在循环外添加一个Console.WriteLine()会改变循环的时间 - 为什么？

Question

请考虑以下代码:

using System;
using System.Diagnostics;

namespace Demo
{
    class Program
    {
        static void Main(string[] args)
        {
            Stopwatch sw = new Stopwatch();

            for (int trial = 0; trial < 4; ++trial)
            {
                sw.Restart();
                loop1();
                Console.WriteLine("loop1() took " + sw.Elapsed);

                sw.Restart();
                loop2();
                Console.WriteLine("loop2() took " + sw.Elapsed);

                sw.Restart();
                loop3();
                Console.WriteLine("loop3() took " + sw.Elapsed);

                // Console.WriteLine(); // <-- Uncomment this and the timings change a LOT!
            }
        }

        static void loop1()
        {
            bool done = false;

            for (int i = 0; i < 100000 && !done; ++i)
            {
                for (int j = 0; j < 100000 && !done; ++j)
                {
                    for (int k = 0; k < 2; ++k)
                    {
                        if (i == 9900)
                        {
                            done = true;
                            break;
                        }
                    }
                }
            }
        }

        static void loop2()
        {
            for (int i = 0; i < 100000; ++i)
            {
                for (int j = 0; j < 100000; ++j)
                {
                    for (int k = 0; k < 2; ++k)
                    {
                        if (i == 9900)
                        {
                            goto exit;
                        }
                    }
                }
            }

        exit: return;
        }

        static void loop3()
        {
            for (int i = 0; i < 100000; ++i)
            {
                for (int j = 0; j < 100000; ++j)
                {
                    for (int k = 0; k < 2; ++k)
                    {
                        if (i == 9900)
                        {
                            k = 2;
                            j = 100000;
                            i = 100000;
                        }
                    }
                }
            }
        }
    }
}

当我使用Visual Studio 2010在Windows 7 x64上编译和运行此版本的RELEASE x86时,我得到以下时序(在Intel Core i7上运行)

loop1() took 00:00:01.7935267
loop2() took 00:00:01.4747297
loop3() took 00:00:05.6677592
loop1() took 00:00:01.7654008
loop2() took 00:00:01.4818888
loop3() took 00:00:05.7656440
loop1() took 00:00:01.7990239
loop2() took 00:00:01.5019258
loop3() took 00:00:05.7979425
loop1() took 00:00:01.8356245
loop2() took 00:00:01.5688070
loop3() took 00:00:05.7238753

这本身就很奇怪 - 为什么loop3()会比其他循环慢得多？无论如何,我然后取消注释指示的行(Console.WriteLine()),我的时间变为:

loop1() took 00:00:01.8229538
loop2() took 00:00:07.8174210
loop3() took 00:00:01.4879274

loop1() took 00:00:01.7691919
loop2() took 00:00:07.4781999
loop3() took 00:00:01.4810248

loop1() took 00:00:01.7749845
loop2() took 00:00:07.5304738
loop3() took 00:00:01.4634904

loop1() took 00:00:01.7521282
loop2() took 00:00:07.6325186
loop3() took 00:00:01.4663219

现在loop2()要慢得多,而loop3()要快得多.我觉得这个最好奇......

所以我有两个问题:

1. 任何人都可以重现这一点,并且
2. 如果是这样,有人可以解释一下吗？

[编辑]我应该补充一点,我可以用秒表验证这些时间,我从命令行运行测试程序(所以我们可以排除Visual Studio干扰它).

