On 10/18/06, Glenn Ramsey <glenn(a)componic.co.nz> wrote:

Hi Ian,

Ian McDonald wrote:

...

I'm wanting to sync two machines to within 1 msec for my research using ntp.

Questions: - is this realistic given how clocks drift? Has anybody done this? - is there a quick way to force them to converge

I've had a quick read of the ntp pages and made one of my machines a broadcast server and the other broadcast clients I think but they seem to be taking a while to converge. The man pages say they will fix at 0.5ms/sec but I'm not seeing that.

Comments appreciated.

NB I don't want my clocks to be deadly accurate to the "real world", just to each other.

A while ago I looked into this a little and from what I recall 1ms is a big ask for a commodity PC, 10ms I think was achievable. As I recall it a major issue is the accuracy of the PC's internal clock. NTP does account for this by working out how much the PC's clock drifts but that still only gets it so far.

I think to get that accuracy you need to equip your PC with a more accurate local clock. I think some sort of GPS card is commonly used.

There's screeds of info about this issue on the NTP related websites. Google should find it.

Also there's another protocol around that is used for syncing PC's but can't remember what it was called.

That probably wasn't that much help, but it might help you to look in the right places.

Glenn

Thanks to everybody's comments and feedback. I've actually managed to code up something myself by building a simplified NTP and it gets me down to around 1 msec. Ian -- Ian McDonald Web: http://wand.net.nz/~iam4 Blog: http://imcdnzl.blogspot.com WAND Network Research Group Department of Computer Science University of Waikato New Zealand