Re: RFC: documentation of the autogroup feature [v2]
From: Michael Kerrisk (man-pages)
Date: Fri Nov 25 2016 - 10:49:44 EST
On 11/25/2016 04:04 PM, Michael Kerrisk (man-pages) wrote:
> Hi Mike,
>
> On 11/25/2016 02:02 PM, Mike Galbraith wrote:
>>> ┌─────────────────────────────────────────────────────┐
>>> │FIXME │
>>> ├─────────────────────────────────────────────────────┤
>>> │How do the nice value of a process and the nice │
>>> │value of an autogroup interact? Which has priority? │
>>> │ │
>>> │It *appears* that the autogroup nice value is used │
>>> │for CPU distribution between task groups, and that │
>>> │the process nice value has no effect there. (I.e., │
>>> │suppose two autogroups each contain a CPU-bound │
>>> │process, with one process having nice==0 and the │
>>> │other having nice==19. It appears that they each │
>>> │get 50% of the CPU.) It appears that the process │
>>> │nice value has effect only with respect to schedul‐ │
>>> │ing relative to other processes in the *same* auto‐ │
>>> │group. Is this correct? │
>>> └─────────────────────────────────────────────────────┘
>>
>> Yup, entity nice level affects distribution among peer entities.
>
> Huh! I only just learned about this via my experiments while
> investigating autogroups.
>
> How long have things been like this? Always? (I don't think
> so.) Since the arrival of CFS? Since the arrival of
> autogrouping? (I'm guessing not.) Since some other point?
> (When?)
Okay, things changed sometime after 2.6.31, at least.
(Just tested on an old box.) So, presumably with the arrival
of either CFS or autogrouping? Next comment certainly applies:
> It seems to me that this renders the traditional process
> nice pretty much useless. (I bet I'm not the only one who'd
> be surprised by the current behavior.)
Cheers,
Michael
--
Michael Kerrisk
Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
Linux/UNIX System Programming Training: http://man7.org/training/