The commends nice/renice have no effect between processes started as foreground process and processes started as background processes. Even with maximum niceness on the foreground processes they get always more attention. While this makes a lot of sense for processes where the user is waitung for the result, it becomes a problem if a long running process is started that way. I thought that Linux should de-prioritize (make slower) long running tasks, but according to the below example, this is not the case: user_foo started all processes as foreground processes, they run now for several weeks. The recently started background tasks of user_bar (they would potentially also use 100% of a CPU if they could) are scheduled with lower priority, despite having niceness -20.
top - 17:12:27 up 31 days, 8:29, 4 users, load average: 33.04, 32.60, 29.48
Tasks: 278 total, 26 running, 252 sleeping, 0 stopped, 0 zombie
Cpu(s): 91.6%us, 0.8%sy, 0.0%ni, 0.0%id, 0.0%wa, 0.0%hi, 0.0%si, 7.5%st
Mem: 16434608k total, 15893028k used, 541580k free, 1168920k buffers
Swap: 24673276k total, 18100k used, 24655176k free, 2255512k cached
PID USER PR NI VIRT RES SHR S %CPU %MEM TIME+ COMMAND
2399 user_foo 39 19 4117m 246m 60m S 102 1.5 34915:56 heavytask
2234 user_foo 39 19 4052m 201m 59m S 101 1.3 39631:48 heavytask
3474 user_foo 39 19 4674m 1.1g 58m S 100 7.2 36057:58 heavytask
3782 user_foo 39 19 4447m 1.3g 58m S 100 8.5 36108:42 heavytask
3128 user_foo 39 19 3112m 182m 60m S 100 1.1 29983:33 heavytask
6644 user_foo 39 19 3637m 705m 59m S 100 4.4 26310:45 heavytask
2571 user_foo 39 19 5220m 1.8g 60m S 99 11.7 14908:44 heavytask
14607 user_bar 0 -20 52488 12m 5368 R 84 0.1 10:42.24 otherTASK
14755 user_bar 0 -20 52488 12m 5368 R 82 0.1 10:35.85 otherTASK
14758 user_bar 0 -20 52488 12m 5368 R 78 0.1 10:54.96 otherTASK
14795 user_bar 0 -20 52488 12m 5368 R 76 0.1 10:01.66 otherTASK
14783 user_bar 0 -20 52488 12m 5368 R 75 0.1 9:32.76 otherTASK
14596 user_bar 0 -20 52488 12m 5368 R 30 0.1 10:40.58 otherTASK
14728 user_bar 0 -20 52488 12m 5368 R 30 0.1 9:39.03 otherTASK
14583 user_bar 0 -20 52488 12m 5368 R 27 0.1 10:07.11 otherTASK
14706 user_bar 0 -20 52488 12m 5368 R 26 0.1 9:38.22 otherTASK
14501 user_bar 0 -20 52488 12m 5368 R 25 0.1 9:40.70 otherTASK
What can be done to degrade the priority of these particular foreground processes? (or to elevate the priority of particular background processes)?
(System is Ubuntu 12.04 LTS with Kernel 3.2.0-32)