How to run code in parallel?

Question

How can i run these two independent loops simultaneously in parallel.

let a1=Seq.map2 (fun a b->(1.0-a)/(a-b)) a2 a3
let b1=Seq.map2 (fun a b->a*b) b2 b3

Answer 1

You can use standard .NET tasks for this - there are no special F# functions or special syntax for spawning a computation in the background:

let a1Work = Task.Factory.StartNew(fun () ->
 Array.map2 (fun a b->(1.0-a)/(a-b)) a2 a3)
let b1 = Array.map2 (fun a b->a*b) b2 b3
let a1 = a1Work.Value

I also changed your Seq.map2 to Array.map2 - computations on sequences are lazy and so running them inside a task would not actually do anything. With arrays, the whole calculation is completed immediately.

• HaagenDaz: is it a1 = a1Work.Value or a1 = a1Work.Result
• HaagenDaz: if there is another loop let c1=Seq.map2 (fun a b->a/b) c2 c3 do i need to set it as task as well?

Answer 2

If this is a pattern you find yourself using regularly, it could be useful to write a generic function that runs two Async computations in parallel, for example like this:

module Async =
 let Parallel2 (a: Async<'T>) (b: Async<'U>) : Async<'T * 'U> =
 async {
 let! res =
 Async.Parallel [|
 async { let! a = a in return box a }
 async { let! b = b in return box b }
 |]
 return (res.[0] :?> 'T, res.[1] :?> 'U)
 }

You can then use it like so:

async {
 let! a1, b1 =
 Async.Parallel2
 (async { return Array.map2 (fun a b->(1.0-a)/(a-b)) a2 a3 })
 (async { return Array.map2 (fun a b->a*b) b2 b3 })
 // ...
}

Answer 3

try Array.zip and Array.Parallel.map?

Array.zip [|1;2;3|] [|2;3;4|] |> Array.Parallel.map (fun (a,b) -> a + b)

or simply use the ParallelSeq(see http://fsprojects.github.io/FSharp.Collections.ParallelSeq/ for more information)