[ntp:questions] Visualization of clock control

David Lord snews at lordynet.org
Thu Jan 12 12:17:13 UTC 2012


Hal Murray wrote:
> In article <20120105125942.GA15654 at localhost>,
>  Miroslav Lichvar <mlichvar at redhat.com> writes:
> 
>>> The other difficulty with respect to real life may be modeling network jitter
>>> as exponential, since I believe the probability distribution for network delays
>>> is heavy-tailed (i.e. with extreme values way over-represented; this is a problem
>>> when using statistics which assume the underlying error distribution is gaussian).
>>> I don't know how to fix that, though.
> 
>> I'd definitely be interested in a better model for network delays. I
>> guess we could try to make a collection of the ntp rawstats logs from
>> various network environments and see how the distribution looks like.
> 
> My 2 cents...
> 
> I have a 384K DSL line.  I'd say it runs in one of 3 modes.
> 
> 1) Mostly idle.  There is a little background traffic: NTP,
>   fetchmail checks and occasional fetches.  yum checks at 3 AM...
> 
> 2) Big download.  Occasionally, I download something like a CD.
> That puts a consistent load on the download direction.  The queueing
> delays go up to ballpark of 1 second.  ntpd doesn't like that.
> After a while it sometimes to a bogus offset.  huffpuff helps,
> but a big download can take a long long time.
> 
> 3) Flurry of web downloads.  If I'm browsing the web, I occasionally
> hit a page with lots of pictures and such.  Mozilla opens lots of 
> TCP connections.  The round trip time goes over 3.5 seconds.
> 
> I don't think any of that fits an exponential tail.
> 
> I've got lots of log files.  Maybe I should make a histogram...
> 

I'm on 2Mbit adsl with maximum upload of 288 kbit. Actual
uploads were topping out at only about 130kbit until I enabled
altq traffic shaping with a cap at about 250kbit. I now hit
250kbit on upload. Certain traffic also gets priority including
ntp. Apart from when my servers are being hammered by abuse
this has worked very well.


David



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