为什么在进行并行计算时打印到控制台会混淆？

Question

在运行一些执行并行计算的代码时，输​​出会出现乱码：不同的消息会混淆。这是一个示例：

Iteration 1
Iteration
Iteration 23 of 19 - Calculating P&L for test window ending at 10/28/1968 12:00:00 AM

 of
Iteration 4
Iteration  of
Iteration 5
Iteration
Iteration 19 - Calculating P&L for test window ending at  of 19 - Calculating P&L for test window ending at 5/29/1974 12:00:00 AM
6 of 878/18/1971 12:00:00 AM19 - Calculating P&L for test window ending at 3/4/1977 12:00:00 AM


 of 19 of
 of 19 - Calculating P&L for test window ending at 6/25/1985 12:00:00 AM

当顺序运行相同的程序时，控制台输出很好，没有乱码。

打印到控制台是通过这个函数完成的：

let windowTrainTest (comm: Communication) critFoo count (model: IModel) (assets: Assets) (paramList: Parameters list) =
    // Deleted some code here
    if comm = Verbose then
        let msg1 = sprintf "\nwindowTrainTestPandL: First date: %A, Last date: %A\nBest Criterion: %.2f\n" fDate lDate bestCriterion
        let msg2 = sprintf "Best Parameters: %A\n" bestParameters 
        printfn "%s" <| msg1 + msg2

    (pandl, wgts), bestParameters, ( ["Criterion", bestCriterion]            |> Map.ofList,
                                     ["FirstDate", fDate; "LastDate", lDate] |> Map.ofList )

并行化由程序的这一部分完成：

let pSeqMapi f (xs: seq<'T>) = xs |> PSeq.mapi f

let trainTest n i (trainSize, fullSize) =
        let takenAssets = assets |> Assets.take (min fullSize len)
        lastDate takenAssets
        |> printfn "\nIteration %d of %d - Calculating P&L for test window ending at %A\n" (i + 1) n
        paramList
        |> windowTrainTest comm' critFoo trainSize model takenAssets

    let mapTrainTest (initSizes: (int * int) list) =
        let f = trainTest initSizes.Length
        match calcType with
        | PSeq -> initSizes |> pSeqMapi f |> List.ofSeq
        | _    -> initSizes |> Seq.mapi f |> List.ofSeq

有没有办法避免这种行为，例如将消息刷新到控制台？

Answer 1

并行计算在不同的线程上运行，如果一个线程在printfn 的中间被中断，而第二个线程在第一个线程再次运行之前运行printfn，那么它们的输出将被交错。

处理此问题的最简单方法是创建一个新函数，该函数将在 printfn 调用周围使用 lock 关键字：

let lockObj = new obj()
let lockedPrintfn msg = lock lockObj (fun _ -> printfn msg)

然后将所有printfn 调用替换为lockedPrintfn，您应该会得到您期望的序列化输出。由于您的线程偶尔会花费一些时间等待 printfn 锁定，因此您的性能会受到一点影响，但只要您的计算花费的时间比打印输出所花费的时间长得多，您实际上就不应该注意到性能稍慢。

Answer 2

我想我找到了解决方案，它不需要锁。我换了行

lastDate takenAssets
|> printfn "\nIteration %d of %d - Calculating P&L for test window ending at %A\n" (i + 1) n

与

let msg = sprintf "\nIteration %d of %d - Calculating P&L for test window ending at %A\n" (i + 1) n (lastDate takenAssets)
printfn "%s" msg

我让那些更有知识的人提供解释。