linux-kernel - Re: [RFC] another scheduler beater

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Message-ID: <20070424081726.GA2714@elte.hu>
Date:	Tue, 24 Apr 2007 10:17:26 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	davidsen@...idon.tmr.com
Cc:	Linux Kernel M/L <linux-kernel@...r.kernel.org>
Subject: Re: [RFC] another scheduler beater

* Bill Davidsen <davidsen@....com> wrote:

> The small attached script does a nice job of showing animation 
> glitches in the glxgears animation. I have run one set of tests, and 
> will have several more tomorrow. I'm off to a poker game, and would 
> like to let people draw their own conclusions.
> 
> Based on just this script as load I would say renice on X isn't a good 
> thing. Based on one small test, I would say that renice of X in 
> conjunction with heavy disk i/o and a single fast scrolling xterm 
> (think kernel compile) seems to slow the raid6 thread measurably. 
> Results late tomorrow, it will be an early and long day :-(

hm, i'm wondering what you would expect the scheduler to do here?

for this particular test you'll get the best result by renicing X to 
+19! Why? Because, as far as i can see this is a partially 'inverted' 
test of X's scheduling.

While the script is definitely useful (you taught me that nice xterm 
-geom trick to automate the placing of busy xterms :), some caveats do 
apply when interpreting the results:

If you have a kernel 3D driver (which you seem to have, judging by the 
glxgears numbers you are getting) then running 'glxgears' wont involve X 
at all. glxgears just gets its own window and then the kernel driver 
draws straight into it, without any side-trips to X. You can see this 
for yourself by starting glitch1.sh from an ssh terminal, and then 
_totally stop_ the X server via "kill -STOP 12345" - all the xterms will 
stop, the X desktop freezes, but the glxgears instance will still 
happily draw its stuff and wheels are happily turning on the screen.

So in this sense glxgears is a 'CPU hog' workload, largely independent 
of X.

now, by renicing X to -10 and running the xterms you'll definitely hurt 
"CPU hogs" - even if it happens to be a glxgears process that draws 3D 
graphics in a window provided by X. But this is precisely what is 
supposed to happen in this case. You should get the best glxgears 
performance by renicing X to _+19_, and that seems to be happening 
according to your numbers - and that's what happens in my own testing 
too.

	Ingo
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