Re: [chrony-dev] SW/HW timestamping on Linux

[Denny Page <dennypage@xxxxxx>]

I still have one significant offset mystery with the latest version.

In the tests described below, the following IP addresses are involved:

  192.168.230.2 Linux box. igb0 connected to main switch at 1Gb.
  192.168.230.3 Linux box. igb0 connected to main switch, speed as described in tests. igb3 connected as described in tests.
  192.168.230.240 hardware NTP server connected to main switch at 100Mb.
  192.168.230.244 hardware NTP server connected to main switch at 100Mb.
  192.168.230.245 hardware NTP server connected to igb3 on 192.168.230.3 as described in tests.

Relevant config from 192.168.230.2:
  server 192.168.230.240 iburst minpoll 0 maxpoll 0
  server 192.168.230.244 iburst minpoll 0 maxpoll 0
  peer 192.168.230.3 iburst minpoll 0 maxpoll 0 xleave noselect
  hwtimestamp igb0

Relevant config from 192.168.230.3:
  server 192.168.230.240 iburst minpoll 0 maxpoll 0
  server 192.168.230.244 iburst minpoll 0 maxpoll 0 
  server 192.168.230.245 iburst minpoll 0 maxpoll 0 noselect
  peer 192.168.230.2 iburst minpoll 0 maxpoll 0 xleave noselect
  hwtimestamp igb0
  hwtimestamp igb3

On 192.168.230.2, igb0 is an Intel I211. On 192.168.230.3, igb0 and igb3 are both ports of an I354. Both Linux boxes show ‘H H’ in the measurements log for all servers/peers past the first few samples. Linux kernel version is 4.4.26, although I’ve done spot checks with 4.8.10 and seen similar results. 

Here are the various test restuls:

  igb0 @ 1Gb; igb3 @ 100Mb direct connect: 192.168.230.245 shows offset of +1230ns
  igb0 @ 100Mb; igb3 @ 100Mb, direct connect: 192.168.230.245 shows no offset
  igb0 @ 1Gb, igb3 @ 1Gb via secondary switch: 192.168.230.245 shows small offset of +75ns
  igb0 @ 1Gb, igb3 @ 100Mb via secondary switch: 192.268.230.245 shows offset of +1300ns
  igb0 @ 100Mb, igb3 @ 100Mb via secondary switch: 192.268.230.245 shows no offset
  igb0 @ 100Mb, igb3 @ 1Gb via secondary switch: 192.268.230.245 shows offset of -1100ns

What is very strange, is that in all tests, the two Linux boxes were closely in-line with each other, with no offset indicated by either side. If there speed connection issues, I would have expected to see an offset with igb0 @ 100Mb.

Is there any possibility of cross pollination of hardware timestamps between the two interfaces?

Thanks,
Denny



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