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
--
To unsubscribe from this list: send the line "unsubscribe kvm" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at http://vger.kernel.org/majordomo-info.html
