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.
> 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
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