[ntp:questions] 1Gb/s network issues - was: Re: OT: GPS18x LVC failure

Kasper Pedersen spam at kasperkp.dk
Mon Oct 12 17:01:34 UTC 2009


On 10/12/2009 12:00 PM, David J Taylor wrote:
>
> "Unruh" <unruh-spam at physics.ubc.ca> wrote in message
> news:QsBAm.47742$Db2.29202 at edtnps83...
> []
>> Actually those Gb ethernet switches seems to have some really bad and
>> variable

> so whether being very simple and unmanaged is likely to mean that it's
> performance is better or worse I leave to speculation or measurement!

I did some tests on Linux. As expected there's variable latency and 
jitter in each direction, in addition to what the network provides. 
Something that gives really large offsets (obviously) is when the 
network stack decides/has to do an ARP, since the other end has one 
second to answer. I measured purely at the receive path: there is the 
jitter from interrupt coalescing in the nic, plus the kernel interrupt 
latency. The receive path is interesting since the kernel provides 
packet receive timestamps, you don't have to wait for the packet to make 
it up to userspace.

I built a device that spits out priority tagged ethernet frames with 
<200ns error to UTC, and did a number of tests with different networks 
and nics, the result of which is in a table here:

   http://n1.taur.dk/etherpps/

On switches that understand priority tagging it makes a world of 
difference. On the corporate network where I had 14us unaccounted 
latency and 18us jitter with the packet prioritized above all other 
traffic, when the prioritization was disabled I got another 200us jitter 
at peak load time (users and iscsi traffic sharing the same path).

I suppose there's a way to make the kernel priority tag outbound ntp 
packets, but I did not find it.

/Kasper Pedersen




More information about the questions mailing list