open System.Threading
open System
// Initialize an array by a parallel init using all available processors // Note, this primitive doesn't support cancellation.
let parallelArrayInit n f =
   let currentLine = ref -1
   let res = Array.zeroCreate n
   let rec loop () =
       let y = Interlocked.Increment(currentLine)
       if y < n then res.[y] <- f y; loop()
   // Start just the right number of tasks, one for each physical CPU
   Async.Parallel [for i in 1 .. Environment.ProcessorCount -> async {do loop()}]
      |> Async.Ignore
      |> Async.RunSynchronously
res

//  example
let rec fib x = if x < 2 then 1 else fib (x - 1) + fib (x - 2)
parallelArrayInit 25 (fun x -> fib x);;