[fedora-virt] is there a best clock to use?

[Glauber Costa]

On Thu, Jan 21, 2010 at 07:42:21AM -0500, Tom Horsley wrote:
> On Thu, 21 Jan 2010 10:18:38 -0200
> Glauber Costa wrote:
> 
> > On Wed, Jan 20, 2010 at 08:42:30PM -0500, Tom Horsley wrote:
> > > Is there a HOWTO anywhere of the different ways to
> > > make time of day clock work in virtual machines and different
> > > ways to influence it from the qemu side of the world?
> > > 
> > > Time keeping is horrible in most of my virtual machines,
> > > and I just wonder if there are settings I can change
> > > in libvirt or kernel boot parameters I can apply in
> > > the linux guest to make things slightly less horrible?
> > > 
> > It basically depends on your guest. What guests are we
> > talking about here?
> > 
> > If you run a fairly recent linux (that has pvclock), you
> > should have no issues.
> 
> I run gazillions of different linux distros, primarily to
> test our software on lots of different platforms, so
> there are a wide variety of different versions of linux
> from old to new. And in fact, time isn't horrible on
> all of them, just on some of them.
> 
> Even in an old linux I have trouble understanding how
> the time can be 200 seconds off as soon as it is booted :-).

It is not as soon as it is booted. It is as soon as you as 
able to do some time measurements. Unfortunately, without pvclock,
situation can get pretty bad, pretty fast. Specially depending
on the clock you are using. pmtimer for example, can make you
see large jumps. You might want to try emulated hpet for them.

It is not quite stable yet, but hey, you can help us knowing
exactly when it breaks :-)