Re: [chrony-dev] Nanosecond timestamps

[Denny Page <dennypage@xxxxxx>]

On Nov 08, 2016, at 03:01, Miroslav Lichvar <mlichvar@xxxxxxxxxx> wrote:

In addition to the kernel, I disable eee and interrupt coalescing on the network interfaces.

Oh, I knew ethernet has some power saving features, but I didn't
realize they could increase latency/jitter. I'll need to experiment
with this :).

The value of wake time is negotiated between the ethernet partners. I believe the max for 100Mb is 30us, and 16us for 1000Mb. In other words, for 100Mb, which is what hardware NTP solutions commonly use, the wake delay can theoretically be larger than the NTP packet transmission time. Usually it ends up being much smaller, but it is still still a significant jitter impact in a local network.

Denny