real time, user time, sys time of a process

針對一個process的時間統計：

real==>指逝去的時間(elapsed time)(time of waiting for I/O to complete + time used by other processes)(透過gettimeofday()或times()收集)
user==>指CPU執行該process的user-mode code的時間(透過 times()收集)
sys==>指CPU執行該process的kernel-mode code的時間(透過times()收集)
User+Sys==>指CPU真正執行該procees的時間(可透clock()收集)

http://www.gnu.org/s/hello/manual/libc/CPU-Time.html#CPU-Time
http://osr600doc.sco.com/en/SDK_c++/_Measuring_Program_Execution_Tim.html


#include stdio.h
#include time.h

#include sys/param.h
#include sys/times.h
#include sys/types.h

int main(){
int i;
struct tms t,u;
long r1,r2;
struct timeval start, end;
clock_t start_c, end_c;
gettimeofday(&start, NULL);
r1 = times(&t);
start_c=clock();
for(i=0;i < 10000 ;i++){
printf("hello!!\n");
}
end_c=clock();
r2 = times(&u);
gettimeofday(&end, NULL);
printf("\nHZ=%d\n",HZ);
printf("user time=%f\n",((float)(u.tms_utime-t.tms_utime))/(HZ));
printf("system time=%f\n",((float)(u.tms_stime-t.tms_stime))/(HZ));
printf("real time=%f\n",((float)(r2-r1))/(HZ));
printf("real time(gettimeofday())=%f\n", (float)((end.tv_sec * 1000000 + end.tv_usec)- (start.tv_sec * 1000000 + start.tv_usec))/1000000);
printf("user+real=%f\n" ,((double) (end_c - start_c)) / CLOCKS_PER_SEC);
return 0;
}


output:
hello!!
.....
hello!!

HZ=100
user time=0.000000
system time=0.020000
real time=0.150000
real time(gettimeofday())=0.151931
user+real=0.020000


clock()的實作:

#include sys/times.h
#include sys/param.h
#include time.h
#include unistd.h

clock_t clock(void) {
struct tms buf;
times(&buf);



#if CLOCKS_PER_SEC == HZ
return (unsigned long) buf.tms_utime + buf.tms_stime;
#elif CLOCKS_PER_SEC % HZ == 0
return ((unsigned long) buf.tms_utime + buf.tms_stime) * (CLOCKS_PER_SEC / HZ);
#elif HZ % CLOCKS_PER_SEC == 0
return ((unsigned long) buf.tms_utime + buf.tms_stime) / (HZ / CLOCKS_PER_SEC);
#else
return ((unsigned long long) buf.tms_utime + buf.tms_stime) * CLOCKS_PER_SEC / HZ;
#endif
}