[ntp:questions] Why does ntp keep changing my conf file?

Daniel Havey dhavey at yahoo.com
Mon Sep 20 18:02:56 UTC 2010


Reject it?  Why would I do that?  I'm just makeing sure of stuff before I decide what to do.  Besides, sometimes it is better to learn a little bit before I just blindly follow the advice of the old school ;^)


> I would have guessed even 802.11a or g AP would be able to
reach a couple thousand packets per second, which could
result in sub-millisecond packet timing, and 802.11n AP
perhaps few hundred thousand packets per second resulting
in a few microseconds packet timing?

Soooo, at 54Mbps sending a 1500 byte packet would take 1500 * 8 / 54M which gives me 0.2222 milliseconds per packet.  Hmmmm, perhaps there is an argument for a PPS source here.  If we are off by 1 ms then we could lose 4 or 5 packets to clock skew.

> ...but latency?  On my control plane?  No way ;^) That lan is soooo
> lightly loaded that any packet can get sent anywhere it wants at any
> time on Gigabit ethernet.

> With Gigabit you may get erratic latency due to buffering and interrupt coalescing.
Just what do you mean by erratic?  This is a modern networking device, it's going send the packet right away.  Sure a few interrupts will coalese but it's not gonna get recieve livelock or anything like that.  The GigE card is gonna get the packet out the door right away, I think any jitter will be small.



--- On Fri, 9/17/10, John Hasler <jhasler at newsguy.com> wrote:

> From: John Hasler <jhasler at newsguy.com>
> Subject: Re: [ntp:questions] Why does ntp keep changing my conf file?
> To: questions at lists.ntp.org
> Date: Friday, September 17, 2010, 5:33 PM
> Daniel Havey writes:
> > ...but latency?  On my control plane?  No
> way ;^) That lan is soooo
> > lightly loaded that any packet can get sent anywhere
> it wants at any
> > time on Gigabit ethernet.
> 
> With Gigabit you may get erratic latency due to buffering
> and interrupt
> coalescing.
> 
> > That might be a little extreme, we don't have a PPS
> source, or roof
> > access for a GPS receiver.
> 
> Since all you want is synchronization among your machines
> your PPS could
> free run.  You should be able to build one.  It's
> trivial: just a stable
> one Hertz pulse generator.
> 
> > If we can get down to a millisecond then we are
> golden.
> 
> Then you don't need a PPS source.
> -- 
> John Hasler 
> jhasler at newsguy.com
> Dancing Horse Hill
> Elmwood, WI USA
> 
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> 


      



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