[ntp:questions] Time reset

Venu Gopal neo.venu at gmail.com
Thu Mar 13 08:39:47 UTC 2008


Hi Martin,

Thanks for the insight.

I was struggling to convince people here (technically!).
Never knew that its a well know issue that has been
discussed several times in the forum.

Thanks to Harlan for his references.

Venu

Martin Burnicki wrote:
> D.Venu Gopal wrote:
>> David Woolley wrote:
>>> Venu Gopal wrote:
>>>
>>>> Its clear that CPU is heavily loaded which might be leading to loss of
>>>> ticks. Yet to check the DMA status for
>>> CPU loading doesn't cause lost timer interrupts. (More  precisely
>>> overruns.)
>>>
>> So its the DISK I/O thats causing loss of ticks ?
>>
>>>> IDE DISK. I'll be out for about a week, after returning I'll
>>>> give few more stats based on combination of CPU load, Disk I/O
>>>> and Network I/O.
> 
> There have been several examples of drivers which kept interrupts disabled
> for too long, so that timer ticks couldn't get through. In most cases
> (AFAIK) this have been drivers for IDE disks, especially if they didn't use
> DMA. 
> 
> This has happened across several operating systems (I remember Linux,
> Windows, and formerly OS/2), with drivers which had not been designed
> properly. So this depends on a specific version of the OS and a specific
> version of a specific driver. You can not say in general that IDE drivers
> cause lost timer ticks, but they are good candidates.
> 
> Unfortunately the new clock routines in the Linux kernel seem to be causing
> problems sometime. This seems to be due to certain combination of a clock
> module which handles a particulare timer on the mainboard and the
> particular timer the implementation of which may vary by the chipset.
> 
> This is not exactly the same as lost timer ticks, but the results are
> similar, i.e. the clock drift can be so large or changing so much that ntpd
> fails to correct it.
> 
> Martin




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