Re: Guest Time Question

[Kay Hayen <kayhayen-Mmb7MZpHnFY@public.gmane.org>]

Hello Eddie,

Am Montag 15 Oktober 2007 06:13:44 schrieb Dong, Eddie:
> >That 0.1% kills me though. do you mean, that every 1 second, 1
> >will be 1ms
> >off? And for every second thereafter, leaving me with 1 second
> >drift after
> >only 1000 seconds?
>
> Oh, that is not my intension :-(
> I just put a rought estimation here. In old Xen
> time, I know there are ~10-30 seconds shift after ~10 hours.

That's about the same, isn't it? That's 30 seconds shift after 36000 seconds 
and really a lot.

> >I am looking for an absolute value for time difference between
> >guest and host
> >that is supposed to be small over longer periods. Is that not
> >achievable for
> >a paravirtual guest?! I really need gettimeofday() calls to be
> >near exactly
> >the same value for applications in guest and host even after months.
>
> Except adding a hypercall to get host time (i.e. pv timer), or
> using network time, I think cetain amount of shift will be always there.

Do you consider that NTP between guest and host is acceptable? I found this 
btw: http://support.ntp.org/bin/view/Support/KnownOsIssues

Quote:  "It appears that Xen just passes time-related system calls to the 
underlying master domain, and does not require any additional changes to 
support time sync into the guest domains."

When I today told my manager (technical background) about this, it was met 
with utter surprise, because "that should be really simple". Incidentally, I 
recently claimed in a meeting that with modern Linux and paravirtualization, 
this problem should no longer exist.

Is this a design problem for KVM (the hypercall missing yet) or just something 
that the KVM community didn't yet have/find the time to get around to?

Best regards,
Kay

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