Re: Time reset
- From: andy.helten@xxxxxxxxxxxx (Andy Helten)
- Date: Fri, 4 Apr 2008 02:22:44 GMT
andy.helten@xxxxxxxxxxxx (Andy Helten) writes:
Another disadvantage with preventing steps is that it isn't really a
supported mode (because it's a "tinker") and, as I've found, it doesn't
always work. When I disable time steps on a linux 2.6.18 kernel, the
drift value goes to +/-500 and can actually swap sign from one run to
the next. This happens even though a time step was never needed (i.e.
offset never went >128ms). With time steps enabled the drift value
settles <90ppm (and again, no step actually occurs).
That certainly sounds like a bug to me.
Me too, but disabling time step is a tinker and tinkers are generally
use at your own risk. Besides, after much testing, I'm fairly certain
the problem is indeed with the kernel -- especially considering I did
not have this problem on an older kernel.
From what I've been able to piece together, this different behaviorbetween step/!step is probably due to the kernel time discipline being
disabled with !step, coupled with a (potential) bug in linux that forces
NTP's "manual" adjustments to have a granularity of 1ms (i.e. somewhere
an adjustment is rounded up or down). I've not verified the bug is
present in my 2.6.18 linux kernel, so don't quote me on it. One might
ask why the kernel time discipline is preemptively disabled in this
manner -- maybe there is a good reason.
AFAIK it is not the kernel that does the time step. Ie, the kernel
discipline is not what demands the step. Also, adjtime certainly does not
have a 1ms granularity.
That is also my understanding, that the kernel does not perform the time
step but it is the kernel that updates the system time every tick. My
understanding of the kernel time discipline is that NTP sets the size of
the update to account for the system clock drift. This mechanism is
apparently disabled when time stepping is disabled. Don't ask me why.
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