附录:

我按如下方式修改了程序,以排除JITTER优化循环的可能性:

using System;
using System.Diagnostics;
using System.Text;

namespace Demo
{
    class Program
    {
        static void Main(string[] args)
        {
            Console.WriteLine(test());
        }

        static string test()
        {
            Stopwatch sw = new Stopwatch();
            int total = 0;
            StringBuilder builder = new StringBuilder();

            for (int trial = 0; trial < 2; ++trial)
            {
                sw.Restart();
                total += loop1();
                builder.AppendLine("loop1() took " + sw.Elapsed);

                sw.Restart();
                total += loop2();
                builder.AppendLine("loop2() took " + sw.Elapsed);

                sw.Restart();
                total += loop3();
                builder.AppendLine("loop3() took " + sw.Elapsed);
                //builder.AppendLine(); // Uncommenting this line makes a big difference!
            }

            builder.AppendLine(total.ToString());

            return builder.ToString();
        }

        static int loop1()
        {
            bool done = false;
            int total = 0;

            for (int i = 0; i < 100000 && !done; ++i)
            {
                for (int j = 0; j < 100000 && !done; ++j)
                {
                    for (int k = 0; k < 2; ++k)
                    {
                        if (i == 9900)
                        {
                            done = true;
                            break;
                        }

                        ++total;
                    }
                }
            }

            return total;
        }

        static int loop2()
        {
            int total = 0;

            for (int i = 0; i < 100000; ++i)
            {
                for (int j = 0; j < 100000; ++j)
                {
                    for (int k = 0; k < 2; ++k)
                    {
                        if (i == 9900)
                        {
                            goto exit;
                        }

                        ++total;
                    }
                }
            }

        exit: return total;
        }

        static int loop3()
        {
            int total = 0;

            for (int i = 0; i < 100000; ++i)
            {
                for (int j = 0; j < 100000; ++j)
                {
                    for (int k = 0; k < 2; ++k)
                    {
                        if (i == 9900)
                        {
                            k = 2;
                            j = 100000;
                            i = 100000;
                        }
                        else
                        {
                            ++total;
                        }
                    }
                }
            }

            return total;
        }
    }
}

现在我的结果如下:

builder.AppendLine()注释掉:

loop1() took 00:00:06.6509471
loop2() took 00:00:06.7322771
loop3() took 00:00:01.5361389
loop1() took 00:00:06.5746730
loop2() took 00:00:06.7051531
loop3() took 00:00:01.5027345
-1004901888

builder.AppendLine()没有注释掉:

loop1() took 00:00:06.9444200
loop2() took 00:00:02.8960563
loop3() took 00:00:01.4759535

loop1() took 00:00:06.9036553
loop2() took 00:00:03.1514154
loop3() took 00:00:01.4764172

-1004901888

当我这样做时,请注意loop2()时序的不同.不计算!

Answer 1

我可以完全重现.另外,我可以像这样消除变化:

    private static void Main(string[] args)
    {
        Stopwatch sw = new Stopwatch();

        for (int trial = 0; trial < 4; ++trial)
        {
            sw.Restart();
            Interlocked.MemoryBarrier();
            loop1();
            Interlocked.MemoryBarrier();
            Console.WriteLine("loop1() took " + sw.Elapsed);

            sw.Restart();
            Interlocked.MemoryBarrier();
            loop2();
            Interlocked.MemoryBarrier();
            Console.WriteLine("loop2() took " + sw.Elapsed);

            sw.Restart();
            Interlocked.MemoryBarrier();
            loop3();
            Interlocked.MemoryBarrier();
            Console.WriteLine("loop3() took " + sw.Elapsed);

            // Console.WriteLine(); // <-- Uncomment this and the timings don't change now.
        }
    }

当我使用MemoryBarriers运行时,无论我以何种方式运行测试,我都会获得第二种模式:

loop1() took ~1 sec
loop2() took ~7 secs
loop3() took ~1 sec

MemoryBarrier定义:

按如下方式同步内存访问:执行当前线程的处理器无法重新排序指令,使得在调用MemoryBarrier之前的内存访问在对MemoryBarrier的调用之后的内存访问之后执行.

由于循环的IL在两个版本之间是相同的,而且MemoryBarrier使方差消失,我认为我们可以得出结论,方差绝对是第一个模式优化的结果......可能是由JITer ...也许是CPU ......不确定那里.

我的环境是一样的,除了我使用的是VS2012和.NET 4.5 RTM.