Re: recommended clock source

Antoine Martin
2009-08-03T09:09:43Z

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>>>>> It seems to be an unsettled issue, but, would any kind soul suggest
>>>>> the current
>>>>> best practice for setting the clock in Ubuntu Linux and Windows guests?
>>>> For Linux the best source clock is the kvm pv clock (exist from 2.6.27
>>>> and above).
> # qemu-system-x86_64 -clock ?
> Available alarm timers, in order of precedence:
> dynticks
> hpet
> rtc
> unix
>
> I see no "pv clock"...
> Which one should I use then?
> Did I miss a ./configure or .config option?
>
>> No, we were talking about different clocks.
>> I was explaining the guest source clock while you wanted some info for
>> the host-qemu clock.
Hah, gotcha. You're talking about the guest kernel as in:
clocksource=[hpet|pit|tsc|acpi_pm|cyclone|scx200_hrt|kvm-clock]

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

So all good, thanks.

>> You can use the default - dynticks. hpet and rtc might be good if you
>> need a fine grain granularity on older < 2.6.24 host kernels.
(I'm on dynticks)
I guess dynticks reduces context switches on the host, but I still get
hundreds per second on guests that are otherwise idle.
How would I go about finding what makes them tick?

Thanks
Antoine

>
>
> Cheers
> Antoine
>
>>>> For windows, standard acpi HAL uses the rtc clock by default. As long as
>>>> you use the -rtc-td-hack it won't drift.
>>>>
>>>> When the tsc is not stable on the host or the host cpu might get into
>>>> deep sleep state (c2), you better use another source clock in the guest
>>>> - for windows it should be the pmtimer (using the boot.ini).
>>>>
>>>>> Thanks!
>>>>>
>>>>> --
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