Re: [Bloat] Graph of bloat

[Rich Brown <richb.hanover@gmail.com>]

On Jul 8, 2015, at 11:55 AM, Dave Taht <dave.taht@gmail.com> wrote:

> On Wed, Jul 8, 2015 at 3:23 AM, Hal Murray <hmurray@megapathdsl.net> wrote:
>> I was monitoring Google's time servers over the recent leap second.  That
>> graph happened to include some good examples of bloat.
>> 
>> http://www.megapathdsl.net/~hmurray/bloat/google-off-smear-bloat.png
>> 
>> I have a slow DSL line with almost 4 seconds of buffering.
>> 
>> The blobs at -20 and -15 seconds are typical of a single large download.  The
>> column at -6 seconds is typical of several active connections.
> 
> That is a very interesting graph! Does ntp adjust system time backward
> based on getting nearly all it's samples with well over a 1/2 second
> of induced delay?

There was a good discussion of this on the NANOG list about a week or so ago, lamenting the leap-second. Lots of learned (and other) people chimed in on the pro's and con's. http://mailman.nanog.org/pipermail/nanog/2015-June/076540.html if you're terminally interested.

I think that chart is the clock offset of the Google time servers compared to the Menlo Park  NTP server (which is "true time"). At 10 hours before midnight (and the arrival of the leap second), the Google servers report the seconds to be a tiny bit longer than a true second, so that by midnight, they're a full 500 msec (half-second) "ahead". When the leap-second drops in, they're a half-second behind "true" time, and it continues the ramp for the next 10 hours to be back "in sync".

(I may have the sense/direction of this wrong, but you get the idea...)

Rich