[ntp:questions] Time reset

Martin Burnicki martin.burnicki at meinberg.de
Thu Mar 13 08:21:56 UTC 2008


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
-- 
Martin Burnicki

Meinberg Funkuhren
Bad Pyrmont
Germany




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