Re: [uml-devel] host cpu frequency scaling and guest bogomips

[Blaisorblade <blaisorblade@yahoo.it>]

On Tuesday 12 July 2005 18:35, Geert Uytterhoeven wrote:
> On Tue, 12 Jul 2005, Blaisorblade wrote:
> > On Tuesday 12 July 2005 07:26, Victor Condino wrote:
> > > just noticed something interesting while testing out my gentoo/uml
> > > root-fs generation script on my laptop. my l/t is an thinkpad T42p,
> > > which has a p3 centrino with speedstep. i use cpufreqd to throttle the
> > > cpu when not on ac between 600 and 1800MHz based on load. what i
> > > noticed is that when I fired up UML while the host's core frequency was
> > > at 600mhz, /proc/cpuinfo reported ~600 bogomips in the guest, but when
> > > i put some load on the host and cpufreq stepped up the core frequency,
> > > UML still reports ~600 bogomips. likewise, when I fired up UML while
> > > the host was under load and it's core at 1200MHz, UML reports higher
> > > bogomips, but then when the host drops back down to the lower core
> > > freq, UML still reports the same higher bogomips.
> > >
> > > Not that this actually affects anything I do or would affect most
> > > people... just thought you guys might want to know =D

> > Hmm, bogomips are calculated at startup and that won't change easily.
> > However, they become inaccurate in a much wider variety of situation, for
> > instance when there's some load on the host; so, using
> > CONFIG_UML_REAL_TIME_CLOCK (IIRC) uses the host clock for time
> > calculation.

> > When you disable this option, I guess that a "sleep 10" when host is at
> > 600, if UML was started at 1800, might last 30 seconds more or less...

> I don't think user space sleep uses the kernel's udelay() or mdelay()...

Sorry, right... they're the only one to use bogomips (just learned), still 
UML_REAL_TIME_CLOCK makes "sleep(1)" in user-space precise.

That option drops the assumption that SIGALRM (or VTALRM) is exact, and 
rather, at ALRM delivery, uses gettimeofday() to discover how long passed 
from last interrupt.
-- 
Inform me of my mistakes, so I can keep imitating Homer Simpson's "Doh!".
Paolo Giarrusso, aka Blaisorblade (Skype ID "PaoloGiarrusso", ICQ 215621894)
http://www.user-mode-linux.org/~blaisorblade

	

	
		
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