Pthreads Hello World，忙等待，互斥量

▶ 一个简单的 Pthreads 程序（不按照《并行程序设计导论》中的程序来写）

● 代码

 #include <stdio.h>
 #include <pthread.h>
 #pragma comment(lib, "pthreadVC2.lib")

 const int thread = ;

 void* work(void* input)
 {
     if (input == nullptr)
         printf("nullptr!\n");
     else
         printf("%d\n", *((int *)input));  // 将得到的 void * 参数转化为需要的数据类型再进行使用
     return nullptr;
 }

 int main()
 {
     pthread_t array[thread];                                // 产生各线程的 pthread_t 对象
     int i, list[thread];

     for (i = ; i < thread; i++)
     {
         list[i] = i * ;
         pthread_create(&array[i], nullptr, work, &list[i]); // 生成各线程来运行函数 work()，函数参数为 list[i]，没有线程属性参数
     }
     for (i = ; i < thread; i++)
         pthread_join(array[i], nullptr);                    // 终止各线程
     getchar();
     return ;
 }

● 输出结果：8 个乱序的数字

● 用到的定义，pthread.h

 typedef struct
 {
     void * p;                                   // Pointer to actual object
     unsigned int x;                             // Extra information - reuse count etc
 } ptw32_handle_t;

 typedef ptw32_handle_t pthread_t;
 typedef struct pthread_attr_t_ * pthread_attr_t;// 没有其他地方提到了 struct pthread_attr_t_ 的成员 

 PTW32_DLLPORT int PTW32_CDECL pthread_create(
     pthread_t * tid,                            // 输入 pthread_t 的指针
     const pthread_attr_t * attr,                // pthread_t 对象的属性
     void *(PTW32_CDECL *start) (void *),        // 工作函数，输入参数和返回类型均为 void *
     void *arg                                   // 工作函数的输入参数
 );

 PTW32_DLLPORT int PTW32_CDECL pthread_join(
     pthread_t thread,                           // 需要等待终止的 pthread_t 对象，注意不是指针
     void **value_ptr                            // ？
 );

▶ 使用直接访问、忙等待和互斥量计算 π 的值，使用公式 π / 4 = 1 - 1 / 3 + 1 / 5 - 1 / 7 + ...，Mathematica 的精确结果为 0.78539816339744830962

● 直接访问临界区，代码

 #include <stdio.h>
 #include <pthread.h>
 #include <time.h>
 #pragma comment(lib, "pthreadVC2.lib")

 const int count = , thread = ;// 使用 8 个线程来计算 2^30 项
 double sum;

 void* threadSum_naïve(void* rank)
 {
     const long long localRank = (long long)rank;// 使用 long long 类型，可以直接与 void* 相互转化
     const int localCount = count / thread;
     int i;
     double sign;
     if (localCount * localRank % )
         sign = -1.0;
     else
         sign = 1.0;
     for (i = localCount*localRank; i < localCount*(localRank + ); i++, sign = -sign)
         sum += sign / ( * i + );
     printf("Thread %2d finished.\n", localRank);
     return nullptr;
 }

 int main()
 {
     pthread_t array[thread];
     int i;
     long long list[thread];
     clock_t time = clock();
     for (i = , sum = 0.0; i < thread; i++)
     {
         list[i] = i;
         pthread_create(&array[i], nullptr, threadSum_naïve, (void *)list[i]);
     }
     for (i = ; i < thread; i++)
         pthread_join(array[i], nullptr);
     time = clock() - time;
     printf("\nsum = %2.10f, time = %d ms\n", sum, time);
     getchar();
     return ;
 }

● 输出结果，发现与精确结果相差很大，这是由于读写冲突造成的

Thread   finished.
Thread   finished.
Thread   finished.
Thread   finished.
Thread   finished.
Thread   finished.
Thread   finished.
Thread   finished.

sum = 0.7852892761, time =  ms

● 忙等待法，代码

 #include <stdio.h>
 #include <pthread.h>
 #include <time.h>
 #pragma comment(lib, "pthreadVC2.lib")

 const int count = , thread = ;
 double sum;
 int flag;

 void* threadSum(void* rank)
 {
     const long long localRank = (long long)rank;
     const int localCount = count / thread;
     int i;
     double sign;
     if (localCount * localRank % )
         sign = -1.0;
     else
         sign = 1.0;
     for (i = localCount*localRank; i < localCount*(localRank + ); i++, sign = -sign)
     {
         while (flag != localRank); // 等待读写标志等于线程编号时进行写入
         sum += sign / ( * i + );
         flag = (flag + ) % thread;// 写入完成调整读写标志以便下一个线程写入
     }
     printf("Thread %2d finished.\n", localRank);
     return nullptr;
 }

 int main()
 {
     pthread_t array[thread];
     int i;
     long long list[thread];
     clock_t time = clock();
     flag = ;
     for (i = , sum = 0.0; i < thread; i++)
     {
         list[i] = i;
         pthread_create(&array[i], nullptr, threadSum, (void *)list[i]);
     }
     for (i = ; i < thread; i++)
         pthread_join(array[i], nullptr);
     time = clock() - time;
     printf("\nsum = %2.10f, time = %d ms\n", sum, time);
     getchar();
     return ;
 }

