linux-kernel - Re: CFS review

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Date:	Tue, 28 Aug 2007 23:37:34 -0400
From:	Bill Davidsen <davidsen@....com>
To:	Al Boldi <a1426z@...ab.com>
CC:	Ingo Molnar <mingo@...e.hu>, Peter Zijlstra <peterz@...radead.org>,
	Mike Galbraith <efault@....de>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	linux-kernel@...r.kernel.org
Subject: Re: CFS review

Al Boldi wrote:
> Ingo Molnar wrote:
>> * Al Boldi <a1426z@...ab.com> wrote:
>>>>> The problem is that consecutive runs don't give consistent results
>>>>> and sometimes stalls.  You may want to try that.
>>>> well, there's a natural saturation point after a few hundred tasks
>>>> (depending on your CPU's speed), at which point there's no idle time
>>>> left. From that point on things get slower progressively (and the
>>>> ability of the shell to start new ping tasks is impacted as well),
>>>> but that's expected on an overloaded system, isnt it?
>>> Of course, things should get slower with higher load, but it should be
>>> consistent without stalls.
>>>
>>> To see this problem, make sure you boot into /bin/sh with the normal
>>> VGA console (ie. not fb-console).  Then try each loop a few times to
>>> show different behaviour; loops like:
>>>
>>> # for ((i=0; i<3333; i++)); do ping 10.1 -A > /dev/null & done
>>>
>>> # for ((i=0; i<3333; i++)); do nice -99 ping 10.1 -A > /dev/null & done
>>>
>>> # { for ((i=0; i<3333; i++)); do
>>>         ping 10.1 -A > /dev/null &
>>>     done } > /dev/null 2>&1
>>>
>>> Especially the last one sometimes causes a complete console lock-up,
>>> while the other two sometimes stall then surge periodically.
>> ok. I think i might finally have found the bug causing this. Could you
>> try the fix below, does your webserver thread-startup test work any
>> better?
> 
> It seems to help somewhat, but the problem is still visible.  Even v20.3 on 
> 2.6.22.5 didn't help.
> 
> It does look related to ia-boosting, so I turned off __update_curr like Roman 
> mentioned, which had an enormous smoothing effect, but then nice levels 
> completely break down and lockup the system.
> 
> There is another way to show the problem visually under X (vesa-driver), by 
> starting 3 gears simultaneously, which after laying them out side-by-side 
> need some settling time before smoothing out.  Without __update_curr it's 
> absolutely smooth from the start.

I posted a LOT of stuff using the glitch1 script, and finally found a 
set of tuning values which make the test script run smooth. See back 
posts, I don't have them here.

-- 
Bill Davidsen <davidsen@....com>
   "We have more to fear from the bungling of the incompetent than from
the machinations of the wicked."  - from Slashdot
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