如何用C计算执行时间?

如何在以下代码中计算执行时间:

#include  /* Core input/output operations */ #include  /* Conversions, random numbers, memory allocation, etc. */ #include  /* Common mathematical functions */ #include  /* Converting between various date/time formats */ #include  #define PI 3.1415926535 /* Known vaue of PI */ #define NDARTS 128 /* Number of darts thrown */ double pseudo_random(double a, double b) { double r; /* Random number */ r = ((b - a) * ((double) rand()/(double) RAND_MAX)) + a; return r; } int main (int argc, char *argv[]) { int n_procs, /* Number of processors */ llimit, /* Lower limit for random numbers */ ulimit, /* Upper limit for random numbers */ n_circle, /* Number of darts that hit the circle */ i; /* Dummy/Running index */ double pi_sum, /* Sum of PI values from each WORKER */ x, /* x coordinate, betwen -1 & +1 */ y, /* y coordinate, betwen -1 & +1 */ z, /* Sum of x^2 and y^2 */ error; /* Error in calculation of PI */ clock_t start_time, /* Wall clock - start time */ end_time; /* Wall clock - end time */ struct timeval stime, starttime1, endtime1; struct timeval tv1, tv2, diff; llimit = -1; ulimit = 1; n_circle = 0; printf("\n Monte Carlo method of finding PI\n\n"); printf(" Number of processors : %d\n", n_procs); printf(" Number of darts : %d\n\n", NDARTS); gettimeofday(&tv1, NULL); gettimeofday(&stime, NULL); srand(stime.tv_usec * stime.tv_usec * stime.tv_usec * stime.tv_usec); for (i = 1; i <= NDARTS; i++) { x = pseudo_random(llimit, ulimit); y = pseudo_random(llimit, ulimit); z = pow(x, 2) + pow(y, 2); if (z <= 1.0) { n_circle++; } } pi_sum = 4.0 * (double)n_circle/(double)NDARTS; pi_sum = pi_sum / n_procs; error = fabs((pi_sum - PI)/PI) * 100; gettimeofday(&tv2, NULL); double timeval_subtract (result, x, y) { result = ((double) x - (double) y ) / (double)CLOCKS_PER_SEC; } double result1 = timeval_subtract(&diff, &tv1, &tv2); printf(" Known value of PI : %11.10f\n", PI); printf(" Average value of PI : %11.10f\n", pi_sum); printf(" Percentage Error : %10.8f\n", error); printf(" Time : \n", clock() ); printf(" Start Time : \n",&tv1); printf(" End Time :\n", &tv2); printf(" Time elapsed (sec) : \n", result1 ); return 0; } 

我使用了timeval_subtract函数,当我执行代码时,我得到了:

 Monte Carlo method of finding PI Number of processors : 16372 Number of darts : 128 Known value of PI : 3.1415926535 Average value of PI : 0.0002004184 Percentage Error : 99.99362048 Time : Start Time : End Time : Time elapsed (sec) : 

首先,我找不到找到处理器数量的错误(我必须得到1个处理器)。

第二个“这是最重要的一点”,为什么我的时间,开始时间,结束时间和时间都过空了?

因为您没有足够的格式字符串,所以需要以’%’开头的内容,例如:

 printf(" Time :%d \n", clock() ); 

n_procs永远不会被初始化,被打印的16372值恰好是之前在堆栈上的值。

C标准库不提供查询处理器数量或高性能计时器的function,因此您必须查看其他查询方法。 例如,POSIX和Windows API都提供了这样的function。

编辑:请参阅以编程方式查找计算机上的核心数,以了解如何初始化n_procs。 看看你如何使用gettimeofday,你可能会使用一些unix变体; “n_procs = sysconf(_SC_NPROCESSORS_ONLN);” 可能就是你想要的。

试试这个:

 printf(" Time : %lu\n", clock() ); printf(" Start Time : %lds %ldus\n", tv1.tv_sec, tv1.tv_usec); printf(" End Time : %lds %ldus\n", tv2.tv_sec, tv2.tv_usec); 

并为:

 double timeval_subtract (result, x, y) 

使用以下命令以微秒为单位返回时间差:

 long timeval_subtract (struct timeval * result, struct timeval * x, struct timeval * y) { long usec = x->tv_sec * 1000000L + x->tv_usec; usec -= (y->tv_sec * 1000000L + y->tv_usec); result->tv_sec = usec / 1000000L; result->tv_usec = usec % 1000000L; return usec; } 

根据两个日期xy的差异,函数timeval_subtract的返回值(不是result !表示的值)可能由于溢出而错误。

假设长为32位宽,这种溢出将发生大于4294s的差异,长时间有64位(应该是64位机器的情况)溢出将在很久以后发生…… 😉

我尝试以下方法:

 int timeval_subtract ( struct timeval *result, struct timeval *x, struct timeval *y ) { if ( x->tv_usec < y->tv_usec ) { int nsec = ( y->tv_usec - x->tv_usec ) / 1000000 + 1; y->tv_usec -= 1000000 * nsec; y->tv_sec += nsec; } if (x->tv_usec - y->tv_usec > 1000000) { int nsec = ( x->tv_usec - y->tv_usec ) / 1000000; y->tv_usec += 1000000 * nsec; y->tv_sec -= nsec; } result->tv_sec = x->tv_sec - y->tv_sec; result->tv_usec = x->tv_usec - y->tv_usec; return x->tv_sec < y->tv_sec; } void Start ( struct timeval *timer_profiling ) { if ( timer_profiling == NULL ) return; gettimeofday ( timer_profiling , NULL ); return; } void End ( struct timeval *timer_profiling , char *msg ) { struct timeval res; struct timeval now; gettimeofday ( &now , NULL ); if ( msg == NULL ) return; timeval_subtract ( &res , &now , timer_profiling ); sprintf ( msg , "[ %ld,%.3ld ms]" , res.tv_sec*1000 + (long)round(res.tv_usec/1000) , res.tv_usec - (long)round(res.tv_usec/1000)*1000); return; } 

用分配的timer_profiling启动(&)一个,然后通过调用End(&s,buff)检索字符串中的结果;