基准测试时如何使缓存失效?

我有这段代码,当交换usingas和usingcast的顺序时,它们的性能也会交换。

using System;
using System.Diagnostics;
using System.Linq;

using System.IO;

class Test
{
    const int Size = 30000000;

    static void Main()
    {
        object[] values = new MemoryStream[Size];



        UsingAs(values);
        UsingCast(values);


        Console.ReadLine();
    }

    static void UsingCast(object[] values)
    {
        Stopwatch sw = Stopwatch.StartNew();
        int sum = 0;
        foreach (object o in values)
        {
            if (o is MemoryStream)
            {
                var m = (MemoryStream)o;
                sum += (int)m.Length;
            }
        }
        sw.Stop();
        Console.WriteLine("Cast: {0} : {1}", sum,
                          (long)sw.ElapsedMilliseconds);
    }

    static void UsingAs(object[] values)
    {
        Stopwatch sw = Stopwatch.StartNew();
        int sum = 0;
        foreach (object o in values)
        {

            if (o is MemoryStream)
            {
                var m = o as MemoryStream;
                sum += (int)m.Length;
            }
        }
        sw.Stop();
        Console.WriteLine("As: {0} : {1}", sum,
                          (long)sw.ElapsedMilliseconds);
    }


}

输出:

As: 0 : 322
Cast: 0 : 281

做这个的时候…

UsingCast(values);
UsingAs(values);

…结果如下:

Cast: 0 : 322
As: 0 : 281

当你这样做的时候…

UsingAs(values);

…结果如下:

As: 0 : 322

当你这样做的时候:

UsingCast(values);

…结果如下:

Cast: 0 : 322

除了独立运行它们之外,如何 使无效 缓存,所以被基准测试的第二个代码不会接收第一个代码的缓存内存?

撇开基准测试不谈,我很喜欢现代处理器实现这种缓存魔力的事实:-)

[编辑]

正如建议尝试这个更快的代码(假设)……

static void UsingAsAndNullTest(object[] values)
{        
    Stopwatch sw = Stopwatch.StartNew();
    int sum = 0;
    foreach (object o in values)
    {
        var m = o as MemoryStream;
        if (m != null)
        {                
            sum += (int)m.Length;
        }
    }
    sw.Stop();
    Console.WriteLine("As and null test: {0} : {1}", sum,
                      (long)sw.ElapsedMilliseconds);
}

……结果是:

As and null test: 0 : 342

比上面两个代码慢

[编辑]:

按照建议,每一个程序都要有自己的副本…

static void UsingAs(object[] values)
{
    object[] a = values.ToArray();

    Stopwatch sw = Stopwatch.StartNew();
    int sum = 0;
    foreach (object o in a)
    {

        if (o is MemoryStream)
        {
            var m = o as MemoryStream;
            sum += (int)m.Length;
        }
    }
    sw.Stop();
    Console.WriteLine("As: {0} : {1}", sum,
                      (long)sw.ElapsedMilliseconds);
}

static void UsingCast(object[] values)
{
    object[] a = values.ToArray();

    Stopwatch sw = Stopwatch.StartNew();
    int sum = 0;
    foreach (object o in a)
    {
        if (o is MemoryStream)
        {
            var m = (MemoryStream)o;
            sum += (int)m.Length;
        }
    }
    sw.Stop();
    Console.WriteLine("Cast: {0} : {1}", sum,
                      (long)sw.ElapsedMilliseconds);
}

…输出:

Cast: 0 : 282
As: 0 : 282

现在他们有同样的结果了,谢谢雷姆斯!

运行cast和独立运行时,它们也会产生相同的结果(即282)。现在,至于他们为什么 更快 (从322毫秒到282毫秒)当他们拿到自己的数组副本时,我无法从中得到任何东西:-)那完全是另一回事