Re: [Bloat] Graph of bloat

[Jan Ceuleers <jan.ceuleers@gmail.com>]

On 08/07/15 18:32, Dave Taht wrote:
> Judging from that graphic... I don't think huff and puff was designed
> for the bufferbloated era! so the question remains, in hal's tests,
> did ntp adjust the clock backwards?

Dave,

No, ntpd did not adjust the time backwards.

What's going on in that graph is that it shows the different approach
that Google has taken to leap second insertion relative to ntpd's
implementation (which aligns with the standard).

Ntpd inserts a leap second by having the last UTC minute of the last day
of (in this case) June 30th last for 61 seconds rather than 60. The
extra second is number 60, after seconds 0 through 59. This is plotted
on the x axis.

The time reported by the Google server is compared to this on the y
axis. 10 hours prior to UTC midnight they slow down the clock by
1/72000, such that at the end of those 72000 seconds (20 hours) one more
"real" second will have passed than is reflected in the time reported by
the clock.

So obviously this means that during the 10 hours prior to UTC midnight
an offset is gradually built up between the Google servers and the rest
of the world, reaching half a second just prior to midnight. After the
leap second insertion on the standard server the sign of the offset is
inverted and the offset between the two begins to decline. 10 hours
after UTC midnight the Google servers return the rate of their clocks to
the normal "1 second per second" rate.

Anyway: Hal shared this because of the artifacts below the main graph
and he posits (quite reasonably) that this was due to bufferbloat.

Jan