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

Brian Utterback brian.utterback at oracle.com
Wed Dec 19 21:49:58 UTC 2012


On 12/19/12 14:05, unruh wrote:
> 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.
>
>

Generally, the PPS signal does not go over the PCI bus. The kernel gets 
its PPS signal via the serial port. You would therefore like the 
controller to have its own PPS signal input, but I don't see one in the 
datasheet.

So you are back to worrying about the sync of the kernel clock and the 
controller clock. It might not matter too much, but it will kind of 
depend on how the receive timestamp is obtained from the card. The 
receive timestamp and transmit timestamp have got to be on the same time 
source or you could run into problems.

Brian

-- 
blu

Always code as if the guy who ends up maintaining your code will be a
violent psychopath who knows where you live. - Martin Golding
-----------------------------------------------------------------------|
Brian Utterback - Solaris RPE, Oracle Corporation.
Ph:603-262-3916, Em:brian.utterback at oracle.com



More information about the questions mailing list