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Consider StartImmediateAsTask instead of StartAsTask to prevent a thread hop #135
Description
In PR #114 (adding support for Async in Bind), @bartelink raised the question whether it would be better to use StartImmediateAsTask instead of StartAsTask.
Specifically, it is about this part:
let mutable awaiter =
Async
.StartAsTask(asyncSource, cancellationToken = sm.Data.cancellationToken)
.GetAwaiter()The current approach was taken to mimic the same approach taken for task in F# Core: https://github.com/dotnet/fsharp/blob/d91b6c5c97accf363d135d1f99816410da4ec341/src/FSharp.Core/tasks.fs#L462, basically:
member inline this.Bind(computation: Async<'TResult1>, continuation: ('TResult1 -> TaskCode<'TOverall, 'TResult2>)) : TaskCode<'TOverall, 'TResult2> =
this.Bind(Async.StartAsTask computation, continuation)For comparison, see what was done in async2, which was poised to be the new async based on resumable code, like task is, but unlike task cold-started like the current async.
So, we have two scenarios:
- use
StartAsTaskto get similar semantics as thetaskCE, but this causes a thread hop - use
StartImmediateAsTask, which uses the current context
I'm not aware of any downside of using the latter. It definitely saves a thread switch in the general case (note that the thread pool may not always cause a physical thread switch).
Activity
abelbraaksma commentedon Dec 19, 2022
This seems relevant as well: dotnet/fsharp#11043 (comment)
That comment by @dsyme basically suggests not to use
StartAsStask. Tbh, I didn't know it started in the thread pool, we haveAsync.Startalready, I figured, as a task, it behaves liketask: stay in the same context.I figured, maybe just report as a bug to F#: dotnet/fsharp#14490
abelbraaksma commentedon Dec 19, 2022
Thanks for bringing this up @bartelink. It required some digging, and writing a context-switching test case, but I'm now convinced this is a bad situation to be in (tbh, as Don mentioned already, these methods should be differently named). Will write a PR to fix it.
Bump version to 4.2.1
abelbraaksma commentedon Dec 28, 2022
The thread hop in F# Core
taskhas been fixed. We should follow suit. See dotnet/fsharp#14499.