Re: kvm guest loops_per_jiffy miscalibration under host load

[Marcelo Tosatti <mtosatti@redhat.com>]

On Tue, Jul 22, 2008 at 01:56:12PM -0600, David S. Ahern wrote:
> I've been running a series of tests on RHEL3, RHEL4, and RHEL5. The
> short of it is that all of them keep time quite well with 1 vcpu. In the
> case of RHEL3 and RHEL4 time is stable for *both* the uniprocessor and
> smp kernels, again with only 1 vcpu (there's no up/smp distinction in
> the kernels for RHEL5).
> 
> As soon as the number of vcpus is >1, time drifts systematically with
> the guest *leading* the host. I see this on unloaded guests and hosts
> (ie., cpu usage on the host ~<5%). The drift is averaging around
> 0.5%-0.6% (i.e., 5 seconds gained in the guest per 1000 seconds of real
> wall time).
> 
> This very reproducible. All I am doing is installing stock RHEL3.8, 4.4
> and 5.2, i386 versions, starting them and watching the drift with no
> time servers. In all of these recent cases the results are for in-kernel
> pit.

David,

You mentioned earlier problems with ntpd syncing the guest time? Can you
provide more details?

I find it _necessary_ to use the RR scheduling policy for any Linux
guest running at static 1000Hz (no dynticks), otherwise timer interrupts
will invariably be missed. And reinjection plus lost tick adjustment is
always problematic (will drift either way, depending which version of
Linux). With the standard batch scheduling policy _idle_ guests can wait
to run upto 6/7 ms in my testing (thus 6/7 lost timer events). Which
also means latency can be horrible.