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.