| Re: [chrony-dev] SW/HW timestamping on Linux |
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On Sun, Nov 27, 2016 at 01:06:20PM -0800, Denny Page wrote:
> 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?
I think the results are consistent with what have you reported
previously. Without the RX correction, the offset in sourcestats was
about -2.1us. Now, with an RX correction of 752 ns on 1gb and 7520 ns
on 100mb, this shifts the asymmetry in delay by 6768 ns, which gives
an offset of 3384 ns. When added to the -2100 ns offset, it's about
1280 ns, which is close to the values you report now. If the offset
should be close to zero, the l2 length used in the calculation of the
correction would need to be about 38 bytes shorter.
It would be very interesting to see how this offset changes with
longer IPv6 packets.
If you are sure the error doesn't come from the switch, I suspect it's
a HW or driver issue. It seems the drivers need to have some
timestamping-specific magic. Look at the following commit for
instance.
http://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/commit/?id=0066c8b6f4050d7c57f6379d6fd4535e2f267f17
I'd suggest to send a detailed report to the intel-wired-lan list and
see if anyone has any suggestions on what could be wrong.
--
Miroslav Lichvar
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