Parallel processing in F# - concurrency

Question

I'm playing around with async in F#. Does this look right, or am I mangling things?
let time f =
let before = System.DateTime.Now
f () |> ignore
let after = System.DateTime.Now
after - before;;
let rec fib = function 0 | 1 -> 1
| n -> fib (n - 1) + fib (n - 2);;
let source = [45; 40; 45; 40]
let synchronous = time <| fun () -> List.map fib source
let para = time <| fun () -> source
|> List.map (fun n -> async {ignore <| fib n})
|> Async.Parallel
|> Async.RunSynchronously
In particular, how do I return results from an async block? Do I have to use mutable state?
Update: here's another approach:
#r "FSharp.PowerPack.Parallel.Seq.dll"
open Microsoft.FSharp.Collections
let pseq = time <| fun () -> source
|> PSeq.map fib
|> PSeq.toList

Answer 1

Firstly, it's a bit of an anti-pattern to use async for parallel CPU processing. See these questions and answers for more information:
Why shouldn't I use F# asynchronous workflows for parallelism?
Task Parallel Library vs Async Workflows
Secondly, your fib function should be re-written to be tail recursive, here's an example from here (including changing to BigInt):
let fib n =
let rec loop acc1 acc2 = function
| n when n = 0I -> acc1
| n -> loop acc2 (acc1 + acc2) (n - 1I)
loop 0I 1I n
Finally, the full code:
let source = [| 45I; 40I; 45I; 40I |]
let sync = time <| fun () -> Array.map fib source
let para = time <| fun () -> Array.Parallel.map fib source
Note that in both cases an Array of the results is returned, you're just throwing it away in your time function. How about a time function that returns both the time and the result?
let time f =
let watch = new System.Diagnostics.Stopwatch()
watch.Start()
let res = f ()
watch.Stop()
(res, watch.ElapsedMilliseconds)
Usage remains the same, but now showing results:
printfn "Sync: %A in %ims" (fst sync) (snd sync)
printfn "Para: %A in %ims" (fst para) (snd para)