It seems to me that the whole nice/priority scheme doesn't actually do anything.
PID USER PR NI VIRT RES SHR S %CPU %MEM TIME+ COMMAND
10084 dick 39 19 1221848 225120 6128 S 407.6 1.4 23478:00 povrayws
5311 dick 20 0 2904912 325728 40324 S 382.1 2.0 133:06.54 qtpovray
Given these two jobs, I would expect povrayws to sit at 0% cpu while qtpovray is busy. And this is what happens when I run this setup on Windows.
povrayws is a cpu/fpu intensive daemon job that will run for months (building an animation). If there is anything else the computer wants to do, I want this job to basically stop.
qtpovray is a sometimes cpu/fpu intensive gui job for developing the animation. If he wants to run, he should preempt the daemon and get close to 800% cpu. (If the daemon is not running, he will take 800% cpu.)
I reniced povrayws in top and ran
chrt -i --pid 0 10084 to set him to idle priority. And still he is getting more cpu than the interactive job. There is no I/O going on to muck up the scheduling. It is just a couple megs of memory and as much cpu as I can throw at it.
And again, when I run this setup on Windows 8, povrayws.exe will sit near 0% when something else is going on.
I wondered if it was my i7's hyperthreading that was confusing the scheduler, but I see the same behavior on a six core i5. (On the i5, Minecraft is fighting with the idle daemon for the cpu.)
IIRC, 20 years ago, this worked as expected, but the last 5 years I'm noticing this 'broken' behavior. It's finally annoying me enough to ask,
has anyone seen idle tasks actually yield to higher priority? It's a pretty edge case to have an 800% cpu, 0% I/O idle task.