openMP学习笔记

本次学习课程来自Intel 高级研究员Tim Mattson

课程视频下载地址(全英文且无字幕):

链接:https://pan.baidu.com/s/1nw6pcRv 密码:aolo

虽然最近量子计算和Nvidia CUDA技术越来越热,但是工业上都采用arm架构的嵌入式设备,负担不起nvidia的成本,所以学好OpenMP、SIMD之类的还是比较重要的。

1.openmp是干什么的

openmp是一个跨平台的、较常用的基于共享内存(地址空间)的并行编程模型,它提供一组编译指令集和库例程给并行应用开发者,openmp只支持fortran,c和c++。Linux下的gcc天然支持openmp(注意版本)。

echo |cpp -fopenmp -dM |grep -i open

openmp的架构层次如下图所示

openMP学习笔记_第1张图片

openmp内线程是如何交互的?

  openmp是一个多线程、共享地址模型,不同线程通过共享变量交互信息。但是随意的数据共享将会导致竞争问题,即每次程序运行的结果都会因OS不同的线程调度执行次序而改变。为了控制竞争问题,openmp采用同步策略来保护数据冲突。但是,同步策略是非常昂贵的,消耗performance的。因此,合格的openmp程序员,将以最小的同步代价来制定数据的访问规则。

2.一则求PI的例子

   我们不来搞hello world这种入门例子了,直接通过对求PI程序的三种优化方法,来感悟下openmp的魅力


   计算机没法求解连续问题,就用数学插值法来离散化求解


openMP学习笔记_第2张图片

原来的串行代码:
#include 
#include 
static long num_steps = 100000000;
double step;
int main ()
{
	  int i;
	  double x, pi, sum = 0.0;
	  double start_time, run_time;

	  step = 1.0/(double) num_steps;

        	 
	  start_time = omp_get_wtime();

	  for (i=1;i<= num_steps; i++){
		  x = (i-0.5)*step;
		  sum = sum + 4.0/(1.0+x*x);
	  }

	  pi = step * sum;
	  run_time = omp_get_wtime() - start_time;
	  printf("\n pi with %ld steps is %lf in %lf seconds\n ",num_steps,pi,run_time);
}	  

SPMD(singleprogram multiple data)优化,每个线程做各自的统计,最后通过atomic同步机制汇总

#include 
#include 

#define MAX_THREADS 4

static long num_steps = 100000000;
double step;
int main ()
{
	  int i,j;
	  double pi, full_sum = 0.0;
	  double start_time, run_time;
	  double sum[MAX_THREADS];

	  step = 1.0/(double) num_steps;


for(j=1;j<=MAX_THREADS ;j++){
   omp_set_num_threads(j);
   full_sum = 0.0;
	  start_time = omp_get_wtime();
#pragma omp parallel private(i)
{
	  int id = omp_get_thread_num();
	  int numthreads = omp_get_num_threads();
	  double x;

	  double partial_sum = 0;

#pragma omp single
	  printf(" num_threads = %d",numthreads);

	  for (i=id;i< num_steps; i+=numthreads){
		  x = (i+0.5)*step;
		  partial_sum += + 4.0/(1.0+x*x);
	  }
#pragma omp critical
		  full_sum += partial_sum;
}
      
	  pi = step * full_sum;
	  run_time = omp_get_wtime() - start_time;
	  printf("\n pi is %f in %f seconds %d threds \n ",pi,run_time,j);
}
}	  

openMP Loop Parallelism 优化,注意openmp for loop 语法,另外注意reduction归约操作

#include 
#include 
static long num_steps = 100000000;
double step;
int main ()
{
	  int i;
	  double x, pi, sum = 0.0;
	  double start_time, run_time;

	  step = 1.0/(double) num_steps;
	 for (i=1;i<=4;i++){
          sum = 0.0;
          omp_set_num_threads(i);
	  start_time = omp_get_wtime();
#pragma omp parallel  
{
#pragma omp single
	  printf(" num_threads = %d",omp_get_num_threads());

#pragma omp for reduction(+:sum)
	  for (i=1;i<= num_steps; i++){
		  x = (i-0.5)*step;
		  sum = sum + 4.0/(1.0+x*x);
	  }
}
	  pi = step * sum;
	  run_time = omp_get_wtime() - start_time;
	  printf("\n pi is %f in %f seconds and %d threads\n",pi,run_time,i);
}
}	  

分治法(Divide and Conquer Pattern)优化,openmp task是高级特性,从openmp3.1后开始支持

openMP学习笔记_第3张图片


3.其他例子

生产者与消费者问题优化

原始的串行代码:

#include 
#ifdef APPLE
#include 
#else
#include 
#endif
#include 

#define N        10000

/* Some random number constants from numerical recipies */
#define SEED       2531
#define RAND_MULT  1366
#define RAND_ADD   150889
#define RAND_MOD   714025
int randy = SEED;

