在进行并行计算时,为什么打印到控制台会混淆？

Question

运行一些执行并行计算的代码时,输​​出变得混乱:不同的消息混淆了.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.

通过此功能打印到控制台:

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 )

并行化是由程序的这一部分完成的:

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a中间被中断printfn而第二个线程printfn在第一个线程再次运行之前运行,则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,该函数将使用lock围绕printfn调用的关键字:

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

然后替换你的所有printfn通话,lockedPrintfn你应该得到你期望的序列化输出.你的性能将受到一点点,因为你的线程会偶尔会花一些时间等待printfn锁,但只要不是花的时间打印输出的计算显著需要更长的时间,你不应该真正注意到稍微慢的性能.