At 16:56 +0100 on 13 Oct (1286988987), Dan Magenheimer wrote:
> I wonder if the "bug" here is that "dependent wall clock" is
> an incredibly poor replacement for NTP (or a Windows Time Server)
> and a hack and really shouldn't even exist?
I've seen Windows Time server do some pretty odd things, but leaving
that aside - the problem with dependent wallclock is that it distributes
only the wallclock time and not the drift parameters, so you get
sawtoothing.
Running NTP in guests has its own problems, particularly around live
migration, where NTP can lose stability quite badly.
There was a paper about this at OSDI last week:
http://www.usenix.org/events/osdi10/tech/full_papers/Broomhead.pdf
Tim.
> I suppose one could
> argue that it makes some sense in a XCP environment, and maybe
> in a server environment where a single physical server is extremely
> isolated, but does it ever make sense in a real world server
> environment?
>
> In other words, I'm arguing that the correct "fix" here is:
> Don't use dependent wallclock.
>
> > -----Original Message-----
> > From: Jan Beulich [mailto:JBeulich@xxxxxxxxxx]
> > Sent: Wednesday, October 13, 2010 6:38 AM
> > To: Hang Du
> > Cc: 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
> >
> > >>> On 13.10.10 at 05:24, "Du, Hang" <hdu@xxxxxxxxxxxx> wrote:
> > > Sorry for my brief description in previous mail and missing
> > > is_initial_xendomain check. The kernel I submit this patch is
> > > linux-2.6.18-xen-3.4.2, I submit the patch again with in_initial-
> > xendomain
> > > check.
> >
> > Actually, I didn't expect you to blindly insert that check, but rather
> > either explain why only DomU-s need the changed behavior (as your
> > original description suggested), or refine the description of the
> > problem you're trying to solve.
> >
> > > In this patch, we support the backward time changing sync to all
> > domUs which
> > > configured to use "dependent wall clock".
> > > Currently, without the backward time syncing, when we change the time
> > > backward in Dom0, the time in DomU would be froze.
> > > And this cause some commands hang and don't executed until the time
> > catch up
> > > with the domU time.
> > > For example:
> > > "rpm -q kernel-xen"
> > > "sleep 1"
> > > Monotonic time should be reset when sync up time from dom0 to domU to
> > > support domU backward time syncing.
> >
> > I can see your point, however ...
> >
> > > diff -urN a/arch/i386/kernel/time-xen.c b/arch/i386/kernel/time-
> > xen.c
> > >
> > > --- a/arch/i386/kernel/time-xen.c 2010-10-11 10:41:06.000000000
> > +0800
> > > +++ b/arch/i386/kernel/time-xen.c 2010-10-11 10:43:32.000000000
> > +0800
> > > @@ -715,6 +715,8 @@
> > > }
> > >
> > > if (shadow_tv_version != HYPERVISOR_shared_info->wc_version) {
> > > + if (!is_initial_xendomain() && !independent_wallclock)
> > > + monotonic_reset();
> > > update_wallclock();
> >
> > ... you definitely need to call monotonic_reset() *after* the
> > update_wallclock() (or else another vCPU, preempted in
> > do_gettimeofday() between the end of the xtime read loop
> > and the acquiring of the monotonic lock, would be able to
> > overwrite monotonic.tv with values obtained before the wall
> > clock update - similar to the secondary problem addressed by
> > c/s 1021).
> >
> > Further, blindly running a reset here doesn't seem nice in
> > the (general) case of the time getting updated forwards.
> >
> > > schedule_clock_was_set_work = 1;
> > > }
> >
> > You'll also want to address Dan's concerns, and you will want to
> > get your patch formatted so that it would actually apply (read:
> > no tab -> space conversion) if you expect it to be committed
> > by Keir at some point.
> >
> > Jan
> >
> >
> > _______________________________________________
> > Xen-devel mailing list
> > Xen-devel@xxxxxxxxxxxxxxxxxxx
> > http://lists.xensource.com/xen-devel
>
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--
Tim Deegan <Tim.Deegan@xxxxxxxxxx>
Principal Software Engineer, XenServer Engineering
Citrix Systems UK Ltd. (Company #02937203, SL9 0BG)
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