F#: Tell me what I'm missing about using Async.Parallel
- by JBristow
ok, so I'm doing ProjectEuler Problem #14, and I'm fiddling around with optimizations in order to feel f# out.
in the following code:
let evenrule n = n / 2L
let oddrule n = 3L * n + 1L
let applyRule n =
if n % 2L = 0L then evenrule n
else oddrule n
let runRules n =
let rec loop a final =
if a = 1L then final
else loop (applyRule a) (final + 1L)
n, loop (int64 n) 1L
let testlist = seq {for i in 3 .. 2 .. 1000000 do yield i }
let getAns sq = sq |> Seq.head
let seqfil (a,acc) (b,curr) = if acc = curr then (a,acc) else if acc < curr then (b,curr) else (a,acc)
let pmap f l =
seq { for a in l do yield async {return f a} }
|> Seq.map Async.RunSynchronously
let pmap2 f l =
seq { for a in l do yield async {return f a} }
|> Async.Parallel
|> Async.RunSynchronously
let procseq f l = l
|> f runRules
|> Seq.reduce seqfil
|> fst
let timer = System.Diagnostics.Stopwatch()
timer.Start()
let ans1 = testlist |> procseq Seq.map // 837799 00:00:08.6251990
printfn "%A\t%A" ans1 timer.Elapsed
timer.Reset()
timer.Start()
let ans2 = testlist |> procseq pmap
printfn "%A\t%A" ans2 timer.Elapsed // 837799 00:00:12.3010250
timer.Reset()
timer.Start()
let ans3 = testlist |> procseq pmap2
printfn "%A\t%A" ans3 timer.Elapsed // 837799 00:00:58.2413990
timer.Reset()
Why does the Async.Parallel code run REALLY slow in comparison to the straight up map? I know I shouldn't see that much of an effect, since I'm only on a dual core mac.
Please note that I do NOT want help solving problem #14, I just want to know what's up with my parallel code.
