Hal Murray wrote:
In article <20120105125942.GA15654@localhost>,
Miroslav Lichvar <mlich...@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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