netdev - Re: Extending socket timestamping API for NTP

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Message-ID: <20170324105549.34b3e778@griffin>
Date:   Fri, 24 Mar 2017 10:55:48 +0100
From:   Jiri Benc <jbenc@...hat.com>
To:     Denny Page <dennypage@...com>
Cc:     Miroslav Lichvar <mlichvar@...hat.com>,
        Richard Cochran <richardcochran@...il.com>,
        netdev@...r.kernel.org,
        "Keller, Jacob E" <jacob.e.keller@...el.com>,
        Willem de Bruijn <willemb@...gle.com>
Subject: Re: Extending socket timestamping API for NTP

On Thu, 23 Mar 2017 10:08:00 -0700, Denny Page wrote:
> I am very surprised at this. The application caching approach
> requires the application retrieve the value via a system call. The
> system call overhead is huge in comparison to everything else. More
> importantly, the application cached value may be wrong. If the
> application takes a sample every 5 seconds, there are 5 seconds of
> timestamps that can be wildly wrong.

You can add a netlink event that is sent on speed change. No need for
polling then and the wrong timestamp window will be very tiny. (It
can't be zero, even with per-packet data.)

ethtool needs to be converted to netlink, anyway.

 Jiri