/* function to fill an array with random numbers */
void fill_rand(int length, double *a)
{
   int i; 
   for (i=0;i

并行优化代码,用标记符号通知消费者生产者是否完成

#include "omp.h"
#ifndef APPLE
#include 
#endif
#include 
#include 

#define N        10000
#define Nthreads 2

/* Some random number constants from numerical recipies */
#define SEED       2531
#define RAND_MULT  1366
#define RAND_ADD   150889
#define RAND_MOD   714025
int randy = SEED;

/* function to fill an array with random numbers */
void fill_rand(int length, double *a)
{
   int i; 
   for (i=0;i


链表遍历、斐波那契递归问题的优化:

原始串行代码:

#include 
#include 
#include 

#ifndef N
#define N 5
#endif
#ifndef FS
#define FS 38
#endif

struct node {
   int data;
   int fibdata;
   struct node* next;
};

int fib(int n) {
   int x, y;
   if (n < 2) {
      return (n);
   } else {
      x = fib(n - 1);
      y = fib(n - 2);
	  return (x + y);
   }
}

void processwork(struct node* p) 
{
   int n;
   n = p->data;
   p->fibdata = fib(n);
}

struct node* init_list(struct node* p) {
    int i;
    struct node* head = NULL;
    struct node* temp = NULL;
    
    head = malloc(sizeof(struct node));
    p = head;
    p->data = FS;
    p->fibdata = 0;
    for (i=0; i< N; i++) {
       temp  =  malloc(sizeof(struct node));
       p->next = temp;
       p = temp;
       p->data = FS + i + 1;
       p->fibdata = i+1;
    }
    p->next = NULL;
    return head;
}

int main(int argc, char *argv[]) {
     double start, end;
     struct node *p=NULL;
     struct node *temp=NULL;
     struct node *head=NULL;
     
	 printf("Process linked list\n");
     printf("  Each linked list node will be processed by function 'processwork()'\n");
     printf("  Each ll node will compute %d fibonacci numbers beginning with %d\n",N,FS);      
 
     p = init_list(p);
     head = p;

     start = omp_get_wtime();
     {
        while (p != NULL) {
		   processwork(p);
		   p = p->next;
        }
     }

     end = omp_get_wtime();
     p = head;
	 while (p != NULL) {
        printf("%d : %d\n",p->data, p->fibdata);
        temp = p->next;
        free (p);
        p = temp;
     }  
	 free (p);

     printf("Compute Time: %f seconds\n", end - start);

     return 0;
}


并行优化代码

看了openmp论坛,递归那边前20个不用并行优化,否则性能反而降低了

#include 
#include 
#include 


#ifndef N
#define N 5
#endif
#ifndef FS
#define FS 38
#endif

typedef struct node {
   int data;
   int fibdata;
   struct node* next;
}node;

node* init_list(node* p);
void processwork(node* p); 
int fib(int n); 

int fib(int n) 
{
   int x, y;
   if (n < 2) {
      return (n);
   } else {
	if(n<20)
		return fib(n-1)+fib(n-2);
#pragma omp task shared(x)
      x = fib(n - 1);
#pragma omp task shared(y)
      y = fib(n - 2);
#pragma omp taskwait
	  return (x + y);
   }
}

void processwork(node* p) 
{
   int n, temp;
   n = p->data;
   temp = fib(n);

   p->fibdata = temp;

}

node* init_list(node* p) 
{
    int i;
    node* head = NULL;
    node* temp = NULL;
    
    head = (node*)malloc(sizeof(node));
    p = head;
    p->data = FS;
    p->fibdata = 0;
    for (i=0; i< N; i++) {
       temp  = (node*)malloc(sizeof(node));
       p->next = temp;
       p = temp;
       p->data = FS + i + 1;
       p->fibdata = i+1;
    }
    p->next = NULL;
    return head;
}

int main() 
{
     double start, end;
     struct node *p=NULL;
     struct node *temp=NULL;
     struct node *head=NULL;

     printf("Process linked list\n");
     printf("  Each linked list node will be processed by function 'processwork()'\n");
     printf("  Each ll node will compute %d fibonacci numbers beginning with %d\n",N,FS);      

     p = init_list(p);
     head = p;

     start = omp_get_wtime();

	#pragma omp parallel 
	{
            #pragma omp master
                  printf("Threads:      %d\n", omp_get_num_threads());

		#pragma omp single
		{
			p=head;
			while (p) {
				#pragma omp task firstprivate(p) //first private is required
				{
					processwork(p);
				}
			  p = p->next;
		   }
		}
	}

     end = omp_get_wtime();
     p = head;
	 while (p != NULL) {
        printf("%d : %d\n",p->data, p->fibdata);
        temp = p->next;
        free (p);
        p = temp;
     }  
	 free (p);

     printf("Compute Time: %f seconds\n", end - start);

     return 0;
}

你可能感兴趣的:(openMP)