【经典问题】现代操作系统经典问题回顾（哲学家就餐问题C#实现）

　　在1971年，著名的计算机科学家艾兹格·迪科斯彻提出了一个同步问题，即假设有五台计算机都试图访问五份共享的磁带驱动器。稍后，这个问题被托尼·霍尔重新表述为哲学家就餐问题。这个问题可以用来解释死锁和资源耗尽。

　　哲学家就餐问题描述：五个哲学家围坐在一张圆桌周围，每个哲学家面前都有一盘通心粉。由于通心粉很光滑，所以需要两把叉子才能夹住，相邻两个盘子之间放有一把叉子。哲学家的生活中有两种交替活动时段：即吃饭和思考（一种抽象而已）。当一个哲学家觉得饿了时，他就试图分两次去取其左边和右边的叉子，每次拿到一把，但不分次序。如果成功地得到了两把叉子，就开始吃饭，吃完后放下叉子继续思考。问题是为哲学家写一段描述其行为的程序，且决不会死锁。

　　源码：

// ************************************************************
// 在1971年，著名的计算机科学家艾兹格·迪科斯彻提出了一个同步问题，
// 即假设有五台计算机都试图访问五份共享的磁带驱动器。稍后，这个问
// 题被托尼·霍尔重新表述为哲学家就餐问题。这个问题可以用来解释死
// 锁和资源耗尽。
// 
// 哲学家就餐问题描述：五个哲学家围坐在一张圆桌周围，每个哲学家面
// 前都有一盘通心粉。由于通心粉很光滑，所以需要两把叉子才能夹住，
// 相邻两个盘子之间放有一把叉子。哲学家的生活中有两种交替活动时段：
// 即吃饭和思考（一种抽象而已）。当一个哲学家觉得饿了时，他就试图
// 分两次去取其左边和右边的叉子，每次拿到一把，但不分次序。如果成功
// 地得到了两把叉子，就开始吃饭，吃完后放下叉子继续思考。问题是为
// 哲学家写一段描述其行为的程序，且决不会死锁。
// 
// 国兴旺 2010/08/21
// ************************************************************

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading;

namespace DiningPhilosophers
{
    /// <summary>
    /// 哲学家就餐问题是现代操作系统中很经典的问题。
    /// 一个哲学家的封装。
    /// 国兴旺 2010/08/21
    /// </summary>
    public class Philosopher : WorkerThread
    {
        public const int READY = 0;
        public const int EATING = 1;
        public const int THINKING = 2;
        public const int FINISHED = 3;

        public Philosopher(object data) : base(data) { }
        public delegate void StateSwitchedHandler(Object sender, StateSwitchedEventArgs args);
        public event StateSwitchedHandler StateSwitch;

        protected override void Run()
        {
            PhilosopherData pd = (PhilosopherData)Data;
            Random rnd = new Random(pd.PhilosopherId);
            StateSwitch(this, new StateSwitchedEventArgs(READY, pd));
            WaitHandle[] forks = new WaitHandle[] { pd.LeftFork, pd.RightFork };

            while (pd.TotalFood > 0)
            {
                // 如果两边的哲学家有拿着叉子的，则等待。 
                WaitHandle.WaitAll(forks);
                // 否则，开始吃通心粉。 
                StateSwitch(this, new StateSwitchedEventArgs(EATING, pd));
                // 饭吃掉一部分移除。 
                pd.TotalFood -= pd.AmountToEat;
                Thread.Sleep(rnd.Next(100, 1000));// 模拟正在吃通心粉

                StateSwitch(this, new StateSwitchedEventArgs(THINKING, pd));
                // 放下左边和右边的叉子开始思考。 
                pd.RightFork.ReleaseMutex();
                pd.LeftFork.ReleaseMutex();

                Thread.Sleep(rnd.Next(100, 1000));// 模拟正在思考
            }

            // 至此，通信粉都吃完了。 
            StateSwitch(this, new StateSwitchedEventArgs(FINISHED, pd));
        }
    }

