[ntp:questions] no ntp synchronisation: 2s to 6s time shift !

Thierry MARTIN thierry-martin at ifrance.com
Fri Feb 15 07:54:48 UTC 2008

Hi all,

Just a few word to keep you informed...

I have been trying acpi_pm clocksource for a few days now and the 
results are quite good :-).

The time drift is less than 1s per day (I would even say less that 500ms 
but this has to be confirmed) which is much better than the default 
config with tsc (>2s /day).

Honestly, I am a bit surprised / worried about these results: I would 
expect that people doing distribution would have the best clocksource 
detected / configured automatically.
Might be because my FC5 base is too old, or ... I Hope I won't find side 

Have a nice day!
Thierry MARTIN

Martin Burnicki a écrit :
> Unruh wrote:
>> Any place where these different clock models is described?
> Hm, I think "clock models" in the sense of NTP is not correct in this
> context. AFAIK there is only one "clock model" in the Linux kernel which
> uses one of those "clocksource" modules as its base for timekeeping,
> similar to the way ntpd uses a refclock as time source.
> Each of the clocksource modules deals with a dedicated timer or counter
> which may or may not be available on a specific hardware architecture, i.e.
> the x86_64 hardware may provide different timers in its chipset than the
> i386 architecture.
>> And defined (what is tsc?)
> The only place I've found to get an overview is the file
> Documentation/kernel-parameters.txt which is part of the Linux kernel
> sources. This also lists which clocksource modules may be available for
> which hardware architecture.
> Here's the relevant part from a 2.6.22 kernel:
>   clocksource=  [GENERIC_TIME] Override the default clocksource
>       Format: <string>
>       Override the default clocksource and use the clocksource
>       with the name specified.
>       Some clocksource names to choose from, depending on
>       the platform:
>       [all] jiffies (this is the base, fallback clocksource)
>       [ACPI] acpi_pm
>       [ARM] imx_timer1,OSTS,netx_timer,mpu_timer2,
>         pxa_timer,timer3,32k_counter,timer0_1
>       [AVR32] avr32
>       [IA-32] pit,hpet,tsc,vmi-timer;
>         scx200_hrt on Geode; cyclone on IBM x440
>       [MIPS] MIPS
>       [PARISC] cr16
>       [S390] tod
>       [SH] SuperH
>       [SPARC64] tick
>       [X86-64] hpet,tsc
> As already mentioned by David Woolley the TSC counter is a register inside
> Pentium CPUs or higher. The tick rate corresponds to the CPU clock, so if
> the CPU clock changes due to power saving efforts this has to be taken into
> account when measuring time intervals.
> The acpi_pm module uses the ACPI Power Management timer which is available
> on every PC which supports ACPI services. 
> The hpet module uses the High Precision Event Timers indroduced by recent
> Intel chip sets. AFAIK it's a replacement for the old Periodic Interrupt
> Timer from the original IBM PC architecture which used to generate periodic
> interrupts at 18.2 Hz under DOS.
> I don't know the exact details or advantages/disadvantages of those timers.
> However, from several postings here in the NG and elsewhere I've seen that
> certain modules may not work properly on certain chipsets. This must not
> necessarily be due to the modules, AFAIK there are also chipsets out there
> where the timers don't work properly and would require specific
> workarounds.
> Martin

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