c – Pthread程序在线程增加时运行较慢

概述我是并行编程的初学者,我尝试用pthread库编写并行程序.我在8处理器计算机上运行程序.问题在于,当我增加NumProcs时,每个线程都会减慢,尽管它们的任务总是相同的.有人可以帮我弄清楚发生了什么吗？ ` #define MAX_NUMP 16using namespace std;int NumProcs;pthread_mutex_t SyncLock; /* mutex */ 我是并行编程的初学者,我尝试用pthread库编写并行程序.我在8处理器计算机上运行程序.问题在于,当我增加NumProcs时,每个线程都会减慢,尽管它们的任务总是相同的.有人可以帮我弄清楚发生了什么吗？
`

#define MAX_NUMP 16using namespace std;int NumProcs;pthread_mutex_t   SyncLock; /* mutex */pthread_cond_t    SyncCV; /* condition variable */int               SyncCount; /* number of processors at the barrIEr so far */pthread_mutex_t   ThreadLock; /* mutex */// used only in solaris. use clock_gettime in linux//hrtime_t          StartTime;//hrtime_t          EndTime;  struct timespec StartTime;struct timespec EndTime;voID barrIEr(){  int ret;  pthread_mutex_lock(&SyncLock); /* Get the thread lock */  SyncCount++;  if(SyncCount == NumProcs) {    ret = pthread_cond_broadcast(&SyncCV);    assert(ret == 0);  } else {    ret = pthread_cond_wait(&SyncCV,&SyncLock);     assert(ret == 0);  }  pthread_mutex_unlock(&SyncLock);}/* The function which is called once the thread is allocated */voID* ThreadLoop(voID* tmp){  /* each thread has a private version of local variables */  long threadID = (long) tmp;   int ret;  int startTime,endTime;  int count=0;  /* ********************** Thread Synchronization*********************** */  barrIEr();  /* ********************** Execute Job ********************************* */  startTime = clock();  for(int i=0;i<65536;i++)    for(int j=0;j<1024;j++)        count++;  endTime = clock();  printf("threadID:%ld,time:%d\n",threadID,endTime-startTime);}int main(int argc,char** argv){  pthread_t*     threads;  pthread_attr_t attr;  int            ret;  int            dx;  if(argc != 2) {    fprintf(stderr,"USAGE: %s <numProcesors>\n",argv[0]);    exit(-1);  }  assert(argc == 2);  NumProcs = atoi(argv[1]);  assert(NumProcs > 0 && NumProcs <= MAX_NUMP);  /* Initialize array of thread structures */  threads = (pthread_t *) malloc(sizeof(pthread_t) * NumProcs);  assert(threads != NulL);  /* Initialize thread attribute */  pthread_attr_init(&attr);  pthread_attr_setscope(&attr,PTHREAD_ScopE_SYstem); // sys manages contention  /* Initialize mutexs */  ret = pthread_mutex_init(&SyncLock,NulL);  assert(ret == 0);  ret = pthread_mutex_init(&ThreadLock,NulL);  assert(ret == 0);  /* Init condition variable */  ret = pthread_cond_init(&SyncCV,NulL);  assert(ret == 0);  SyncCount = 0;  Count = 0;  /* get high resolution timer,timer is expressed in nanoseconds,relative   * to some arbitrary time.. so to get delta time must call gethrtime at   * the end of operation and subtract the two times.   */  //StartTime = gethrtime();  ret = clock_gettime(CLOCK_MONOTONIC,&StartTime);  for(dx=0; dx < NumProcs; dx++) {    /* ************************************************************     * pthread_create takes 4 parameters     *  p1: threads(output)     *  p2: thread attribute     *  p3: start routine,where new thread begins     *  p4: arguments to the thread     * ************************************************************ */    ret = pthread_create(&threads[dx],&attr,ThreadLoop,(voID*) dx);    assert(ret == 0);  }  /* Wait for each of the threads to terminate */  for(dx=0; dx < NumProcs; dx++) {    ret = pthread_join(threads[dx],NulL);    assert(ret == 0);  }  //EndTime = gethrtime();  ret = clock_gettime(CLOCK_MONOTONIC,&EndTime);  printf("Time = %ld nanoseconds\n",EndTime.tv_nsec - StartTime.tv_nsec);  pthread_mutex_destroy(&ThreadLock);  pthread_mutex_destroy(&SyncLock);  pthread_cond_destroy(&SyncCV);  pthread_attr_destroy(&attr);  return 0;}

解决方法 你的意见是预期的.

通常影响这种情况的主要因素(工人在本地计算上旋转)是：

>比率nb_threads / nb_available_machine_cores
>每个线程的亲和力

这里的最佳方案是当比率为1时,每个线程与其中一个核心具有唯一的亲和力.

我们的想法是最大化每个核心吞吐量.你可以通过在每个核心上运行一个且只有一个线程来实现.如果增加线程数(比率> 1),多个线程将共享同一个内核,迫使内核(通过任务调度程序)在每个线程的执行之间切换.这就是你所观察到的.

每次内核必须 *** 作这样的开关时,您需要支付上下文切换.它可能会成为明显的开销.

注意：

您可以使用pthread_setaffinity设置线程的亲缘关系.

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