Miroslav, I believe that the hardware NTP device, chrony, or both, are striking/calculating timestamps incorrectly. I have a test in mind that will allow me to determine if this is correct, and if so which. Back to you soon.
Denny > On Nov 18, 2016, at 00:00, Miroslav Lichvar <mlich...@redhat.com> wrote: > > On Thu, Nov 17, 2016 at 05:49:44PM -0800, Denny Page wrote: >> This port speed differential appears to result in a asymmetry in >> transmit/receive time which significantly affects the calculations. If I >> lock the monitor host port at 100Mb, all three units show precise >> synchronization, both with hardware and software time stamping. As noted >> previously, with the monitor host port at 1Gb, I see ~300ns (positive) with >> software and ~2200ns (negative) with hardware. > > Very interesting! > >> I’ve spent many years on latency in networks, but have never come across >> this specific issue. I would like to get my head around how the asymmetry >> comes about, and how much it is. I am continuing to research this. I believe >> I generally understand how asymmetry affects the calculations, but would >> appreciate any guidance you can offer in terms of quantifying how much >> asymmetry is required to produce the offsets seen. Also any reason that you >> can think of for the offset to be positive with software timestamps, but >> negative with hardware timestamps. > > The general rule is that in order to see a positive increase in offset > of d, the delay of packets from the server to the client needs to > increase by 2 * d. So, in your case if we take the offset of the local > unit as a reference, we see an increase of 600ns in the client->server > delay with SW timestamping and an increase of 4400ns in the > server->client direction with HW timestamping. > > I don't know much about networking HW and I can only speculate. I > suspect that if the link speeds don't match, the switch is forced to > buffer the data and this buffering takes longer when going from 100mb > to 1gb than when going from 1gb to 100mb. This might explain the > offset with HW timestamping. > > In the case with SW timestamping, maybe the lower speed of the link to > the local unit increases the delay of the RX interrupt for some > reason? Maybe coallescing is not completely disabled and the delay > takes into account the link speed? I've no idea. It would be great to > hear from someone who is familiar with the HW and network driver. > > -- > Miroslav Lichvar > > -- > To unsubscribe email chrony-dev-requ...@chrony.tuxfamily.org with > "unsubscribe" in the subject. > For help email chrony-dev-requ...@chrony.tuxfamily.org with "help" in the > subject. > Trouble? Email listmas...@chrony.tuxfamily.org. > -- To unsubscribe email chrony-dev-requ...@chrony.tuxfamily.org with "unsubscribe" in the subject. For help email chrony-dev-requ...@chrony.tuxfamily.org with "help" in the subject. Trouble? Email listmas...@chrony.tuxfamily.org.