On 10/12/2009 12:00 PM, David J Taylor wrote: > > "Unruh" <[email protected]> wrote in message > news:qsbam.47742$db2.29...@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 _______________________________________________ questions mailing list [email protected] https://lists.ntp.org/mailman/listinfo/questions
