[ntp:questions] high precision tracking: trying to understand sudden jumps

David Woolley david at ex.djwhome.demon.co.uk.invalid
Sun Mar 30 23:00:58 UTC 2008


Unruh wrote:
>> 
> 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 
measure.
> 
> 
> 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 
here.




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