Determine the average time a system spends iobound from boot (useful for seeing how stacked & oversold low-cost VMs are)

iowait.c

/* See how much time a system spends in iowait since boot (as a percentage)
 * Useful to alert on iobound processes
 * Written by Tim Post, March 2008                               
 * Hereby donated to the public domain, or use under the terms of the MIT license.                          
 */                                                              

#include <stdio.h>
#include <time.h> 
#include <string.h>
#ifndef USER_HZ
#define USER_HZ 100
#endif

#define BUF_S  64
#define BUF_M  256
#define BUF_L  512
#define BUF_XL 1024

/* Long double is used for precision */
static long double get_average_iowait(void)
{                                          
    FILE *fp;                          
    char buff[BUF_XL];                 
    char item[BUF_S];                  
    time_t c_time, u_time, time_val;
    long user = 0, nice = 0, system = 0, idle = 0, iowait = 0, btime = 0;

    fp = fopen("/proc/stat", "r");
    if (! fp) {
            fprintf(stderr, "Unable to open /proc/stat\n");
            return -1;
    }
    fgets(buff, BUF_XL, fp);
    sscanf(buff, "%s %ld %ld %ld %ld %ld",
            item, &user, &nice, &system, &idle, &iowait);
    for (;;) {
        memset(buff, 0, sizeof(buff));
        fgets(buff, BUF_M, fp);
        if (feof(fp))
            break;
        sscanf(buff, "%s %ld", item, &time_val);
        if (! strcmp(item, "btime")) {
            btime = time_val;
            break;
        }
    }

    fclose(fp);

    if (btime == 0)
        return -1;
    c_time = time(NULL);
    u_time = c_time - btime;

    return (100 * ((long double)(iowait / USER_HZ) / u_time));
}

int main(void)
{
    long double io = 0;

    io = get_average_iowait();
    if (io < 0) {
        fprintf(stderr, "Unable to determine boot time from /proc/stat\n");
        return 1;
    }
    printf("This system has spent %-2.5Lf %% of its time in IOWAIT since boot.\n",
            io);
    return 0;
}