On 2010-09-20, Daniel Havey <[email protected]> wrote: > 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.
Experiment says you are wrong. I used to have my machines on regular ethernet (110MB) and the time delay was 140usec almost withoug fail. With a GB switch and GB cards, it is all o\ver the place, with a bimodal ( 140 and about 300usec roundtrip ) up to 10ms round trip. It is really really horrible for timing purposes. > > > > --- On Fri, 9/17/10, John Hasler <[email protected]> wrote: > >> From: John Hasler <[email protected]> >> Subject: Re: [ntp:questions] Why does ntp keep changing my conf file? >> To: [email protected] >> 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 >> [email protected] >> Dancing Horse Hill >> Elmwood, WI USA >> >> _______________________________________________ >> questions mailing list >> [email protected] >> http://lists.ntp.org/listinfo/questions >> > > > = _______________________________________________ questions mailing list [email protected] http://lists.ntp.org/listinfo/questions
