Large and apparently permanent step in PPMerror - confused? - NTP

This is a discussion on Large and apparently permanent step in PPMerror - confused? - NTP ; Hi, I am syncing a Linux 2.6.26 kernel to a Serial DCD PPS GPS source. The kernel time has been running for quite some time with a ~160ppm frequency error however inexplicably the ppm error over night simply jumped to ...

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Thread: Large and apparently permanent step in PPMerror - confused?

  1. Large and apparently permanent step in PPMerror - confused?

    Hi,
    I am syncing a Linux 2.6.26 kernel to a Serial DCD PPS GPS source.
    The kernel time has been running for quite some time with a ~160ppm
    frequency error however inexplicably the ppm error over night simply
    jumped to ~105ppm. The system has for the last couple of day tracked in
    a stable manner at that ~105ppm rather than the ~160ppm offset it used
    to be.

    There has been no change to the system that I am aware of. The machine
    was not rebooted. There has been no dramatic change in temperature
    although I think temperature change would be very dramatic cause 55ppm jump.
    (In fact you can see the daily temperature variation in the ppm error
    which is in the order of a couple of ppm over time)

    I have a plot of the kernel offset and ppm error as well as the time
    offset to a stable GPS steered internet peer in my area.
    http://dcureton.com/ppm_jump.jpg

    I am at a loss to explain why this jump occurred and whether to expect
    it to occur in the future. Does anyone have any ideas to the cause of
    this jump in PPM error. System is on a UPS so unlikely to be a power
    supply issue. I am stumped?

    Thanks,
    David

  2. Re: Large and apparently permanent step in PPM error - confused?

    David Cureton wrote:
    > Hi,
    > I am syncing a Linux 2.6.26 kernel to a Serial DCD PPS GPS source.
    > The kernel time has been running for quite some time with a ~160ppm
    > frequency error however inexplicably the ppm error over night simply
    > jumped to ~105ppm. The system has for the last couple of day tracked
    > in
    > a stable manner at that ~105ppm rather than the ~160ppm offset it used
    > to be.

    []
    > I am at a loss to explain why this jump occurred and whether to expect
    > it to occur in the future. Does anyone have any ideas to the cause of
    > this jump in PPM error. System is on a UPS so unlikely to be a power
    > supply issue. I am stumped?
    >
    > Thanks,
    > David


    Crystals can sometimes jump in frequency, although I'm surprised to see a
    modern unit do so, and that is quite a large step. How old is the system?

    David


  3. Re: Large and apparently permanent step in PPM error - confused?

    David J Taylor wrote:
    []
    > Crystals can sometimes jump in frequency, although I'm surprised to
    > see a modern unit do so, and that is quite a large step. How old is
    > the system?
    > David


    Also - it could be that a capacitor in the oscillator circuit has either
    become open circuit, or has become disconnected. As long as the
    oscillator restarts consistently, I would not worry about that. I /might/
    be a little more concerned about crystal frequency steps.

    Cheers,
    David


  4. Re: Large and apparently permanent step in PPM error - confused?

    In article <490BA73F.7070500@dcureton.com>,
    david.cureton@dcureton.com (David Cureton) writes:
    >Hi,
    > I am syncing a Linux 2.6.26 kernel to a Serial DCD PPS GPS source.
    >The kernel time has been running for quite some time with a ~160ppm
    >frequency error however inexplicably the ppm error over night simply
    >jumped to ~105ppm. The system has for the last couple of day tracked in
    >a stable manner at that ~105ppm rather than the ~160ppm offset it used
    >to be.
    >
    >There has been no change to the system that I am aware of. The machine
    >was not rebooted. There has been no dramatic change in temperature
    >although I think temperature change would be very dramatic cause 55ppm jump.
    >(In fact you can see the daily temperature variation in the ppm error
    >which is in the order of a couple of ppm over time)


    50 ppm is a huge change. That's either something quite rare in
    the hardware or an interesting software quirk.

    Is there anything interesting in syslog at about that time?
    Did it get warm and shift to a slower CPU frequency?

    There is a bug in the Linux 2.6 kernels. The TSC calibration
    routine doesn't get the same answer each time. So you could get
    something like that if you rebooted. But you said it wasn't
    rebooted so this seems unlikely.

    How long has the system been up? I wonder if some parameter
    finally overflowed N bits and is now taking a slightly different
    path through some code.

    --
    These are my opinions, not necessarily my employer's. I hate spam.


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