1
votes

I have a C program that parses the /proc//stat directory to calculate the average CPU utilization over a period of 5 seconds:

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <string.h>

#define ITERATIONS 5

int main(int argc, char *argv[])
{
    if (argc != 2) {
        printf( "usage: %s <PID>\n", argv[0] );
        return(-1);
    }

    long double a[4], b[4];
    long double pidTime = 0.0;
    long int clk;
    int i;
    FILE *fp;
    char stat[1024];

    clk = sysconf(_SC_CLK_TCK);

    if (clk == -1) {
        printf("Could not determine clock ticks per second");
        return(-1);
    }

    char *pidPath = malloc(strlen("/proc/stat/")+strlen(argv[1])+1);
    if (pidPath == NULL) {
        printf("Could not allocate memory for str\n");
        return(-1);
    } else {
        strcpy(pidPath, "/proc/");
        strcat(pidPath, argv[1]);
        strcat(pidPath, "/stat");
    }

    for(i = 0; i < ITERATIONS; i++)
    {
        fp = fopen(pidPath,"r");
        if (fp == NULL) {
            perror(pidPath);
            return(-1);
        } else {
            fgets(stat, sizeof(stat), fp);
            sscanf(stat,"%*d %*s %*c %*d %*d %*d %*d %*d %*u %*lu %*lu %*lu %*lu %Lf %Lf %Lf %Lf %*ld %*ld %*ld %*ld %*llu",&a[0],&a[1],&a[2],&a[3]);
            fclose(fp);
            sleep(1);
        }

        fp = fopen(pidPath,"r");
        if (fp == NULL) {
            perror(pidPath);
            return(-1);
        } else {
            fgets(stat, sizeof(stat), fp);
           sscanf(stat,"%*d %*s %*c %*d %*d %*d %*d %*d %*u %*lu %*lu %*lu %*lu %Lf %Lf %Lf %Lf %*ld %*ld %*ld %*ld %*llu",&b[0],&b[1],&b[2],&b[3]);
            fclose(fp);
        }

        pidTime += (((b[0]+b[1]+b[2]+b[3]) - (a[0]+a[1]+a[2]+a[3])));
    }

    pidTime = (pidTime / (clk * ITERATIONS));
    printf("pidCPU=%Lf\n", pidTime);
    printf("%ld", clk);
    free(pidPath);
    return(0);
}

From what I understand the relevant fields in stat are:

  • int utime; /** user mode jiffies **/

  • int stime; /** kernel mode jiffies **/

  • int cutime; /** user mode jiffies with childs **/

  • int cstime; /** kernel mode jiffies with childs **/

For a single process, this works great, but when I have a process that forks, or is multithreaded, this breaks down. Do the cutime and cstime counters only work when the parent is waiting for the child processes? How can I calculate the total usage of the process tree rooted at PID?

1
Why not output your errors on stderr ? - Ascam
@Ascam I should, I think I was just being lazy at this point. You're right though. - anthozep
Anyway, just following logic, since you're tracking a PID, you'll only be given infos about this PID. Each child process being a... process, you refer to them by their own ID. My final point is: yes, it seems normal that you're getting only some wait activity watching parents. - Ascam

1 Answers

1
votes

Yes, the parent needs to wait for the CPU time of the children to be added in (see manual entry for getrusage link). Also see this answer for more details.