● 输出结果，发现花费的时间非常长，这是因为每个线程每次向结果中写入一个数，造成了极长的等待队列

Thread   finished.
Thread   finished.
Thread   finished.
Thread   finished.
Thread   finished.
Thread   finished.
Thread   finished.
Thread   finished.

sum = 0.7853981632, time =  ms

● 忙等待法 + 局部和，代码

 #include <stdio.h>
 #include <pthread.h>
 #include <time.h>
 #pragma comment(lib, "pthreadVC2.lib")

 const int count = , thread = ;
 double sum;
 int flag;

 void* threadSum(void* rank)
 {
     const long long localRank = (long long)rank;
     const int localCount = count / thread;
     int i;
     double sign, localSum;
     if (localCount * localRank % )
         sign = -1.0;
     else
         sign = 1.0;
     for (i = localCount * localRank, localSum = 0.0; i < localCount * (localRank + ); localSum += sign / ( * i + ), i++, sign = -sign);
     for (; flag != localRank;);// 仍然使用忙等待，但是每个线程仅向总和中写入一次部分和
     sum += localSum;
     flag = (flag + ) % thread;    

     printf("Thread %2d finished.\n", localRank);
     return nullptr;
 }

 int main()
 {
     pthread_t array[thread];
     int i;
     long long list[thread];
     clock_t time = clock();
     flag = ;
     for (i = , sum = 0.0; i < thread; i++)
     {
         list[i] = i;
         pthread_create(&array[i], nullptr, threadSum, (void *)list[i]);
     }
     for (i = ; i < thread; i++)
         pthread_join(array[i], nullptr);
     time = clock() - time;
     printf("\nsum = %2.10f, time = %d ms\n", sum, time);
     getchar();
     return ;
 }

● 输出结果，速度大为加快

Thread   finished.
Thread   finished.
Thread   finished.
Thread   finished.
Thread   finished.
Thread   finished.
Thread   finished.
Thread   finished.

sum = 0.7853981632, time =  ms

● 使用互斥量，代码

 #include <stdio.h>
 #include <pthread.h>
 #include <time.h>
 #pragma comment(lib, "pthreadVC2.lib")

 const int count = , thread = ;
 double sum;
 pthread_mutex_t pmt;

 void* threadSum(void* rank)
 {
     const long long localRank = (long long)rank;
     const int localCount = count / thread;
     int i;
     double sign, localSum;
     if (localCount * localRank % )
         sign = -1.0;
     else
         sign = 1.0;
     for (i = localCount * localRank, localSum = 0.0; i < localCount * (localRank + ); localSum += sign / ( * i + ), i++, sign = -sign);
     pthread_mutex_lock(&pmt);         // 与使用忙等待相同的办法使用互斥量
     sum += localSum;
     pthread_mutex_unlock(&pmt);

     printf("Thread %2d finished.\n", localRank);
     return nullptr;
 }

 int main()
 {
     pthread_t array[thread];
     int i;
     long long list[thread];
     clock_t time = clock();
     pthread_mutex_init(&pmt, nullptr);// 初始化互斥量
     for (i = , sum = 0.0; i < thread; i++)
     {
         list[i] = i;
         pthread_create(&array[i], nullptr, threadSum, (void *)list[i]);
     }
     for (i = ; i < thread; i++)
         pthread_join(array[i], nullptr);
     pthread_mutex_destroy(&pmt);     // 销毁互斥量
     time = clock() - time;
     printf("\nsum = %2.10f, time = %d ms\n", sum, time);
     getchar();
     return ;
 }

● 输出结果，速度与使用忙等待相近

Thread   finished.
Thread   finished.
Thread   finished.
Thread   finished.
Thread   finished.
Thread   finished.
Thread   finished.
Thread   finished.

sum = 0.7853981632, time =  ms

● 用到的定义，pthread.h

 typedef struct pthread_mutex_t_ * pthread_mutex_t;
 typedef struct pthread_mutexattr_t_ * pthread_mutexattr_t;

 PTW32_DLLPORT int PTW32_CDECL pthread_mutex_init(pthread_mutex_t * mutex, const pthread_mutexattr_t * attr);
     // 初始化互斥量，输入已经声明的一个 pthread_mutex_t 变量的指针及一个 pthread_mutexattr_t 属性指针，初始化完成后互斥量为解锁状态

 PTW32_DLLPORT int PTW32_CDECL pthread_mutex_destroy(pthread_mutex_t * mutex);   // 销毁用完的互斥量

 PTW32_DLLPORT int PTW32_CDECL pthread_mutex_lock(pthread_mutex_t * mutex);      // 锁定互斥量以访问临界区

 PTW32_DLLPORT int PTW32_CDECL pthread_mutex_unlock(pthread_mutex_t * mutex);    // 解锁互斥量以离开临界区访问