[ntp:questions] A proposal to use NIC launch time support to improve NTP

unruh unruh at invalid.ca
Wed Dec 19 19:05:43 UTC 2012

On 2012-12-19, Hal Murray <hal-usenet at ip-64-139-1-69.sjc.megapath.net> wrote:
> In article <50D1C5B9.8020701 at oracle.com>,
>  Brian Utterback <brian.utterback at oracle.com> writes:
>>No, you are missing the point. You have two clocks in this scenario, the 
>>kernel clock and the network controller clock. If one gets a good time 
>>then you have to set the other from it. This means that this time will 
>>have to travel over the PCI bus which will introduce jitter.
>>Now, if you have a PPS signal available and can provide it to both the 
>>network controller and the kernel, then you don't have this problem 
>>since the PPS signal will sync the time to an accuracy better than the 
>>jitter that was introduced.
> Doesn't the PPS signal to the kernel have to go over the same PCI bus?
> I'd guess that you would get better results from a network card.
> That's assuming it has a good clock.  All you have to do is read
> a counter.  There is no interrupt latency.  You can also read it
> several times and pick the best one.

Pick the best one? How would you know what the best one was?
Not sure what you mean by a "good clock". It certainly will not be an
accurate clock. It may be one whose drift rate is not too bad, although
I suspect it will change with temperature. 


More information about the questions mailing list