[ntp:hackers] Of Linux and jitter

David L. Mills mills at udel.edu
Tue Oct 19 16:08:31 PDT 2004


Albert segfaulted well into the protocol dance with the debug trace 
turned on. With the debug trace turned off, iit faulted within a couple 
seconds. I have not concluded anything about the serial port code, just 
that it has gotten far worse in recent Solaris versions and not in other 
systems. And, I don't think it is the radio; I have tried different 
radios on different Sun boxes and the radios all look the same.

I don't know what to think about the worsening nature of Sun I/O. First 
it was the audio codec/mixer jitter, which went from 20 microseconds to 
ten milliseconds, not the serial port, which went from 100 microseconds 
to a second and a half. I cannot gaurantee the jitter is not some silly 
aspect of the code which something in recent Solaris has exposed, but 
from all evidence it sure looks like something in the kernel. A 
1.5-second popcorn spike is just not credible.


mayer at gis.net wrote:

>----- Original Message Follows -----
>>1. All the backroom and campus machines are now running ntp-dev and
>>all  seem to work except Linux albert on the campus wire. It segfaults
>>after  a few minutes. I am dumb with Linux debugging tools, so I'll
>>leave the  carcass on the hope some Linux jockey can look at it.
>We've seen that. We suspect a bad resolver library. Harlan may have
>more to say about this.
>>2. I'm very suspicious about the Solaris jitter using a serial port
>>and  any of several radios. The jitter can be over one second(!) and
>>varies  in intensity with time. I can't believe the port interrupt
>>routine can  have jitter that high. The jitter does not occur with a
>>SunOS SPARC IPC  running ntp-dev, which is now the best timekeeper we
>>have. The jitter  first showed up on Ultra-1 pogo running Solaris 8/9
>>and Blade 1500  running Solaris 9. Could there be something in the
>>ntpd I/O code causing  this? It's only in the serial port code; there
>>is no such jitter in the  packet I/O code.
>I haven't touch ANY of the serial port code, on the packet code. Maybe
>changes in one of the refclocks affects this?
>>hackers mailing list
>>hackers at support.ntp.org

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