[Xen-users] ntpd under Xen Dom0 exhibits extremely high jitter/noise? runs stable/quiet under non-xen kernel.

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On a selection of boxes, ntpd running in Xen Dom0 reproducibly
exhibits extermely high noise/jitter.
Switching back to -default, non-xen kernel ntpd runs with very low jitter/noise.

Question -- how can I 'tame' ntpd noise & jitter when running in Dom0?
 Is the problem a config issue, or a bug?

Already reported this downstream; everybody's "stumped".

Running

 uname -a
  Linux test 2.6.31.8-0.1 #1 SMP 2009-12-15 23:55:40 +0100 x86_64
x86_64 x86_64 GNU/Linux

with @ grub,

 kernel /vmlinuz root=/dev/V0/ROOT rootfstype=ext4
rootflags=journal_checksum resume=/dev/V0/SWAP showopts splash=silent
selinux=0 SELINUX_INIT=NO elevator=cfq vga=0x31a console=tty0
console=ttyS0,57600n8

 cat /sys/devices/system/clocksource/clocksource0/current_clocksource
   tsc

on 5 different mobos, all with AMD Phenom II X4 Deneb Cores
(http://en.wikipedia.org/wiki/List_of_AMD_Phenom_microprocessors#.22Deneb.22_.28C2.2C_45_nm.2C_quad-core.29)

e.g.,

 grep "model name" /proc/cpuinfo
   model name      : AMD Phenom(tm) II X4 920 Processor
 grep flags /proc/cpuinfo
   flags           : fpu de tsc msr pae mce cx8 apic mtrr mca cmov pat
clflush mmx fxsr sse sse2 ht syscall nx mmxext fxsr_opt lm 3dnowext
3dnow constant_tsc rep_good nonstop_tsc pni cx16 popcnt hypervisor
lahf_lm cmp_legacy extapic cr8_legacy abm sse4a misalignsse
3dnowprefetch

running ntpd, within ~ 5 minutes of launch,

 ntpq -p -c rv

returns expected low-jitter, low-offset, full-reach results,

               remote           refid      st t when poll reach   delay   offset
        jitter
        
==============================================================================
          *clock.fmt.he.ne .PPS.            1 u    2  128  377   14.472    6.176
        21.235
          +otc2.psu.edu    128.4.1.1        2 u   68  128  377   98.808   17.478
        33.898
          +clock.isc.org   216.200.141.8    2 u   67  128  377   15.491   -4.421
        22.236
          +clock.sjc.he.ne .CDMA.           1 u   66  128  377   14.973   14.981
        23.539
          +zorro.sf-bay.or 216.218.254.202  2 u   77  128  377   14.861   -5.981
        14.786
          +rrcs-64-183-56- .GPS.            1 u  124  128  377   34.705   11.295
        36.253
          +nist1.aol-ca.tr .ACTS.           1 u   62  128  377   16.302    8.190
        24.812

rebooting to,

 uname -a
  Linux test 2.6.31.8-0.1-xen #1 SMP 2009-12-15 23:55:40 +0100 x86_64
x86_64 x86_64 GNU/Linux

with @ grub,

 kernel /xen.gz loglvl=all loglvl_guest=all dom0_mem=1024M
vga=gfx-1280x1024x32 console=vga,com1 com1=57600,8n1 iommu=1
cpufreq=xen:performance cpuidle
 module /vmlinuz-xen root=/dev/V0/ROOT rootfstype=ext4
rootflags=journal_checksum resume=/dev/V0/SWAP showopts splash=silent
selinux=0 SELINUX_INIT=NO elevator=cfq vga=0x31a console=tty0
console=xvc0,57600 iommu=off noirqdebug clocksource=xen

and,

 cat /sys/devices/system/clocksource/clocksource0/current_clocksource
   xen

 cat /etc/init.d/boot.local
   xenpm set-scaling-governor ondemand
   xenpm set-scaling-governor performance

after boot,

 cat /proc/cpuinfo | grep MHz
  cpu MHz         : 2800.000
  cpu MHz         : 2800.000
  cpu MHz         : 2800.000
  cpu MHz         : 2800.000

 xenpm get-cpufreq-para | grep governor
  current_governor     : performance
  current_governor     : performance
  current_governor     : performance
  current_governor     : performance

ntpd @ Dom0 return extremely high offset & jitter, never dropping to
'normal', non-xen levels; e.g., even after 2-3 days of running,

              remote           refid      st t when poll reach   delay   offset 
 jitter
        
==============================================================================
          +clock.fmt.he.ne .PPS.            1 u   60   64  377   21.185  -1690.8
        658.866
          +otc2.psu.edu    128.4.1.1        2 u   33   64  377  109.992  -1754.1
        659.825
          +clock.isc.org   216.200.141.8    2 u    2   64  377   13.304  -1827.8
        676.431
          *clock.sjc.he.ne .CDMA.           1 u   46   64  377   21.193  -1724.5
        655.151
           zorro.sf-bay.or 216.218.254.202  2 u   53   64  377   15.396  -1711.7
        653.852
          +rrcs-64-183-56- .GPS.            1 u   28   64  377   53.359  -1766.9
        657.547
          +nist1.aol-ca.tr .ACTS.           1 u   37   64  377   22.613  -1742.4
        665.713

Also, with ntpd *off*,

@ boot to kernel-default
  sntp clock.isc.org && date && hwclock --show
      2009 Dec 28 10:38:49.000867 + 0.250226 +/- 0.023099 secs
      Mon Dec 28 10:38:50 PST 2009
      Mon 28 Dec 2009 10:38:51 AM PST  -0.998094 seconds

@ boot to kernel-xen
  sntp clock.isc.org && date && hwclock --show
!     2009 Dec 28 10:31:32.000206 + 28800.725203 +/- 0.013941 secs
!     Mon Dec 28 18:31:32 PST 2009
      Mon 28 Dec 2009 10:31:33 AM PST  -0.638684 seconds

note the dramatic differences in 'sntp' & 'date' results.  These
results can be reproduced swithicn back and forth between -xen &
-default kernel boots.

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