Parallelization with PLINQ

The past couple of days I've been interested in exploring PLINQ with F#.  But Since I’m using F# 2.0 for Visual Studio 2008 (i.e. .NET 2.0 build), I had to 1) Install the 3.5 SP1 Reactive Extensions (Rx) for .NET which include a back-port of PLINQ (http://msdn.microsoft.com/en-us/devlabs/ee794896.aspx), and 2) Download the F# PowerPack 2.0 May 2010 source code (http://fsharppowerpack.codeplex.com/SourceControl/list/changesets) and compile the Parallel.Seq project after adding a reference to System.Threading (which was augmented by Rx), then 3) reference the dll in my ProjectEuler project and open Microsoft.FSharp.Collections.

Problem 10 is a perfect candidate for parallelization, since addition is commutative.  The following parallelized version of Problem 10 literally runs twice as fast on my dual core processor than the (nearly) identical non-parallelized version (b).

let problem10e = 
    ({3..2..1999999}
    |> PSeq.filter isPrime
    |> PSeq.sumBy int64) + 2L

To highlight how simple and effective PLINQ is, consider the following attempt at manually parallelizing using F# async workflows.  Because of the overhead involved in trying to parallelize 2 million tasks in one batch, this version actually runs 3 times slower than version (b).

let problem10c =
    let asyncIsPrime i = async { if i |> isPrime then return i else return 0 }
    let tasks = seq {for i in 3..2..1999999 -> asyncIsPrime i}
    ((Async.RunSynchronously (Async.Parallel tasks)) |> Seq.filter((<) 0) |> Seq.sumBy(int64)) + 2L