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. > > catchup: deliver at higher rate to catchup > > merge: ticks merged into 1 single tick > > discard: all missed ticks are discarded > > But those interpretations aren't stated in the docs. That makes it hard > to map them on individual hypervisors - or model proper new hypervisor > interfaces accordingly. That's not a real problem, now I notice they are missing the docs, I can just add them in. Daniel -- |: http://berrange.com -o- http://www.flickr.com/photos/dberrange/ :| |: http://libvirt.org -o- http://virt-manager.org :| |: http://autobuild.org -o- http://search.cpan.org/~danberr/ :| |: http://entangle-photo.org -o- http://live.gnome.org/gtk-vnc :|
