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

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Date:	Wed, 4 Nov 2009 13:28:14 -0800 (PST)
From:	Dan Magenheimer <dan.magenheimer@...cle.com>
To:	john stultz <johnstul@...ibm.com>, Avi Kivity <avi@...hat.com>
Cc:	Jeremy Fitzhardinge <jeremy@...p.org>,
	Glauber Costa <glommer@...hat.com>,
	Jeremy Fitzhardinge <jeremy.fitzhardinge@...rix.com>,
	kurt.hackel@...cle.com, the arch/x86 maintainers <x86@...nel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Glauber de Oliveira Costa <gcosta@...hat.com>,
	Xen-devel <xen-devel@...ts.xensource.com>,
	Keir Fraser <keir.fraser@...citrix.com>, zach.brown@...cle.com,
	chris.mason@...cle.com, Ingo Molnar <mingo@...hat.com>
Subject: RE: [Xen-devel] Re: [PATCH 3/5] x86/pvclock: add vsyscall
  implementation

> From: john stultz [mailto:johnstul@...ibm.com]
> On Thu, Oct 29, 2009 at 7:07 AM, Avi Kivity <avi@...hat.com> 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,
> CLOCK_MONOTONIC_COARSE (as well as CLOCK_REALTIME_COARSE), which
> 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
changed.

Dan
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