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F # parallelism task under Mono does not "appear" to execute in parallel

I have the following dummy code for testing TPL in F #. (Mono 4.5, Xamarin Studio, Quad-Core MacBook Pro)

To my surprise, all processes run in a single thread. No parallelism at all.

open System
open System.Threading
open System.Threading.Tasks


let doWork (num:int) (taskId:int) : unit =
    for i in 1 .. num do
        Thread.Sleep(10)
        for j in 1 .. 1000 do
            ()
        Console.WriteLine(String.Format("Task {0} loop: {1}, thread id {2}", taskId, i, Thread.CurrentThread.ManagedThreadId)) 

[<EntryPoint>]
let main argv = 

    let t2 = Task.Factory.StartNew(fun() -> doWork 10 2)
    //printfn "launched t2"
    Console.WriteLine("launched t2")
    let t1 = Task.Factory.StartNew(fun() -> doWork 8 1)
    Console.WriteLine("launched t1")
    let t3 = Task.Factory.StartNew(fun() -> doWork 10 3)
    Console.WriteLine("launched t3")
    let t4 = Task.Factory.StartNew(fun() -> doWork 5 4)
    Console.WriteLine("launched t4")
    Task.WaitAll(t1,t2,t3,t4)
    0 // return an integer exit code

However, if I increase the disconnect time of the stream from 10 to 100 ms, I can see a bit of parallelism.

How am I wrong? What does it mean? I considered the possibility that the CPU shut down before TPL could start the task in a new thread. But for me it does not make sense. I can increase the internal dummy loop for j in 1 .. 1000 do ()to over 1000 cycles. The result is the same: no parallelism ( thread.sleep10 ms set).

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let doWork (num:int) (taskId:int) : unit =
    for i in 1 .. num do
        Thread.Sleep(10)
        for j in 1 .. 1000 do
            Debug.WriteLine("x")
        Console.WriteLine(String.Format("Task {0} loop: {1}, thread id {2}", taskId, i, Thread.CurrentThread.ManagedThreadId))

Update:
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+5

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