linux-kernel - Re: [PATCH] CFS: Fix missing digit off in wmult table

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Message-ID: <Pine.LNX.4.64.0707171633510.1817@scrub.home>
Date:	Tue, 17 Jul 2007 17:12:39 +0200 (CEST)
From:	Roman Zippel <zippel@...ux-m68k.org>
To:	Ingo Molnar <mingo@...e.hu>
cc:	Matt Mackall <mpm@...enic.com>, James Bruce <bruce@...rew.cmu.edu>,
	Thomas Gleixner <tglx@...utronix.de>,
	Mike Galbraith <efault@....de>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andi Kleen <andi@...stfloor.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org,
	Arjan van de Ven <arjan@...radead.org>,
	Chris Wright <chrisw@...s-sol.org>
Subject: Re: [PATCH] CFS: Fix missing digit off in wmult table

Hi,

On Tue, 17 Jul 2007, Ingo Molnar wrote:

> * Roman Zippel <zippel@...ux-m68k.org> wrote:
> 
> > > > It's nice that these artifacts are gone, but that still doesn't 
> > > > explain why this ratio had to be increase that much from around 
> > > > 1:10 to 1:69.
> > > 
> > > More dynamic range is better? If you actually want a task to get 20x 
> > > the CPU time of another, the older scheduler doesn't really allow 
> > > it.
> > 
> > You can already have that, the complete range level from 19 to -20 was 
> > about 1:80.
> 
> But that is irrelevant: all tasks start out at nice 0, and what matters 
> is the dynamic range around 0.
> 
> So the dynamic range has been made uniform in the positive from 
> 1:10...1:20...1:30 to 1:69 for nice +19, and from 1:8 to 1:69 in the 
> minus. (with 1:86 nice -20) If you look at the negative nice levels 
> alone it's a substantial increase but if you compare it with positive 
> nice levels you'll similar kinds of dynamic ranges were already present 
> in the old scheduler and you'll see why we've done it.

So let's look at them:

for (i=0;i<20;i++) print i, " : ", (20-i)*5, " : ", 100*1.25^-i, " : ", e(l(2)*(-i/5))*100, "\n";
0 : 100 : 100 : 100.00000000000000000000
1 : 95 : 80.00000000000000000000 : 87.05505632961241391300
2 : 90 : 64.00000000000000000000 : 75.78582832551990411700
3 : 85 : 51.20000000000000000000 : 65.97539553864471296900
4 : 80 : 40.96000000000000000000 : 57.43491774985175034000
5 : 75 : 32.76800000000000000000 : 50.00000000000000000000
6 : 70 : 26.21440000000000000000 : 43.52752816480620695700
7 : 65 : 20.97152000000000000000 : 37.89291416275995205900
8 : 60 : 16.77721600000000000000 : 32.98769776932235648400
9 : 55 : 13.42177280000000000000 : 28.71745887492587517000
10 : 50 : 10.73741824000000000000 : 25.00000000000000000000
11 : 45 : 8.58993459200000000000 : 21.76376408240310347800
12 : 40 : 6.87194767360000000000 : 18.94645708137997602900
13 : 35 : 5.49755813888000000000 : 16.49384888466117824200
14 : 30 : 4.39804651110400000000 : 14.35872943746293758500
15 : 25 : 3.51843720888320000000 : 12.50000000000000000000
16 : 20 : 2.81474976710656000000 : 10.88188204120155173900
17 : 15 : 2.25179981368524800000 : 9.47322854068998801400
18 : 10 : 1.80143985094819840000 : 8.24692444233058912100
19 : 5 : 1.44115188075855872000 : 7.17936471873146879200

(nice level : old % : new % : my suggested %)

Your levels divert very quickly from what they used to be (upto a factor 
of 7), it's also not really easy to remember what the individual levels 
mean.
I at least try to keep them somewhat in the range they used to be (and 
the difference is limited to a factor of about 2), also every 5 levels the 
amount of cpu time is halved, which is very easy to remember.

If you need more dynamic range, is there a law that prevents us from going 
beyond 19? For example:

for (i=20;i<=30;i++) print i, " : ", (20-i)*5, " : ", 100*1.25^-i, " : ", e(l(2)*(-i/5))*100, "\n";
20 : 0 : 1.15292150460684697600 : 6.25000000000000000000
21 : -5 : .92233720368547758000 : 5.44094102060077586900
22 : -10 : .73786976294838206400 : 4.73661427034499400700
23 : -15 : .59029581035870565100 : 4.12346222116529456000
24 : -20 : .47223664828696452100 : 3.58968235936573439600
25 : -25 : .37778931862957161700 : 3.12500000000000000000
26 : -30 : .30223145490365729300 : 2.72047051030038793400
27 : -35 : .24178516392292583400 : 2.36830713517249700300
28 : -40 : .19342813113834066700 : 2.06173111058264728000
29 : -45 : .15474250491067253400 : 1.79484117968286719800
30 : -50 : .12379400392853802700 : 1.56250000000000000000

setpriority() accepts such values without error.

bye, Roman
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