為什么訪問當前執行緒之外的變數很慢？

我有以下兩種方法的代碼：

using System.Diagnostics;

internal class Program
{
    public static double SeqArraySum(double[] x)
    {
        var watch = new Stopwatch();
        watch.Start();
        double sum1 = 0;
        double sum2 = 0;

        for (int i = 0; i < x.Length / 2; i  )
            sum1  = x[i];

        for (int i = x.Length / 2; i < x.Length; i  )
            sum2  = x[i];

        double sum = sum1   sum2;
        watch.Stop();
        Console.WriteLine($"Seq Milliseconds: {watch.ElapsedMilliseconds}, sum:{sum}");
        return sum;
    }

    public static double ParArraySum(double[] x)
    {
        var watch = new Stopwatch();
        watch.Start();
        double sum1 = 0;
        double sum2 = 0;

        var t1 = new Thread(() =>
        {
            for (int i = 0; i < x.Length / 2; i  )
                sum1  = x[i];
        });
        t1.Start();

        for (int i = x.Length / 2; i < x.Length; i  )
            sum2  = x[i];

        t1.Join();
        double sum = sum1   sum2;
        watch.Stop();
        Console.WriteLine($"Par Milliseconds: {watch.ElapsedMilliseconds}, sum:{sum}");
        return sum;
    }

    static void Main()
    {
        var arr = new double[100000000];
        for (int i = 0; i < arr.Length; i  )
            arr[i] = i / 100000000000d;

        for(int i = 1; i <= 5; i  )
        {
            Console.WriteLine($"Attempt {i}");
            var resultSeq = SeqArraySum(arr);
            var resultPar = ParArraySum(arr);
        }
    }
}

如果我運行這個程式，我會得到以下結果（.NET Core 6.0，除錯模式）：

Attempt 1
Seq Milliseconds: 226, sum:49999,999499999976
Par Milliseconds: 448, sum:49999,999499999976
Attempt 2
Seq Milliseconds: 194, sum:49999,999499999976
Par Milliseconds: 409, sum:49999,999499999976
Attempt 3
Seq Milliseconds: 194, sum:49999,999499999976
Par Milliseconds: 463, sum:49999,999499999976
Attempt 4
Seq Milliseconds: 195, sum:49999,999499999976
Par Milliseconds: 460, sum:49999,999499999976
Attempt 5
Seq Milliseconds: 192, sum:49999,999499999976
Par Milliseconds: 457, sum:49999,999499999976

我希望 ParArraySum 運行得更快，但它沒有發生。但如果更換

        var t1 = new Thread(() =>
        {
            for (int i = 0; i < x.Length / 2; i  )
                sum1  = x[i];
        });

有了這個：

        var t1 = new Thread(() =>
        {
            var tempSum = 0d;
            for (int i = 0; i < x.Length / 2; i  )
                tempSum  = x[i];
            sum1 = tempSum;
        });

我得到了預期的加速。

Attempt 1
Seq Milliseconds: 217, sum:49999,999499999976
Par Milliseconds: 130, sum:49999,999499999976
Attempt 2
Seq Milliseconds: 241, sum:49999,999499999976
Par Milliseconds: 118, sum:49999,999499999976
Attempt 3
Seq Milliseconds: 199, sum:49999,999499999976
Par Milliseconds: 115, sum:49999,999499999976
Attempt 4
Seq Milliseconds: 191, sum:49999,999499999976
Par Milliseconds: 115, sum:49999,999499999976
Attempt 5
Seq Milliseconds: 192, sum:49999,999499999976
Par Milliseconds: 115, sum:49999,999499999976

為什么我的原始代碼這么慢？是否可以重復訪問 sum1 而不會減速？

uj5u.com熱心網友回復：

通過做這個：

double sum1 = 0;
var t1 = new Thread(() =>
{
    for (int i = 0; i < x.Length / 2; i  )
        sum1  = x[i];
});

您在閉包中捕獲sum1區域變數。為了使其作業，編譯器必須重寫代碼，使其sum1不再是常規的區域變數，而是編譯器生成的類的欄位，然后將該類的實體傳遞給閉包（另一個執行緒運行的代碼）。

在這種情況下，這對于性能很重要，因為訪問存盤在堆疊上的“常規”區域變數在對大型陣列求和的情況下效率更高，并且編譯器可以使用各種優化。一遍又一遍地訪問某個實體的欄位效率較低。

在這種情況下：

var t1 = new Thread(() =>
{
    var tempSum = 0d;
    for (int i = 0; i < x.Length / 2; i  )
        tempSum  = x[i];
    sum1 = tempSum;
});

您在傳遞給新執行緒的方法內創建區域變數，因此它將是“常規”區域變數，并且重復訪問它是有效的。然后，您只能在回圈結束時訪問捕獲的變數sum1一次，這不會對性能產生負面影響。

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