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Date: Mon, 30 Dec 2002 12:28:38 +0100
To: linux-kernel@vger.kernel.org
Subject: How much we can trust packet timestamping
Message-ID: <20021230112838.GA928@pusa.informat.uv.es>
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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

                Debian GNU/Linux: a dream come true
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