is there a best clock to use?

[Tom Horsley]

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?

Sometimes I get the impression that the guest time is
already two or three minutes off as soon as it is
booted, and I can't imagine how that happens since I
would assume it gets a copy of the host machine time
for the emulated rtc clock in the guest. How can it
already be broken at boot time?

[Glauber Costa]

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.

[Tom Horsley]

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 :-).

[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 :-)

[Tom Horsley]

On Thu, 21 Jan 2010 07:42:21 -0500
Tom Horsley wrote:

>  Even in an old linux I have trouble understanding how
>  the time can be 200 seconds off as soon as it is booted :-). 
DOH! I just figured this bit out: I have two host machines,
and one of them wasn't running ntpd, so the time on the
host was in fact about 200 seconds off. Fixing that should
at least fix the most horrible time keeping bit :-).

Maybe my guest machine dhclients will stop letting leases
expire now.