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

Ulf Samuelsson ulf at invalid.com
Thu Dec 20 11:36:07 UTC 2012


On 2012-12-20 10:32, Hal Murray wrote:
> In article <beoAs.13296$KS4.12744 at newsfe11.iad>,
>   unruh <unruh at invalid.ca> writes:
>
>> Pick the best one? How would you know what the best one was?
>
> Read TSC, read clock, read TSC.  Subtract the TSC values to
> get an upper limit on how long it took to read the clock.
> Best is fastest.
>
>
>> 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.
>
> I was thinking that the error in a clock on the network card after
> reading by the CPU would be better than one done with interrupts
> on the CPU.  Yes, I was assuming something like a sane crystal
> and yes it would probably drift with temperature.
>
>

Luckily there is never a need to read the SYSTIM counter directly,
since you read H/W timestamps in CMSGs.

BR
Ulf Samuelsson



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