[vdr] Re: Time jumps

[Wolfgang Fritz <wolfgang.fritz@gmx.net>]

Sergei Haller wrote:

> On Wed, 25 Dec 2002, Wolfgang Fritz (WF) wrote:
> 
> WF> >>one possibility would be to do the following:
> WF> >>   if the system clock is off by at least XX seconds (what is a
> good
> WF> >>   value? 2? 10? 20?), then set the clock, causing a time jump
> on the
> WF> >>   system clock.
> WF> >>   if the diff is less than XX seconds, use adjtime.
> WF>
> WF> You  may  estimate  the  systematic  drift  of  your  system 
> clock by
> WF> comparing  it  to  a reliable external clock source (don't know if
> the
> WF> satellite  time  is  good  enough)  and then compensate the
> systematic
> WF> drift  with  adjtimex  (see  man  adjtimex).  This way the system
> time
> WF> should  always  be  in  the  "adjtime range" and no clock jumps
> should WF> happen.
> WF>
> WF> I am doing this on a server by checking the time against an NTP
> server
> WF> once  a  day.  The  systematic drift of my clock is abt 24
> seconds/day WF> (!!) and I am able to compensate it to less than
> 25ms/day.
> 
> I  assume,  your server is running 24h/day. What happens if the
> machine is shut down? I assume it still drifts by 24sec/day while it
> is switched off. Most (many?) vdr machines are switched off more than
> 12h/day (I assume so)

Yes ist is. But you can use the hardware clock to adjust the system 
clock on boot with the hwclock command (see man hwclock). The drift of 
the hardware clock is stored in /etc/adjtime (confusing name, this file 
is for hwclock, not for adjtimex)

> 
> Is the correction amount stored in some file and is used
> automatically? Or do you have to call adjtimex each time you boot?
>
I think you have to call adjtimex on each boot. I would store the adjust 
parameters in a file and call adjtimex from an init script together 
with hwclock.
 
> anyway,  it's  a  good idea (-> only one correction per power cycle of
> the vdr machine would be needed)
>
I would try the following setup to set and keep a good system clock:

1. Adjust the system clock from the CMOS clock on boot:
   hwclock --hctosys. If you have a valid /etc/adjtime file, you will
   already get a rather good system time on boot. The SuSE distro does
   that already in its standard init scripts, I don't know what
   other distros do.
2. Setup a periodic check of the system time against a reliable
   clock source. From this you can determine the systematic drift of
   the (soft) system clock. Set the CMOS clock to the system clock
   in this script with hwclock --systohc directly after you
   have adjusted the soft clock. This generates the
   /etc/adjtime file. The best way to do this is via a NTP server.  
   I have not tried to use a time reference with low resolution
   as the transponder time seems to be (1 second). 

> could  you  send  me  an  example  how  you're  using  adjtimex? (I do
> not undestand exactly, what all of the modes are doing)
>
 
I'm only using the --tick and --frequency parameters. I calculate the 
values with a perl script from the output of the ntpdate command which 
contains the clock offset. I call ntpdate when I go online (dialup 
connection), but maximum 1 times per day.

> BTW.:  the  glibc  implementation of adjtime is just calling adjtimex
> with mode ADJ_OFFSET_SINGLESHOT and an offset. but this seems not to
> make the changes permanent.
> 
No, adjtime(x) adjusts only the software clock and changes are lost on 
power down. You 

> another  BTW.:  adjtimex  is  Linux  specific  and  should  not be
> used in
> programs  intended  to be portable. Is VDR a linux-only program or
> does it run on other unix systems.
>

Yes, it is linux specific. I don't know if other OSes provide a similar 
functionality.

Wolfgang
> c ya
>         Sergei




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