Re: RFC: documentation of the autogroup feature [v2]

From: Michael Kerrisk (man-pages)
Date: Fri Nov 25 2016 - 11:10:13 EST


On 11/25/2016 04:51 PM, Mike Galbraith wrote:
> On Fri, 2016-11-25 at 16:04 +0100, Michael Kerrisk (man-pages) 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?)
>
> Always. Before CFS there just were no non-peers :)

Well that's one way of looking at it. So, the change
that I'm talking about came in 2.6.32 with CFS then?

>> 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.)
>
> Yup, group scheduling is not a single edged sword, those don't exist.
> Box wide nice loss is not the only thing that can bite you, fairness,
> whether group or task oriented cuts both ways.

Understood. But again I'll say, I bet a lot of old-time users
(and maybe many newer) would be surprised by the fact that
nice(1) / setpriority(2) have effectively been rendered no-ops
in many use cases. At the very least, it'd have been nice
if someone had sent a man pages patch or at least a note...

Cheers,

Michael



--
Michael Kerrisk
Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
Linux/UNIX System Programming Training: http://man7.org/training/