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I am learning about the CFS in Linux. So i decided to see how the nice value can give a lower priority for background (i.e. nice) processes.

I ran two CPU processes: taskset 01 python3 loop.py in two terminals. Then i renice -n 19 -p 12954 for one of the processes.

  PID USER      PR  NI    VIRT    RES    SHR S  %CPU %MEM     TIME+ COMMAND     
12963 garyF     20   0   29620   9248   5256 R  49.8  0.1   2:05.64 python3     
12954 garyF     39  19   29620   9420   5428 R  50.2  0.1   2:12.68 python3     

But even though the nice values are different, top says that the processes are getting equal share (50%) of the CPU. Even running these processes for about five minutes, the TIME+ column shows similar cpu time for each. I would expect that the time slice of the nice process is much lower than the normal process, and thus get a lower CPU share. Am i missing something?

This is my loop.py:

#!/bin/python3

from datetime import datetime;
from datetime import timedelta;
import signal;
import sys;

def sigint_handler(signal, frame):
    dt = datetime.now() - start_time;
    ms = (dt.days * 24 * 60 * 60 + dt.seconds) * 1000 + dt.microseconds/1000.0;
    print('You pressed Ctrl+C!');
    print('Time elapsed is ', end='')
    print(ms, end='');
    print('ms.');
    sys.exit(0);
pass;

signal.signal(signal.SIGINT, sigint_handler);
start_time = datetime.now();
val = 3;
while True:
    val %= 2;
    val += 2;
pass;
  • Got the answer. Nice has no effect if used in different terminals. Or rather, different groups. Linux groups processes to prevent CPU hogging and allow for more responsiveness. But this effectively making per-process nice values useless, unless applied on the group; or disable autogroup in kernel. superuser.com/questions/805599/… (Can't even flag this as duplicate, LoL.) – garyF Apr 4 '18 at 19:15

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