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RE: [Xen-devel] [PATCH] time-xen : Reset monotonic time when sync up tim

To: Tim Deegan <Tim.Deegan@xxxxxxxxxx>
Subject: RE: [Xen-devel] [PATCH] time-xen : Reset monotonic time when sync up time from dom0 to domU
From: Dan Magenheimer <dan.magenheimer@xxxxxxxxxx>
Date: Fri, 15 Oct 2010 07:53:53 -0700 (PDT)
Cc: Jeremy Fitzhardinge <jeremy@xxxxxxxx>, xen-devel@xxxxxxxxxxxxxxxxxxx, Keir Fraser <keir@xxxxxxx>, Julien, Shunli Yi <syi@xxxxxxxxxxxx>, Darryl Veitch <dveitch@xxxxxxxxxxxxxx>, Jan Beulich <JBeulich@xxxxxxxxxx>, Saipu Liu <saliu@xxxxxxxxxxxx>, Hang Du <hdu@xxxxxxxxxxxx>, Timothy Broomhead <tim@xxxxxxxxxxxxxxx>, Ridoux <jridoux@xxxxxxxxxxxxxx>
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(RADclock authors cc'ed in attempt to re-merge the two threads...
Tim, if it's not too late, please reply to this email rather
than the previous one without the full cc list)

> From: Dan Magenheimer
> Sent: Friday, October 15, 2010 8:47 AM
> To: Tim Deegan
> Cc: Jeremy Fitzhardinge; Jan Beulich; Hang Du; Keir Fraser; Saipu Liu;
> Shunli Yi; xen-devel@xxxxxxxxxxxxxxxxxxx
> Subject: RE: [Xen-devel] [PATCH] time-xen : Reset monotonic time when
> sync up time from dom0 to domU
> > > Also, for those certain enterprise applications that want
> > > to sample time 10000+ samples/second/processor, and need
> > > to know "immediately" when a sample might be bad (due
> > > to, for example, live migration), I think each sample would
> > > need to check xenstore.  Is xenstore up to that kind of
> > > pounding (and is it fast enough)?
> >
> > No it's certainly not (either of those things), but:
> >  - the problem of turning a distributed wallclock into something
> > suitable
> >    for timestamping that aggressively is orthogonal to the problem of
> >    distributing that wallclock in the first place; and
> Agreed that this is a better solution to a very important problem.
> I'm just trying to determine if it helps solve another time-related
> real world problem.
> >  - all the user really needs is a generation counter to know that the
> >    clock correction values are stale, and the kernel can provide that
> >    alongside the stime.
> Agreed that a generation counter solves this.  BUT...
> (1) afaik there is no generation counter in any time-related
> kernel API; and
> (2) afaict this generation counter would need to be "pushed"
> from the dom0 kernel so would either (a) require domU kernels
> to read from xenstore, or (b) require some kind of privileged
> hypercall from dom0 to the hypervisor telling the hypervisor
> to change the generation counter (and scaling values?) for all
> domUs.

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