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

From Peter Zijlstra <peterz@infradead.org>
Newsgroups linux.kernel
Subject Re: RFC: documentation of the autogroup feature [v2]
Date 2016-11-25 17:20 +0100
Message-ID <sHrod-2Rp-3@gated-at.bofh.it> (permalink)
References (5 earlier) <sGJnb-7Id-17@gated-at.bofh.it> <sHa41-8qT-7@gated-at.bofh.it> <sHoJH-15u-7@gated-at.bofh.it> <sHqBQ-2hw-27@gated-at.bofh.it> <sHrex-2Lj-15@gated-at.bofh.it>
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On Fri, Nov 25, 2016 at 05:04:56PM +0100, Peter Zijlstra wrote:
> That is; the whole thing
> becomes, where l denotes the level in the hierarchy and i an
> entity on that level:
> 
>                  l      w_g,i
>   dt_l,i = dt \Prod  ----------
>                 g=0  \Sum w_g,j
> 
> 
> Or more concretely, for E:
> 
>                       w_E
>   dt_1,E = dt_0,C -----------
>                   (w_E + w_F)
> 
>                         w_C               w_E
>          = dt ----------------------- -----------
>               (w_A + w_B + w_C + w_D) (w_E + w_F)
> 

And this also immediately shows one of the 'problems' with it. Since we
don't have floating point in kernel, these fractions are evaluated with
fixed-point arithmetic. Traditionally (and on 32bit) we use 10bit fixed
point, recently we switched to 20bit for 64bit machines.

That change is what bit you on the nice testing.

But it also means that once we run out of fractional bits things go
wobbly. The fractions, as per the above, increase the deeper the group
hierarchy goes but are also affected by the number of CPUs in the system
(not immediately represented in that equation).

Not to mention that many scheduler operations become O(depth) in cost,
which also hurts. An obvious example being task selection, we pick a
runnable entity for each level, until the resulting entity has no
further children (iow. is a task).

