为什么后续的直接方法调用比第一次调用要快得多？

Question

正如本MSDN文章中所解释的,当使用.NET Reflection API InvokeMember时,由于元数据的缓存,第一次调用比后续调用花费更多时间.

当我在不使用Reflection的情况下测试直接方法调用时,我也看到了对Mono和.NET 4的类似效果.

第一个数字是操作的结果," - "后面的第二个数字是以ms为单位的此操作所花费的时间.我使用'< - '标记来标识第一个方法调用.

300 - 0.192 <--
300 - 0.004
300 - 0.004
-100 - 0.096 <--
-100 - 0.004
-100 - 0.004

为什么是这样？我可以理解第一次调用可能比较慢,但速度慢50倍并不是我的预期.

附件是获得此结果的源代码.

图书馆

namespace MyClass
{
    public class Calculator
    {
        public int Value1 {get; set;}
        public int Value2 {get; set;}
        public Calculator()
        {
            Value1 = 100;
            Value2 = 200;
        }

        public int Add(int val1, int val2)
        {
            Value1 = val1; Value2 = val2;
            return Value1 + Value2;
        }

        public int Sub(int val1, int val2)
        {
            Value1 = val1; Value2 = val2;
            return Value1 - Value2;
        }
    }
}

调用此库的代码

// http://msdn.microsoft.com/en-us/magazine/cc163759.aspx
using System;
using System.IO;
using System.Reflection;
using System.Diagnostics;
using System.Collections.Generic;
using MyClass;

class TestOne
{
    static void DirectTest()
    {
        Stopwatch sw;
        Calculator t = new Calculator();

        sw = Stopwatch.StartNew();
        int value1 = t.Add(100,200);
        sw.Stop();
        double time1 = sw.Elapsed.TotalMilliseconds;

        sw = Stopwatch.StartNew();
        int value2 = t.Add(100,200);   
        sw.Stop();
        double time2 = sw.Elapsed.TotalMilliseconds;

        sw = Stopwatch.StartNew();
        int value3 = t.Add(100,200); 
        sw.Stop();
        double time3 = sw.Elapsed.TotalMilliseconds;

        Console.WriteLine("{0} - {1}", value1, time1);
        Console.WriteLine("{0} - {1}", value2, time2);
        Console.WriteLine("{0} - {1}", value3, time3);

        sw = Stopwatch.StartNew();
        value1 = t.Sub(100,200);
        sw.Stop();
        time1 = sw.Elapsed.TotalMilliseconds;

        sw = Stopwatch.StartNew();
        value2 = t.Sub(100,200);  
        sw.Stop();
        time2 = sw.Elapsed.TotalMilliseconds;

        sw = Stopwatch.StartNew();
        value3 =  t.Sub(100,200); 
        sw.Stop();
        time3 = sw.Elapsed.TotalMilliseconds;

        Console.WriteLine("{0} - {1}", value1, time1);
        Console.WriteLine("{0} - {1}", value2, time2);
        Console.WriteLine("{0} - {1}", value3, time3);
    }
    static void Main()
    {
        DirectTest();
        DirectTest();
    }
}

Answer 1

这是因为用于.NET应用程序的Just In Time(JIT)编译方法.该MSIL字节码是由JIT编译器翻译成机器代码一次,该代码的后续执行速度更快,因为本机版本已经生成并缓存.

运行代码时,您需要支付一次罚款,但JIT编译器还可以对当前体系结构执行优化,如果代码是从一开始就是本机代码,则无法执行该优化.但是,您可以通过调用RuntimeHelpers.PrepareMethod来强制执行JIT传递.