Red Hat Bugzilla – Bug 449187
Clock drift in certain KVM guest
Last modified: 2009-07-14 11:21:25 EDT
Description of problem:
in certain KVM guests (let's call it WUI) time drifts significantly (slows down)
while in the other guest (clet's call it NODE) on the same host, time stays
stable. One significant difference is:
NODE - LOC: 100321 Local timer interrupts
WUI - LOC: 1088230 Local timer interrupts
This happens even with clocksource=acpi_pm boot param
( TSC is unstable, from WUI dmesg: Clocksource tsc unstable (delta = 4686864172 ns)
Version-Release number of selected component (if applicable):
Steps to Reproduce:
1. build WUI guest:
sh create-wui-appliance.sh -v -t
2. on WUI: service ntpd stop; ntpd -q -d
3. wait ~10min; ntpd -q -d
clock drifted, ntpd -q -d output shows (last line): e.g.
ntpd: time set +74.993754s
clock doesn't drift: e.g.
ntpd: time slew -0.054264s
For the record, what is the current clocksource in your guest?
(As the output of cat
since I have clocksource=acpi_pm but it was also falling back to this b/c of
unstable tsc (Clocksource tsc unstable w/ few sec delta in dmesg)
Important data I missed in the report is CPU:
model name : Intel(R) Core(TM)2 Quad CPU Q9450 @ 2.66GHz
stepping : 6
This stepping doesn't have C2 support.
Mobo is Q35 beta
On suggestion from chrisw I'm testing with:
Author: Marcelo Tosatti <email@example.com>
Date: Fri Apr 11 14:53:26 2008 -0300
KVM: hlt emulation should take in-kernel APIC/PIT timers into account
After running for few hours, the same VM doesn't exhibit such time drift,
ntpd -q -d says: time slew +0.039053s
Adding to comment 3 - after I left VM idling for few more hourse, time drifted
again ( > 1000s so it was too much even for ntpd -q )
Will try now on a different CPU...
Just for another data point, I have a Core2 Due E6850, running the WUI, and ntp
seems to be keeping the time perfectly in sync for me.
Another data point. I have a Core2 Duo E6850 running the WUI and when I disable
NTP my guest loses time rapidly (1 minute lost out of every 10).
I tried a test where I disabled ACPI in the Guest via libvirt config, and now
time is kept perfectly in sync with the guest.
I can confirm that on Q9450 with ACPI disabled in WUI guest, I get stable time,
ntpd -q after ~15min shows:
ntpd: time slew +0.051113
And this is with original F9 KVM kmods:
I see this too. F10 x86_64 host on Athlon X2, F10 i686 guests. clocksource=acpi_pm
How little drift is reasonable to expect?
If time on the host is well kept then there should be nothing for ntpd in the guests to do. The guests should just hwclock --hctosys, ntpdate or similar once at boot and then time in the guest should stay in sync with the host from then on, because there should be no clock drift to compensate for. But can we hope for acpi_pm emulation to become that good?
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I am using KVM with 64 bit fedora 11 and 64 bit guests (maybe the version should get updated) and had similar problem.
With fedora 11 guest 'cat /sys/devices/system/clocksource/clocksource0/current_clocksource' shows kvm-clock and all works fine (ntpd).
With CentOS kernel 2.6.18-128.1.14.el5 I get jitter > 5000 and ntpd refuses to work. Clock source is jiffies (the only one available).
With CentOS kernel 2.6.18-128.1.14.el5 with 'notsc divider=10' added to the kernel boot line I get jitter from 18 to 20 on three virtual machines and ntpd works fine.
Fedora 9 changed to end-of-life (EOL) status on 2009-07-10. Fedora 9 is
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