On Mon, Nov 28, 2016 at 09:22:42AM -0800, Denny Page wrote:
> Before I try and make a case to the driver and hardware folk, I think I need 
> to be able to explain how stamps on both two linux systems can sometimes be 
> in agreement with stamps on the second interface and sometimes not. Given 
> just the following two tests:
> 
>  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
> 
> I cannot explain why the two linux systems do not disagree on stamps in the 
> second test. Can you think of something that the driver or hardware could be 
> doing that would explain that?

I'm not sure I understand the question. How do you compare timestamps
from the two linux systems? You mean that the offset they see when
peering with each other is zero even though they use different
hardware, with presumably different errors in timestamps?

An answer to that would be that it doesn't matter how big is the error
if you are comparing them over the same interface as they use for
synchronization with their server. The clocks on the two systems could
be far from each other, but as the error on the path to the server is
same as the error on the path to the peer, it will cancel out. You
would need another reference to see how accurate their clocks really
are.

Does that answer your question?

-- 
Miroslav Lichvar

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