[ntp:questions] A proposal to use NIC launch time support to improve NTP
mlichvar at redhat.com
Wed Dec 19 16:41:43 UTC 2012
On Wed, Dec 19, 2012 at 11:05:57AM -0500, Brian Utterback wrote:
> On 12/19/12 10:12, Ulf Samuelsson wrote:
> >The desired launchtime is compared to the network controller
> >timestamp counter in H/W, so again there is no need to synchronize
> >with the system time.
> Yes there is. The ntpd program has to set a timestamp in the
> outgoing packet and then specify the launchtime when it writes the
> packet. The goal here is to have the timestamp written in the packet
> exactly match the time the packet actually hits the wire. So, the
> timestamp in the packet must be a little in the future when it is
> written so that by the time the controller gets it the packet can be
> delayed until the right time. Since ntpd cannot access the clock in
> the controller, this requires that the kernel time be relatively
> close to the controller time.
ntpd can read the clock, much more slowly than the system clock, but
still fast enough to send tens of thousands of packets per second.
I think it makes more sense to have one loop controlling just the PHC
and another, much tighter, syncing the system clock from the PHC,
rather than trying to sync the system clock through the PHC.
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