On Tue, Jan 12, 2010 at 06:14:57PM -0800, mail ignored wrote:
> 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".
>
How many vcpus does your dom0 have? Have you configured domain weights
(ie. guaranteed cpu time for dom0?)
Have you tried dedicating a single cpu core only for dom0?
-- Pasi
> 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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