Change niceness of all processes by niceness

Question

I am using Debian, is there a way to change the niceness of all running process based on their current niceness? For instance change all currently running processes that have a niceness of -20 or -19 to -10. Renice can change a process, and processes for certain users. But as far as I can tell it can't do it based on current niceness.

I am trying to run a program with -20 niceness to try and get around some timing spikes that seem to occur semi-regularly. These might be caused by certain processes with the same priority taking resources. I was hoping to check this with some niceness fiddling.

Answer 1

Something to start with in C:

#include <stdio.h>
#include <sys/types.h>
#include <dirent.h>
#include <stdlib.h>
#include <string.h>

static char *prstatname(char *buf, char **endptr)
{
    /* parse process name */
    char *ptr = buf;
    while (*ptr && *ptr != '(') ++ptr;
    ++ptr;
    if (!ptr) return 0;

    char *name = ptr;
    while (*ptr)
    {
        if (*ptr == ')' && *(ptr+1) && *(ptr+2) && *(ptr+3)
                && *(ptr+1) == ' ' && *(ptr+3) == ' ')
        {
            *ptr = 0;
            *endptr = ptr + 1;
            return name;
        }
        ++ptr;
    }
    return 0;
}

int main(void)
{
    DIR *proc = opendir("/proc");
    if (!proc) return 1;

    struct dirent *ent;

    while ((ent = readdir(proc)))
    {
        /* check whether filename is all numeric, then it's a process id */
        char *endptr;
        int pid = strtol(ent->d_name, &endptr, 10);
        if (*endptr) continue;

        /* combine to '/proc/{pid}/stat' to get information about process */
        char statname[64] = {0,};       
        strcat(statname, "/proc/");
        strncat(statname, ent->d_name, 52);
        strcat(statname, "/stat");

        FILE *pstat = fopen(statname, "r");
        if (!pstat) continue;

        /* try to read process info */
        char buf[1024];
        if (!fgets(buf, 1024, pstat))
        {
            fclose(pstat);
            continue;
        }
        fclose(pstat);

        char *name = prstatname(buf, &endptr);
        if (!name) continue;

        /* nice value is in the 17th field after process name */
        int i;
        char *tok = strtok(endptr, " ");
        for (i = 0; tok && i < 16; ++i) tok = strtok(0, " ");
        if (!tok || i < 16) continue;

        int nice = strtol(tok, &endptr, 10);
        if (*endptr) continue;

        printf("[%d] %s -- nice: %d\n", pid, name, nice);
    }
}

If you understand this program, you can easily modify it to do what you wanted.