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Re: [Xen-users] [Xen-devel] Xen timekeeping best practices

Hi Ian,

What are the current timekeeping best practices now?

pvops kernels behave as if independent_wallclock == 1, so the existing
       set /proc/sys/xen/independent_wallclock to 1 and run ntp on
still applies.

O, thanks!

The thing is, we have several servers in Xen 4.0.1 using NTP in dom0 and domU. We didn't have any problem keeping a correct time with Xen 3 on the same servers, but with Xen 4 we have a recurring problem where the clock goes out of sync for an hour or so, in dom0 and domU at the same time.

We have done more tests since friday, and it seems that this issue does not occur on servers having a Intel Xeon L5630, but on older servers having E5405 or L5410 processors.

Looking at the output from xm dmesg, olders servers have :
(XEN) checking TSC synchronization across 8 CPUs: passed.
(XEN) Platform timer appears to have unexpectedly wrapped 10 or more times.
(XEN) Platform timer is 3.579MHz ACPI PM Timer
(XEN) CPUIDLE: disabled due to no HPET. Force enable with 'cpuidle'.

where newer have :

(XEN) TSC is reliable, synchronization unnecessary
(XEN) Platform timer is 14.318MHz HPET

I'm wondering if my issue could simply be linked to the fact that HPET is not available ?

On newer servers, the ouput of
# xm debug-key s; xm dmesg | tail
is :
(XEN) TSC marked as reliable, warp = 0 (count=4)
(XEN) dom3: mode=0,ofs=0x42f57c62a36,khz=2133464,inc=1
(XEN) dom4: mode=0,ofs=0x430b996b94f,khz=2133464,inc=1
(XEN) dom5(hvm): mode=0,ofs=0x470ba656d26,khz=2133464,inc=1
(XEN) dom10(hvm): mode=0,ofs=0x6baa148c81f66,khz=2133464,inc=1
(XEN) No domains have emulated TSC

whereas it is on olders servers :
(XEN) Platform timer appears to have unexpectedly wrapped 10 or more times
(XEN) TSC has constant rate, deep Cstates possible, so not reliable, warp=2310 (count=8) (XEN) dom1: mode=0,ofs=0x4379529ffa3,khz=2000068,inc=1,vtsc count: 5579056116 kernel, 18906545 user (XEN) dom2: mode=0,ofs=0x43a55ddb79e,khz=2000068,inc=1,vtsc count: 12684066480 kernel, 25277162 user (XEN) dom3: mode=0,ofs=0x43bd882b29e,khz=2000068,inc=1,vtsc count: 35568040315 kernel, 27356321 user

Any idea would be appreciated !


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