linux-kernel - Re: (ondemand) CPU governor regression between 2.6.23 and 2.6.24

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Message-Id: <200801262238.19222.toralf.foerster@gmx.de>
Date:	Sat, 26 Jan 2008 22:38:15 +0100
From:	Toralf Förster <toralf.foerster@....de>
To:	Tomasz Chmielewski <mangoo@...g.org>
Cc:	linux-kernel@...r.kernel.org
Subject: Re: (ondemand) CPU governor  regression between 2.6.23 and 2.6.24

It seems to be rather a scheduler issue than a governor issue b/c
the issue went away after unsetting CONFIG_FAIR_GROUP_SCHED.

If I unselect CONFIG_FAIR_GROUP_SCHED then the %CPU value raises 80%
- which forces the ondemand governor do speed up the CPU frequency:


  PID USER      PR  NI  VIRT  RES  SHR S %CPU %MEM    TIME+  COMMAND
 7137 tfoerste  20   0  1796  488  428 R 95.5  0.0   0:01.40 factor
 7083 dnetc     39  19   664  348  264 R  2.1  0.0   3:08.33 dnetc
 4033 root      20   0 97252 9420 4008 R  0.7  0.9   0:09.43 X
 

Am Samstag, 26. Januar 2008 schrieben Sie:
> Toralf Förster wrote:
> 
> > I use a 1-liner for a simple performance check : "time factor 819734028463158891"
> > Here is the result for the new (Gentoo) kernel 2.6.24:
> > 
> > With the  ondemand governor of the  I get:
> > 
> > tfoerste@n22 ~/tmp $ time factor 819734028463158891
> > 819734028463158891: 3 273244676154386297
> > 
> > real    0m32.997s
> > user    0m15.732s
> > sys     0m0.014s
> > 
> > With the ondemand governor the CPU runs at 600 MHz,
> > whereas with the performance governor I get :
> > 
> > tfoerste@n22 ~/tmp $ time factor 819734028463158891
> > 819734028463158891: 3 273244676154386297
> > 
> > real    0m10.893s
> > user    0m5.444s
> > sys     0m0.000s
> > 
> > (~5.5 sec as I expected) b/c the CPU is set to 1.7 GHz.
> > 
> > The ondeman governor of previous kernel versions however automatically increased
> > the CPU speed from 600 MHz to 1.7 GHz.
> > 
> > My system is a ThinkPad T41, I'll attach the .config 
> 
> During the test, run top, and watch your CPU usage. Does it go above 80% 
> (the default for 
> /sys/devices/system/cpu/cpu0/cpufreq/ondemand/up_threshold).
> 
> ondemand CPUfreq governor has a few tunables, described in 
> Documentation/cpu-freq. One of them is up_threshold:
> 
> up_threshold: defines what the average CPU usaged between the samplings
> of 'sampling_rate' needs to be for the kernel to make a decision on
> whether it should increase the frequency. For example when it is set
> to its default value of '80' it means that between the checking
> intervals the CPU needs to be on average more than 80% in use to then
> decide that the CPU frequency needs to be increased.
> 
> What CPUFreq processor driver are you using?
> 
> 
> I had a similar problem with CPUfreq and dm-crypt (slow reads), see 
> (more setup problem than something kernel-related):
> 
> http://blog.wpkg.org/2008/01/22/cpufreq-and-dm-crypt-performance-problems/
> 
> 

-- 
MfG/Sincerely

Toralf Förster
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