F#: PSeq.sumBy, What's Gone Wrong?

As per my previous blog posting, I have cashflow generation and hence NPV being calculated for my IRS rate. I’ve move from a List to a Seq for storage of the InterestRateStream’s. Primarily this move was so that I could take advantage of PSeq.

If we look at my current code, it’s processes each stream sequentially:

    let valueDate = new DateTime(2010,1,1)
    let trade  = createTrade 101
    let cashFlows = 
        trade.SwapStream |> Seq.sumBy (fun x -> generateCashflow(x,trade, valueDate))
    printfn "IRS NPV %A" cashFlows

Since the cashflow generation of each leg is separate, I’d like to parallelize the processing, so I change the code to use PSeq:

    let valueDate = new DateTime(2010,1,1)
    let trade  = createTrade 101
    let cashFlows = 
        trade.SwapStream |> PSeq.sumBy (fun x -> generateCashflow(x,trade, valueDate))
    printfn "IRS NPV %A" cashFlows

Unfortunately this doesn’t generate the same NPV for the trade.

I was expecting the function generateCashflow() to be called twice, one for each leg, in parallel, with the result of each function being summed into cashFlows. Maybe I’ve miss-understood what PSeq.sumBy should do? Or maybe I have a parallel execution bug.

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~ by mdavey on March 11, 2010.