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

To: Avi Kivity <avi@xxxxxxxxxx>
Subject: RE: [Xen-devel] Re: [PATCH 3/5] x86/pvclock: add vsyscall implementation
From: Dan Magenheimer <dan.magenheimer@xxxxxxxxxx>
Date: Mon, 2 Nov 2009 07:46:06 -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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> > I don't have any public data available for this DB usage, 
> Sorry, that doesn't explain anything.

Well for now just consider the DB usage as another use
of profiling.  But one can easily draw scenarios where
a monotonic timestamp is also used to guarantee transaction

> > Search for "flight recorder".  This feature is intended to
> > be enabled all the time, but with non-vsyscall gettimeofday
> > the performance impact is unacceptably high, so they are using  
> For profiling work fast timestamping is of course great, but surely 
> there is no monotonicity requirement?

Yes and no.  Monotonicity is a poor substitute for a more
generic mechanism that might provide an indication that a
discontinuity has occurred (forward or backward); if an app
could get both the timestamp AND some kind of "continuity
generation counter" (basically a much more sophisticated
form of TSC_AUX that changes whenever the timestamp is
coming from a different source), perhaps all problems could be solved.

> I don't think we'll be able to provide monotonicity with vsyscall on 
> tsc-broken hosts, so we'll be limited to correcting the tsc frequency 
> after migration for good-tsc hosts.

True, though clock_gettime(CLOCK_MONOTONIC) can provide
the monotonicity where it is required.


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