[ntp:questions] Effect of Gigabit Interrupt coalescence on ntp timing

Terje Mathisen "terje.mathisen at tmsw.no" at ntp.org
Wed Jun 13 06:22:35 UTC 2012


unruh wrote:
> On 2012-06-12, David Malone <dwmalone at walton.maths.tcd.ie> wrote:
>> some of these cards actually timestamped the frames when received
>> and then the timestamp provided by SO_TIMESTAMP or similar could
>> be corrected. It seems only a few cards can do this though.
>
> It would be nice. But then what clock would they use to timestamp the
> packets. If it is an onboard network card clock one would then also have to
> discipline that network card clock ( or at least calbrate it, and as we
> know that calibration changes so it would have to be another continuous
> calibration of that clock.)

We would _NOT_ need to tune that clock, a static measurement of the rate 
is more than enough!

Assume a really bad on-NIC crystal, supposed to be 10 MHz, but actually 
off by 1000 ppm, i.e. an order of magnitude worse than most of the 
really cheap motherboard crystals:

The maximum time from packet arrival until the interrupt service routine 
can grab it seems to be around a ms, while most packets are handled in a 
us or two, right?

Since the NIC clock only needs to measure the time between packet 
arrival and ISR read, an error of 1000 ppm over a full ms interval 
corresponds to 1 us total, while for all packets that are serviced in 
less than 100 us, the measurement error will be 100 ns, since that is 
the resolution of the 10 MHz timer.

OK?

BTW, all the 1588-capable NICs out there, and there must be quite a few 
of them now, will do more than this, and with a better on-board timer.

Terje
-- 
- <Terje.Mathisen at tmsw.no>
"almost all programming can be viewed as an exercise in caching"




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