000 batches of 10 operations each), I get:

Future (uncontested): 1993ms
Task (uncontested): 4126ms
Future (contested): 5487ms
Task (contested): 6787ms

So our custom awaitable object is faster… but I’m just not convinced that it is enough of an improvement to justify changing away from the Task API. This call density is somewhat artificial, and we’re talking less than a ┬Ás per-operation difference.

Conclusions

In some ways I’m pleasantly surprised with the results; if Task is keeping up (more or less), even outside of it’s primary design case, then I think we should forget about it; use Task, and move on to the actual meat of the problem we are trying to solve.

However, I’ll leave my experimental Future/Future<T> code as reference only off on the side of BookSleeve – in case anybody else feels the need for a non-TPL implementation. I’m not saying mine is ideal, but it works reasonably.

But: I will not be changing away from Task / Task<T> at this time. I’m passionate about performance, but I’m not (quite) crazy; I’ll take the more typical and more highly-tested Task API that has been put together by people who really, really understand threading optimisation, to quite ludicrous levels.