Re: Timedrift in KVM guests after livemigration.

[Espen Berg <espen@monsternett.no>]

Den 18.04.2010 11:22, skrev Dor Laor:
>>> What do you mean by "adopts" ? Note that the cpu frequency
>>> means nothing for all the modern operating systems, at least
>>> since the days of common usage of MS-DOS which relied on CPU
>>> frequency for its time functions. All interesting things are
>>> now done using timers instead, and timers (which don't depend
>>> on CPU frequency again) usually work quite well.
>> The assumption that frequency of the ticks was calculated by the hosts
>> MHz, was based on the fact that grater clock frequency differences
>> caused higher time drift. 60 MHz difference caused about 24min drift,
>> 332 MHz difference caused about 2h25min drift.
>>> What complicates things is that the most cheap and accurate
>>> enough time source is TSC (time stamp counter register in
>>> the CPU), but it will definitely be different on each
>>> machine. For that, 0.12.3 kvm and 2.6.32 kernel (I think)
>>> introduced a compensation. See for example -tdf kvm option.
>> Ah, nice to know. :)
> That's two different things here:
> The issue that Espen is reporting is that the hosts have different
> frequency and guests that relay on the tsc as a source clock will notice
> that post migration. The is indeed a problem that -tdf does not solve.
> -tdf only adds compensation for the RTC clock emulation.
>
> What's the guest type and what's the guest's source clock?

All guest are Debian lenny with latest upstream kernel, hvm/kvm.

We are using kvm-clock as guest source clock.

cat /sys/devices/system/clocksource/clocksource0/current_clocksource
kvm-clock


Regards
Espen