    /// <summary>
    /// 哲学家相当于一个线程，这是工作线程抽象类，作为对线程操作的封装。
    /// 国兴旺 2010/08/21
    /// </summary>
    public abstract class WorkerThread
    {
        private object _threadData;
        private Thread _rawThread;

        public object Data
        {
            get { return _threadData; }
            set { _threadData = value; }
        }

        public WorkerThread(object data)
        {
            _threadData = data;
        }

        public WorkerThread()
        {
            _threadData = null;
        }

        public void Start()
        {
            _rawThread = new Thread(new ThreadStart(this.Run));
            _rawThread.Start();
        }

        protected abstract void Run();
    }

    /// <summary>
    /// 封装哲学家数据的结构。
    /// 国兴旺 2010/08/21
    /// </summary>
    public struct PhilosopherData
    {
        public int PhilosopherId;   // 代表哲学家
        public Mutex RightFork;     // 代表右边的叉子
        public Mutex LeftFork;      // 代表左边的叉子
        public int AmountToEat;     // 吃掉的通心粉
        public int TotalFood;       // 盘中的总的通心粉
    }

    /// <summary>
    /// 事件：用来通知外部现在哲学家的状态。
    /// 国兴旺 2010/08/21
    /// </summary>
    public class StateSwitchedEventArgs : EventArgs
    {
        public int type;
        public PhilosopherData philosopherData;
        public StateSwitchedEventArgs(int t, PhilosopherData pd)
        {
            type = t;
            philosopherData = pd;
        }
    }
}


using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading;

namespace DiningPhilosophers
{
    class Program
    {
        static void Main(string[] args)
        {
            // 准备数据
            Mutex[] forks = new Mutex[N];
            for (int i = 0; i < N; i++)
            {
                forks[i] = new Mutex(false);// 开始时叉子放在桌子上，没有哲学家拿到。
            }

            Philosopher[] p = new Philosopher[N];
            for (int i = 0; i < N; i++)
            {
                PhilosopherData pd;
                pd.PhilosopherId = i;
                pd.RightFork = forks[(i + 1) % N];// 右边的叉子编号
                pd.LeftFork = forks[(i + N - 1) % N];// 左边的叉子编号
                pd.AmountToEat = N_EAT;// 暂时大锅饭，大家都一样。
                pd.TotalFood = N_TOTAL;// 暂时大锅饭，大家都一样。
                p[i] = new Philosopher(pd);
                p[i].StateSwitch +=
                    new Philosopher.StateSwitchedHandler(
                        (sender, data) =>
                        {
                            Console.WriteLine("Philosopher:{0} IS {1}.",
                                data.philosopherData.PhilosopherId,
                                TranslateState(data.type));
                        });
            }

            for (int i = 0; i < N; i++)
                p[i].Start();

            Console.ReadKey();
        }

        private static string TranslateState(int state)
        {
            switch (state)
            {
                case Philosopher.READY:
                    return "READY";
                case Philosopher.EATING:
                    return "EATING";
                case Philosopher.THINKING:
                    return "THINKING";
                case Philosopher.FINISHED:
                    return "FINISHED";
                default:
                    throw new Exception("*NOT* FOUND.");
            }
        }

        public const int N = 5;// 哲学家或叉子数目。
        public const int N_EAT = 2;// 每次吃掉的通心粉数量。
        public const int N_TOTAL = 20;// 盘中通心粉总数。
    }
}

 

