[ntp:questions] high precision tracking: trying to understand sudden jumps
david at ex.djwhome.demon.co.uk.invalid
Sun Mar 30 23:00:58 UTC 2008
> I expect that he means the offsets that ntp measures. NTP does NOT correct
I suspect that too.
> random offsets. Ie, if there is noise source which makes the offsets vary
It averages them so as to reduce their effective size.
> by 500usec ntp will not get rid of them. You will see them in the offsets
> as measured by ntp. Now, the time keeping might (or might not) be more
> accurate than that, but those offsets are what I suspect he means.
The question is about "measured errors" that significantly exceed the
random offsets. In any case the systematic error can also greatly
exceed the measured offset - that represents an error that ntpd cannot
> Almost all disk drives on Linux now use dma.
They need to do both and the drivers that caused this problem were
capable of using DMA. The problem was, I believe, that certain chipsets
were unsafe with DMA, so the default, at least used to be, the
unconditional one of doing programmed transfers; you could enable DMA at
your own risk.
My impression is that there are still enough systems with lost disk
interrupts that someone reporting one tick backward steps can reasonably
be assumed to have that problem, and it is a reasonable probability for
someone who doesn't report the direction of the step. The other common
cause of steps, which are balanced in both directions, is not applicable
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