module Async =
    open System.Threading
    let inline ParallelWithThrottle (millisecondsTimeout:int) (limit:int) (items:'a seq)  (operation:'a->Async<'b>) =
        let semaphore = new System.Threading.SemaphoreSlim(limit, limit)
        let mutable count = (items |> Seq.length) 
        items 
        |> Seq.map(fun item -> async {
                let! isHandleAquired = Async.AwaitTask <| semaphore.WaitAsync(millisecondsTimeout=millisecondsTimeout)
                if isHandleAquired then 
                    try
                        return! operation item
                    finally 
                        if Interlocked.Decrement(&count) = 0 then
                            semaphore.Dispose()
                        else semaphore.Release() |> ignore
                else return! failwith "Failled to aquire handle" })
        |> Async.Parallel 

// Example
let rec fib n =
    match n with
    | 1 | 2 -> 1
    | n -> fib(n - 2) + (fib - 1)

let op n =  async{
      let fibN = fib n
      return (n, fibN)   }

Async.ParallelWithThrottle 10000 4 [30..45] op 
    |> Async.RunSynchronously
    |> Seq.iter(fun (n,fibN) ->  printfn "The fib number of %d is %d" n fibN)