　　运行结果：

Philosopher:0 IS READY.
Philosopher:0 IS EATING.
Philosopher:1 IS READY.
Philosopher:1 IS EATING.
Philosopher:3 IS READY.
Philosopher:2 IS READY.
Philosopher:4 IS READY.
Philosopher:1 IS THINKING.
Philosopher:4 IS EATING.
Philosopher:0 IS THINKING.
Philosopher:3 IS EATING.
Philosopher:3 IS THINKING.
Philosopher:4 IS THINKING.
Philosopher:2 IS EATING.
Philosopher:1 IS EATING.
Philosopher:1 IS THINKING.
Philosopher:3 IS EATING.
Philosopher:2 IS THINKING.
Philosopher:4 IS EATING.
Philosopher:3 IS THINKING.
Philosopher:0 IS EATING.
Philosopher:4 IS THINKING.
Philosopher:1 IS EATING.
Philosopher:1 IS THINKING.
Philosopher:4 IS EATING.
Philosopher:0 IS THINKING.
Philosopher:3 IS EATING.
Philosopher:4 IS THINKING.
Philosopher:2 IS EATING.
Philosopher:2 IS THINKING.
Philosopher:4 IS EATING.
Philosopher:3 IS THINKING.
Philosopher:0 IS EATING.
Philosopher:0 IS THINKING.
Philosopher:3 IS EATING.
Philosopher:4 IS THINKING.
Philosopher:3 IS THINKING.
Philosopher:1 IS EATING.
Philosopher:2 IS EATING.
Philosopher:2 IS THINKING.
Philosopher:0 IS EATING.
Philosopher:1 IS THINKING.
Philosopher:4 IS EATING.
Philosopher:4 IS THINKING.
Philosopher:0 IS THINKING.
Philosopher:2 IS EATING.
Philosopher:3 IS EATING.
Philosopher:3 IS THINKING.
Philosopher:1 IS EATING.
Philosopher:1 IS THINKING.
Philosopher:2 IS THINKING.
Philosopher:4 IS EATING.
Philosopher:0 IS EATING.
Philosopher:4 IS THINKING.
Philosopher:1 IS EATING.
Philosopher:1 IS THINKING.
Philosopher:1 IS EATING.
Philosopher:0 IS THINKING.
Philosopher:2 IS EATING.
Philosopher:2 IS THINKING.
Philosopher:1 IS THINKING.
Philosopher:3 IS EATING.
Philosopher:4 IS EATING.
Philosopher:4 IS THINKING.
Philosopher:2 IS EATING.
Philosopher:3 IS THINKING.
Philosopher:1 IS EATING.
Philosopher:2 IS THINKING.
Philosopher:0 IS EATING.
Philosopher:0 IS THINKING.
Philosopher:2 IS EATING.
Philosopher:1 IS THINKING.
Philosopher:3 IS EATING.
Philosopher:2 IS THINKING.
Philosopher:4 IS EATING.
Philosopher:3 IS THINKING.
Philosopher:0 IS EATING.
Philosopher:4 IS THINKING.
Philosopher:1 IS EATING.
Philosopher:1 IS THINKING.
Philosopher:4 IS EATING.
Philosopher:0 IS THINKING.
Philosopher:3 IS EATING.
Philosopher:3 IS THINKING.
Philosopher:0 IS EATING.
Philosopher:4 IS THINKING.
Philosopher:1 IS EATING.
Philosopher:1 IS THINKING.
Philosopher:4 IS EATING.
Philosopher:0 IS THINKING.
Philosopher:3 IS EATING.
Philosopher:3 IS THINKING.
Philosopher:0 IS EATING.
Philosopher:1 IS FINISHED.
Philosopher:4 IS THINKING.
Philosopher:0 IS THINKING.
Philosopher:3 IS EATING.
Philosopher:2 IS EATING.
Philosopher:4 IS FINISHED.
Philosopher:2 IS THINKING.
Philosopher:3 IS THINKING.
Philosopher:2 IS EATING.
Philosopher:3 IS FINISHED.
Philosopher:2 IS THINKING.
Philosopher:0 IS EATING.
Philosopher:0 IS THINKING.
Philosopher:2 IS EATING.
Philosopher:0 IS FINISHED.
Philosopher:2 IS THINKING.
Philosopher:2 IS FINISHED.

 

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