[ntp:questions] problem with synchronizing two comps to each other

Martin Burnicki martin.burnicki at meinberg.de
Fri May 20 13:18:17 UTC 2005


Brad Knowles wrote:
> At 4:25 PM +0000 2005-05-19, Bjorn Gabrielsson wrote:
>>  PPSKit-patch implements the kernel PPSAPI which is useful when you
>>  have a good GPS with PPS-signal attached to the computer. In which way
>>  does this relate to loosing timer interrupts? or the OPs question?
> Sorry, I was under the impression that the PPSKit patches also
> helped with the overall problem of losing interrupts.  My bad.

Yes, Bjorn is right. As the name says, PPSKit is only required if you want
PPS support for your kernel, i.e. if you have a refclock which generates
PPS pulses.
>>  Lots of people run NTP on Linux with no problems.
> And lots of people have problems with NTP on Linux.
>>                                                     Lots of companys
>>  sell Linux based NTP-servers.
> How many of those are not patched with the PPSKit?  And of those
> that are not patched, how many work well?  And how many people have
> resolved the networking issues with Linux?
> I'm not saying that Linux won't work.  However, it has a number
> of well-known areas where it tends to have difficulties relative to
> NTP.  

I think machines without a refclock don't require the PPSkit patch. If
machines are configured as standard clients which poll the time from an NTP
server, I'm not aware of any problems with Linux.

> In addition, there are a much greater number of people using 
> Linux who are neophytes, and an there is increased ratio of neophytes
> to experts.  There are also some issues with the networking code in
> current versions of Linux.
> The result is a serious issue that needs to be addressed any time
> someone talks about doing NTP using Linux.

AFAIK there are some problems with broadcast/multicast modes because things
have been implemented in Linux in a different way as commonly known for
other Unices. The legacy way works perfectly.
> If they're already an expert and they know what the PPSKit
> patches do and whether or not they need them, then we're down to
> things like the networking code, or problems inherent to anyone
> trying to use desktop-grade computers as servers, or perhaps issues
> internal to NTP itself.

I've also seen that the ACPI implementation in a PC BIOS affects the quality
of time keeping. With ACPI enabled there's been some jitter in the system
clock, but after ACPI had been disabled by the BIOS programmers, the jitter
disappeared. You can't blame things like this on Linux.
Best regards,

Martin Burnicki

Meinberg Funkuhren
Bad Pyrmont

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