[ntp:hackers] odd behaviour (one more time)

David L. Mills mills at udel.edu
Tue Sep 9 16:25:22 UTC 2008


Jim,

There have been three versions of the kernel code since 1992, the 
microkernel (Solairs), nanokernel (1994) and nanokernel with TAI (1998). 
So far as I know, Linux has used the 1998 version but without TAI. The 
microkernel version and nanokernel versions differed in the scaling of 
poll interval to time constant. In the former the time constant is 
reduced by four; in the latter the time constant is equal to the poll 
interval.

The generic timex.h header file correctly accounts for all three 
versions, but the various OS versions have adopted their own version 
with and without errors. However, the STA_NANO should be present in the 
timex.h file so that the ntp_loopfilter.c knows what interface and what 
scaling should be used. The STA_PLL bit in the status word is no longer 
used, as the capability to to microsecond precision is inconsistent with 
the hardware clock interpolation.

Another nit with Linux is that some configurations come up with the 
timer frequency of 1000 Hz rather that the design parameter of 100 Hz. 
It is possible, and indeed is the case for Ultrix (256 Hz) and OSF/1 
(1024 Hz) to change the loop parameters to match any frequency, but 
Linux hasn't done that.

Dave

Jim Houston wrote:

> On Tuesday 09 September 2008 03:07:25 clemens at dwf.com wrote:
>
>> Jim:-
>> I had also looked at the list of things that KERNEL_PLL depended on 
>> in the
>> configuration code, and was sure that none of those were defined, again,
>> wrong timex.h .
>>
>> Mumph.
>
> ...
>
>> So, what is the problem here, and what is the solution?
>> Are wrong time constants being used in the Linux kernel when STA_NANO is
>> set? Should this code NOT be being included for Linux?
>> And why in my p127 but not p115?
>> Confused.
>
>
> Hi Reg,
>
> The problem is that ntpd and the kernel need to come to a common
> understanding of the meaning of the time_constant value. I don't
> know how to make that happen.
>
> The current use of #ifdef STA_NANO to decide if the time_constant
> should be sys_poll or sys_poll-4 means that the timex.h and the kernel
> need to match. In previous versions of ntpd there was code which checked
> if the STA_NANO (or perhaps MOD_NANO) flag could be set. It used this test
> to choose between the microsecond or nanosecond interface. This allowed
> switching between microsecond and nanosecond versions of the kernel
> code without having to have two versions of ntpd built with two versions
> of timex.h.
>
> I got a bounce message from ntp:hackers on my last message
> saying, that it was waiting for the moderator to decide if it should be
> sent to the group. Maybe you could summarize our discussion and
> post it.
>
> I hope this helps.
>
> Jim Houston - Concurrent Computer Corp.
>
>
>
>
>
>
>



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