[ntp:questions] Re: System clock

David Woolley david at djwhome.demon.co.uk
Fri Oct 7 22:43:24 UTC 2005


In article <43467245.644D67E3 at ssd.usa.alcatel.com>,
ssubrama at ssd.usa.alcatel.com (SivaKumar Subramani) wrote:

> Actually my requirement is to run a clock  which can be controlled by NTP
> daemon process. I  want NTP to manage the clock functionality to do on my
> clock not on the system clock.

In theory this could be done by accumulating the phase correction that
would have been accumulated on a properly controlled clock (including
interpolating the effects of kernel PLLs, etc).  But see below.

>                               Actually a virtual clock shall be maintained by
                                                         ^^^^^
> the NTP daemon process with system clock functionality. Because the system

"the NTP daemon" obvious has a special meaning in this specification,
we need to know what that is as it doesn't have the meaning that would
normally be understood; whoever wrote this, rather legalistic,
specification has no authority to specify that's behaviour.

The general style of this specification looks like it was written by
someone who felt they had to justify themselves as a specification writer,
or was written by a competing bidder in order to load the requirement in
favour of their solution.  (I've seen both over specification through
inexperience and overspecification by a consultant whose bidding.)

> clock shall be controlled by some other program in the network, this shall
> distribute the TOD to the entire network including my processor. This TOD
> clock sometime may be set by the user manually.This TOD process shall get the

This is a killer.  It means the only way that you can meet the requirement
is to completely duplicate the software clock processing; that can't be
done within a user space program like ntpd, it can normally only be done
by modifying the kernel source code.  To track the accumulated correction,
you have to assume that no corrections of any kind are being applied.

Again, we have to ask:  "what is the real requirement?", and by that we
don't mean what the specification document says but the reasons that 
forced the specification document to say them.

> I've attached the architecture diagram to make it further clear on the

EIther you didn't attach it or it got stripped by the gateway.  This
is primarily a newsgroup and it is a discussion newsgroup, so binaries
would not have been acceptable.




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