How much we can trust packet timestamping

uaca
2002-12-30T11:20:22Z

Hi all

IMHO The problem is quite complicated because

+ common hardware is not designed for real time:

- sends multiple PDUs within one interrupt, and can be delayed
- Host adapter bus & infraestructure is not designed to garantee latency
etc...

+ software is also not designed for realtime

- drivers may timestamp in softirq's
- irqs has no deterministic latency either
etc...

So even if do_gettimeofday() has 1/CPUfreq resolution by using TSC register
packet timestamping meassurement is biased, how much?

anybody has studied this? are there reports/doc about this topic?

of course, this can be avoided by using specialiced hardware, but I'm not
interested on that and I would like to know how much I can trust this
timestamps

Any comment would be greatly appreciated

Thanks

Ulisses

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