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

To: Keir Fraser <keir.fraser@xxxxxxxxxxxxx>
Subject: Re: [Xen-devel] How to increase timer resolution?
From: "Robert Kaiser (FH)" <kaiser@xxxxxxxxxxxxxxxxxxxxxxxxxx>
Date: Sun, 14 Sep 2008 17:23:06 +0200
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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.


(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?

 -- 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

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?



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