Friday, December 30, 2011

Solving F# Async.StartChild Leak: Futures

I discovered a memory leak in Async.StartChild and here discuss a workaround based on a Future abstraction.

I noticed what appears to be a memory leak in F# standard library function Async.StartChild. This happened in a context of a socket server, where I attempted to perform socket reading and writing in parallel. It seems that memory use slowly grows and memory profiler points to some CancellationTokenSource-related objects not being released.

As a non-leaking alternative, I used my own abstractions. The basic idea is to use synchronizable events. Unfortunately Event is already used in F# to mean something different, so I will use the word Future instead. If you know F# events, the key problem is that subscribing to events after they happen is meaningless, for example this code procuces nothing:

let test () =
    let e = Event<_>()
    e.Publish.Add(printf "%i")

In contrast, Future objects retain the value. For simplicity, I allow subscribing and triggering only once. In addition, the sample includes Executor code. I found by experimentation that running short-lived coordination tasks on a single thread instead of the ThreadPool is beneficial. Enjoy:


  1. Can you give an example how you would do that? My understanding is that you can indeed use Lazy for coordination: forcing a Lazy value will make the consumer thread wait until the producer thread will compute the value. The reason this does not make me happy is because it consumes a whole .NET-level thread. With a callback-based Future, only Async-level thread is waiting, which consumes a lot less resources.