Measuring time of parent and children processes

Question

I've written a program which forks in a loop. The only thing children processes do is to increase a counter and exit, whereas a parent process waits for each of them.

My goal is to measure user and system time of parent process and all his children separately. I've succeded with parent process using times() function and struct tms. Surprisingly, the same aproach to children processes isn't working. What is the mistake that I'm doing? How to measure those times?

I've also tried getrusage() and I/it failed.

My code:

#include <stdio.h>
#include <sys/types.h>
#include <unistd.h>
#include <sys/wait.h>
#include <sys/time.h>
#include <sys/resource.h>
#include <sys/times.h>
#include <time.h>

#ifndef COUNT
#define COUNT 100000
#endif



int counter;


int main(){

struct tms time1,time2;
times(&time1);

int count = COUNT;
pid_t pid;
while(count--){
    if((pid=fork())<0){
        printf("fork error\n");
    } else if(pid==0){ /* child */
        counter++;
        _exit(0);
    } else {
        waitpid(pid,NULL,0); /*wait()*/
    }
}
printf("COUNTER: %d\n",counter);



times(&time2);

long double clktck=sysconf(_SC_CLK_TCK);
double user=(time2.tms_utime-time1.tms_utime)/(double)clktck;
double system=(time2.tms_stime-time1.tms_stime)/(double)clktck;
double cuser=(time2.tms_cutime-time1.tms_cutime)/(double)clktck;
double csystem=(time2.tms_cstime-time1.tms_cstime)/(double)clktck;

printf("USER:%lf\nSYSTEM:%lf\n",user,system);
printf("CUSER:%lf\nCSYSTEM:%lf\n",cuser,csystem);



return 0;
}

Answer 1

I think the problem is that your children are executing too quickly ; they don't take enough time to execute, so the sum of their time is plenty of zeros. To test this theory, I slightly changed your program:

#include <stdio.h>
#include <sys/types.h>
#include <unistd.h>
#include <stdlib.h>
#include <sys/wait.h>
#include <sys/time.h>
#include <sys/resource.h>
#include <sys/times.h>
#include <time.h>

#ifndef COUNT
#define COUNT 100
#endif



int counter;


int main(){

struct tms time1,time2;
times(&time1);

int count = COUNT;
pid_t pid;
while(count--){
    if((pid=fork())<0){
        printf("fork error\n");
    } else if(pid==0){ /* child */
        int i;
        for (i=0; i<10000; i++) {
            printf("in child %i\n", getpid());
        }
        exit(0);
    } else {
        waitpid(pid,NULL,0); /*wait()*/
    }
}
printf("COUNTER: %d\n",counter);



times(&time2);

printf("%lu %lu %lu %lu\n", time2.tms_utime, time2.tms_stime, time2.tms_cutime, time2.tms_cstime);

long double clktck=sysconf(_SC_CLK_TCK);
double user=(time2.tms_utime-time1.tms_utime)/(double)clktck;
double system=(time2.tms_stime-time1.tms_stime)/(double)clktck;
double cuser=(time2.tms_cutime-time1.tms_cutime)/(double)clktck;
double csystem=(time2.tms_cstime-time1.tms_cstime)/(double)clktck;

printf("USER:%lf\nSYSTEM:%lf\n",user,system);
printf("CUSER:%lf\nCSYSTEM:%lf\n",cuser,csystem);



return 0;
}

You'll see that I drastically cut down on the number of children, and made the children do some real work; 10_000 printf(... getpid()) operations. Now the times amount to something:

$ time ./times
...
in child 16181
COUNTER: 0
1 0 24 95
USER:0.010000
SYSTEM:0.000000
CUSER:0.240000
CSYSTEM:0.950000

real    0m2.234s
user    0m0.250s
sys 0m0.950s

I'm afraid your children just didn't have enough work to do to amount to anything. (Odd, sounds like parenting advice.)

Answer 2

Each child is given their own address space. The code will not work because it will increment it's own local copy of counter and quit, leaving the version in the parent process/all other children untouched.

Also, you are very likely to get some errors with that many children.

Sorry I could only help with half the program :(.