OpenMP 并行化处理测试

OpenMP 并行化处理测试

#pragma  omp parallel for 这条语句是用来指定后面的for循环语句变成并行执行的,将for循环里的语句变成并行执行后效率会不会提高呢?还是测试一 下吧，测试的时候，循环最好大一些，增加计算量，不然程序耗时太短，很难区分。

 1 #include <omp.h>  

 2 #include <stdio.h>  

 3 #include <sys/time.h>  

 4 void test() {  

 5     int a = 0;  

 6     for(int i = 0; i < 10000000; i++) {  

 7         a = i + 1;  

 8     }  

 9 }  

10 int main() {  

11     int nthrds = 4;  

12     omp_set_num_threads(nthrds);  

13     timeval tStart,cTime;  

14     long long tPassed = 0;  

15     gettimeofday(&tStart, 0);  

16     for (int i = 0; i < 100; i++) {  

17         test();  

18     }  

19     gettimeofday(&cTime, 0);  

20     cTime.tv_sec -= tStart.tv_sec;   

21     cTime.tv_usec -= tStart.tv_usec;  

22     tPassed = 1000000LL * cTime.tv_sec + cTime.tv_usec;   

23     tPassed /= 1000;  

24     printf("Time = %lld/n", tPassed);  

25     gettimeofday(&tStart, 0);  

26     #pragma omp parallel for  

27     for (int i = 0; i < 100; i++) {  

28         test();  

29     }  

30     gettimeofday(&cTime, 0);  

31     cTime.tv_sec -= tStart.tv_sec;  

32     cTime.tv_usec -= tStart.tv_usec;  

33     tPassed = 1000000LL * cTime.tv_sec + cTime.tv_usec;   

34     tPassed /= 1000;  

35     printf("Time = %lld/n", tPassed);  

36     return 1;  

37 }  

 result:

1 [root@c0108 zlt]# g++ omp.c -o omp -fopenmp  

2 [root@c0108 zlt]# ./omp  

3 Time = 2859  

4 Time = 718  

5 [root@c0108 zlt]#   

在四核的机器上，开了四个线程，加速比基本上是4，果然不错。 

此外，BS一下clock()函数，我向来觉得它一无是处。。。

#include <omp.h>
#include <stdio.h>
#include <time.h>
void test() {
 int a = 0;
 for(int i = 0; i < 10000000; i++) {
  a = i + 1;
 }
}
int main() {
 int nthrds = 4;
 omp_set_num_threads(nthrds);
 long start = clock();
 for (int i = 0; i < 100; i++) {
  test();
 }
 long end = clock();
 printf("Time = %ld/n", end - start);
 start = clock();
 #pragma omp parallel for
 for (int i = 0; i < 100; i++) {
  test();
 }
 end = clock();
 printf("Time = %ld/n", end - start);
 return 1;
}

结果：

1  

2 

3 [root@c0108 zlt]# g++ omp.c -o omp -fopenmp  

4 [root@c0108 zlt]# ./omp  

5 Time = 2860000  

6 Time = 2860000  

7 [root@c0108 zlt]#   

8  

 

clock有三个问题:
1)如果超过一个小时,将要导致溢出.
2)函数clock没有考虑CPU被子进程使用的情况.
3)也不能区分用户空间和内核空间.