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Thread

RFC: documentation of the autogroup feature "Michael Kerrisk (man-pages)" <mtk.manpages@gmail.com> - 2016-11-22 17:20 +0100
  [patch] sched/autogroup: Fix 64bit kernel nice adjustment Mike Galbraith <efault@gmx.de> - 2016-11-23 11:40 +0100
    Re: [patch] sched/autogroup: Fix 64bit kernel nice adjustment "Michael Kerrisk (man-pages)" <mtk.manpages@gmail.com> - 2016-11-23 14:50 +0100
      Re: [patch] sched/autogroup: Fix 64bit kernel nice adjustment Mike Galbraith <efault@gmx.de> - 2016-11-23 15:20 +0100
        Re: [patch] sched/autogroup: Fix 64bit kernel nice adjustment "Michael Kerrisk (man-pages)" <mtk.manpages@gmail.com> - 2016-11-23 15:30 +0100
          Re: [patch] sched/autogroup: Fix 64bit kernel nice adjustment Mike Galbraith <efault@gmx.de> - 2016-11-23 17:00 +0100
    [tip:sched/urgent] sched/autogroup: Fix 64-bit kernel nice level  adjustment tip-bot for Mike Galbraith <tipbot@zytor.com> - 2016-11-24 07:30 +0100
  Re: RFC: documentation of the autogroup feature Mike Galbraith <efault@gmx.de> - 2016-11-23 12:50 +0100
    Re: RFC: documentation of the autogroup feature "Michael Kerrisk (man-pages)" <mtk.manpages@gmail.com> - 2016-11-23 15:00 +0100
      Re: RFC: documentation of the autogroup feature Mike Galbraith <efault@gmx.de> - 2016-11-23 16:40 +0100
        Re: RFC: documentation of the autogroup feature "Michael Kerrisk (man-pages)" <mtk.manpages@gmail.com> - 2016-11-23 17:10 +0100
          Re: RFC: documentation of the autogroup feature Mike Galbraith <efault@gmx.de> - 2016-11-23 18:20 +0100
            Re: RFC: documentation of the autogroup feature "Michael Kerrisk (man-pages)" <mtk.manpages@gmail.com> - 2016-11-23 23:50 +0100
        Re: RFC: documentation of the autogroup feature "Michael Kerrisk (man-pages)" <mtk.manpages@gmail.com> - 2016-11-23 17:10 +0100
          Re: RFC: documentation of the autogroup feature Mike Galbraith <efault@gmx.de> - 2016-11-23 18:20 +0100
            RFC: documentation of the autogroup feature [v2] "Michael Kerrisk (man-pages)" <mtk.manpages@gmail.com> - 2016-11-24 22:50 +0100
              Re: RFC: documentation of the autogroup feature [v2] Afzal Mohammed <afzal.mohd.ma@gmail.com> - 2016-11-25 14:00 +0100
                Re: RFC: documentation of the autogroup feature [v2] "Michael Kerrisk (man-pages)" <mtk.manpages@gmail.com> - 2016-11-25 14:10 +0100
              Re: RFC: documentation of the autogroup feature [v2] Mike Galbraith <efault@gmx.de> - 2016-11-25 14:30 +0100
                Re: RFC: documentation of the autogroup feature [v2] "Michael Kerrisk (man-pages)" <mtk.manpages@gmail.com> - 2016-11-25 16:30 +0100
                Re: RFC: documentation of the autogroup feature [v2] "Michael Kerrisk (man-pages)" <mtk.manpages@gmail.com> - 2016-11-25 16:50 +0100
                Re: RFC: documentation of the autogroup feature [v2] Mike Galbraith <efault@gmx.de> - 2016-11-25 17:00 +0100
                Re: RFC: documentation of the autogroup feature [v2] "Michael Kerrisk (man-pages)" <mtk.manpages@gmail.com> - 2016-11-25 17:20 +0100
                Re: RFC: documentation of the autogroup feature [v2] Peter Zijlstra <peterz@infradead.org> - 2016-11-25 17:20 +0100
                Re: RFC: documentation of the autogroup feature [v2] "Michael Kerrisk (man-pages)" <mtk.manpages@gmail.com> - 2016-11-25 17:40 +0100
                Re: RFC: documentation of the autogroup feature [v2] "Michael Kerrisk (man-pages)" <mtk.manpages@gmail.com> - 2016-11-25 22:00 +0100
                Re: RFC: documentation of the autogroup feature [v2] Peter Zijlstra <peterz@infradead.org> - 2016-11-25 22:50 +0100
                Re: RFC: documentation of the autogroup feature [v2] "Michael Kerrisk (man-pages)" <mtk.manpages@gmail.com> - 2016-11-29 08:50 +0100
                Re: RFC: documentation of the autogroup feature [v2] Peter Zijlstra <peterz@infradead.org> - 2016-11-29 12:50 +0100
                Re: RFC: documentation of the autogroup feature [v2] "Michael Kerrisk (man-pages)" <mtk.manpages@gmail.com> - 2016-11-29 14:50 +0100
                Re: RFC: documentation of the autogroup feature [v2] Peter Zijlstra <peterz@infradead.org> - 2016-11-25 17:10 +0100
                Re: RFC: documentation of the autogroup feature [v2] Peter Zijlstra <peterz@infradead.org> - 2016-11-25 17:20 +0100
                Re: RFC: documentation of the autogroup feature [v2] "Michael Kerrisk (man-pages)" <mtk.manpages@gmail.com> - 2016-11-25 17:40 +0100
                Re: RFC: documentation of the autogroup feature [v2] Peter Zijlstra <peterz@infradead.org> - 2016-11-25 23:50 +0100
        Re: RFC: documentation of the autogroup feature "Michael Kerrisk (man-pages)" <mtk.manpages@gmail.com> - 2016-11-27 22:20 +0100
          Re: RFC: documentation of the autogroup feature Mike Galbraith <efault@gmx.de> - 2016-11-28 02:50 +0100
            Re: RFC: documentation of the autogroup feature "Michael Kerrisk (man-pages)" <mtk.manpages@gmail.com> - 2016-11-29 10:20 +0100
              Re: RFC: documentation of the autogroup feature Mike Galbraith <efault@gmx.de> - 2016-11-29 14:50 +0100

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