linux-kernel - Re: RFC: documentation of the autogroup feature [v2]

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Message-ID: <cf0a6f4b-ba96-9ebe-ad29-ba8ccef5741c@gmail.com>
Date:   Fri, 25 Nov 2016 16:48:50 +0100
From:   "Michael Kerrisk (man-pages)" <mtk.manpages@...il.com>
To:     Mike Galbraith <efault@....de>
Cc:     mtk.manpages@...il.com, Peter Zijlstra <a.p.zijlstra@...llo.nl>,
        Ingo Molnar <mingo@...nel.org>,
        linux-man <linux-man@...r.kernel.org>,
        lkml <linux-kernel@...r.kernel.org>,
        Thomas Gleixner <tglx@...utronix.de>
Subject: Re: RFC: documentation of the autogroup feature [v2]

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/