Re: [Xen-devel] How to increase timer resolution?

["Robert Kaiser (FH)" <kaiser@xxxxxxxxxxxxxxxxxxxxxxxxxx>]

Hi Keir,

thanks for your quick response

Keir Fraser schrieb:
> set_timer() and friends are already high-resolution, so long as your system
> has a local APIC. This is undoubtedly the case if running a multi-processor
> system. On a uni-processor system you might need to forcibly enable use of
> the APIC with 'lapic' as a Xen boot parameter.
>
> Either way, early during boot you should find a message saying local APIC is
> found and enabled, or not found, or whatever.
>   
Quote:

(XEN) Local APIC disabled by BIOS -- reenabling.
(XEN) Could not enable APIC!

Seems like I have to switch to a different testing machine then, the old 
notebook I was using doesn't seem to have an APIC :-(
Do you know what resolution I can expect to get on an APIC-enabled machine?

Rob
>  -- Keir
>
> On 14/9/08 07:06, "Robert Kaiser" <kaiser@xxxxxxxxxxxxxxxxxxxxxxxxxx> wrote:
>
>   
> > Hi everyone,
> >
> > I'm using Xen 3.2.1 in my experimental setup, trying to find relationships
> > between switch frequencies and overheads. For that, it would be good if I
> > could increase the resolution of the  timer (i.e. the one that you access
> > with set_timer(), stop_timer() and friends). Currently, this seems to be
> > fixed at 10 milliseconds, but I would like to change it to 1ms or maybe even
> > down to a few hundred microseconds. I'm aware that this will affect
> > performance --  that is part of what I'd like to measure.
> >
> > Can you experts give me some hints how to do this, i.e. is it just a matter of
> > giving the kernel the right commandline options (which?), can it be done by
> > changing a few compile time constants   (which? where?), or is there more
> > involved?
> >
> > Ideally, I would implement a new control function by which dom0 could change
> > the frequency dynamically at run-time, but even having to re-compile Xen
> > between experiments might be an acceptable approach. However, the code base
> > is so dauntingly huge for a newbie like myself, I'm having difficulties in
> > finding the right set-screws.  Can anyone help me out?
> >
> > Cheers
> >
> > Rob
> >     
>
>   

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