On 2012-01-20 13:54, Daniel P. Berrange wrote: > On Fri, Jan 20, 2012 at 01:51:20PM +0100, Jan Kiszka wrote: >> On 2012-01-20 13:42, Daniel P. Berrange wrote: >>> On Fri, Jan 20, 2012 at 01:00:06PM +0100, Jan Kiszka wrote: >>>> On 2012-01-20 12:45, Daniel P. Berrange wrote: >>>>> On Fri, Jan 20, 2012 at 12:13:48PM +0100, Jan Kiszka wrote: >>>>>> On 2012-01-20 11:25, Daniel P. Berrange wrote: >>>>>>> On Fri, Jan 20, 2012 at 11:22:27AM +0100, Jan Kiszka wrote: >>>>>>>> On 2012-01-20 11:14, Marcelo Tosatti wrote: >>>>>>>>> On Thu, Jan 19, 2012 at 07:01:44PM +0100, Jan Kiszka wrote: >>>>>>>>>> On 2012-01-19 18:53, Marcelo Tosatti wrote: >>>>>>>>>>>> What problems does it cause, and in which scenarios? Can't they be >>>>>>>>>>>> fixed? >>>>>>>>>>> >>>>>>>>>>> If the guest compensates for lost ticks, and KVM reinjects them, guest >>>>>>>>>>> time advances faster then it should, to the extent where NTP fails to >>>>>>>>>>> correct it. This is the case with RHEL4. >>>>>>>>>>> >>>>>>>>>>> But for example v2.4 kernel (or Windows with non-acpi HAL) do not >>>>>>>>>>> compensate. In that case you want KVM to reinject. >>>>>>>>>>> >>>>>>>>>>> I don't know of any other way to fix this. >>>>>>>>>> >>>>>>>>>> OK, i see. The old unsolved problem of guessing what is being executed. >>>>>>>>>> >>>>>>>>>> Then the next question is how and where to control this. Conceptually, >>>>>>>>>> there should rather be a global switch say "compensate for lost ticks of >>>>>>>>>> periodic timers: yes/no" - instead of a per-timer knob. Didn't we >>>>>>>>>> discussed something like this before? >>>>>>>>> >>>>>>>>> I don't see the advantage of a global control versus per device >>>>>>>>> control (in fact it lowers flexibility). >>>>>>>> >>>>>>>> Usability. Users should not have to care about individual tick-based >>>>>>>> clocks. They care about "my OS requires lost ticks compensation, yes or no". >>>>>>> >>>>>>> FYI, at the libvirt level we model policy against individual timers, for >>>>>>> example: >>>>>>> >>>>>>> <clock offset="localtime"> >>>>>>> <timer name="rtc" tickpolicy="catchup" track="guest"/> >>>>>>> <timer name="pit" tickpolicy="delay"/> >>>>>>> </clock> >>>>>> >>>>>> Are the various modes of tickpolicy fully specified somewhere? >>>>> >>>>> There are some (not all that great) docs here: >>>>> >>>>> http://libvirt.org/formatdomain.html#elementsTime >>>>> >>>>> The meaning of the 4 policies are: >>>>> >>>>> delay: continue to deliver at normal rate >>>> >>>> What does this mean? The timer stops ticking until the guest accepts its >>>> ticks again? >>> >>> It means that the hypervisor will not attempt to do any compensation, >>> so the guest will see delays in its ticks being delivered & gradually >>> drift over time. >> >> Still, is the logic as I described? Or what is the difference to "discard". > > With 'discard', the delayed tick will be thrown away. In 'delay', the > delayed tick will still be injected to the guest, possibly well after > the intended injection time though, and there will be no attempt to > compensate by speeding up delivery of later ticks. OK, let's see if I got it: delay - all lost ticks are replayed in a row once the guest accepts them again catchup - lost ticks are gradually replayed at a higher frequency than the original tick merge - at most one tick is replayed once the guest accepts it again discard - no lost ticks compensation Jan -- Siemens AG, Corporate Technology, CT T DE IT 1 Corporate Competence Center Embedded Linux -- 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
