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RE: [Xen-devel] Re: [PATCH 3/5] x86/pvclock: add vsyscall implementation

To: john stultz <johnstul@xxxxxxxxxx>, Avi Kivity <avi@xxxxxxxxxx>
Subject: RE: [Xen-devel] Re: [PATCH 3/5] x86/pvclock: add vsyscall implementation
From: Dan Magenheimer <dan.magenheimer@xxxxxxxxxx>
Date: Wed, 4 Nov 2009 13:28:14 -0800 (PST)
Cc: Jeremy Fitzhardinge <jeremy@xxxxxxxx>, Jeremy Fitzhardinge <jeremy.fitzhardinge@xxxxxxxxxx>, kurt.hackel@xxxxxxxxxx, Glauber Costa <glommer@xxxxxxxxxx>, the arch/x86 maintainers <x86@xxxxxxxxxx>, Linux Kernel Mailing List <linux-kernel@xxxxxxxxxxxxxxx>, Glauber de Oliveira Costa <gcosta@xxxxxxxxxx>, Xen-devel <xen-devel@xxxxxxxxxxxxxxxxxxx>, Keir Fraser <keir.fraser@xxxxxxxxxxxxx>, zach.brown@xxxxxxxxxx, Ingo Molnar <mingo@xxxxxxxxxx>, chris.mason@xxxxxxxxxx
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> From: john stultz [mailto:johnstul@xxxxxxxxxx]
> On Thu, Oct 29, 2009 at 7:07 AM, Avi Kivity <avi@xxxxxxxxxx> wrote:
> >
> > Out of interest, do you know (and can you relate) why those 
> apps need
> > 100k/sec monotonically increasing timestamps?
> This is sort of tangential, but depending on the need, this might be
> of interest:  Recently I've added a new clock_id,
> return a HZ granular timestamp (same granularity as filesystem
> timestamps). Its very fast to access, since there's no hardware to
> touch, and is accessible via vsyscall.
> The idea being, if your hitting clock_gettime 100k/sec but you really
> don't have the need for nsec granular timestamps, it might provide a
> really nice performance boost.
> Here's the commit:

Hi John --

Yes, possibly of interest.  But does it work with CONFIG_NO_HZ?
(I'm expecting that over time NO_HZ will become widespread
for VM OS's, though interested in if you agree.)

Also very interested in your thoughts about a variation
that returns something similar to a TSC_AUX to notify
caller that the underlying reference clock has/may have


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