[time-nuts] sync computer clock ticks

[Bruce Griffiths]

M. Warner Losh wrote:
> In message: <477E816D.5010803 at vogel.cx>
>             Christian Vogel <vogelchr at vogel.cx> writes:
> : For example, do you want to timestamp interrupts to synchronize 
> : machinery? Or just note down the timestamps of bittorrent downloads very 
> : precisely? Just synchronizing an otherwise idling computer is probably 
> : much easier than a machine that is doing a lot of additional work that 
> : can mess up the timekeeping by clogging the processor or just creating 
> : varying stress on the power-supply lines.
>
> That's the real question.  Like I said before, when you are down to
> the microsecond level, what good is that?  The jitter from system
> calls and such is going to be high enough to make that not completely
> useful.  And if you are trying to do things to hardware with software
> that requires that level of synchronization, you aren't going to get
> it without timestamping done in hardware of some mutually observable
> event (pps, packets on the network, etc).
>
> Warner
>   
PTP achieves submicrosecond synchronisation over small LANs without
network switches.
It uses hardware time stamping of LAN packets.
Someone has even implemented it in software (with degraded performance).
See:
http://ieee1588.nist.gov/

Bruce