The mean end-to-end (from writing to a socket to reading from a socket),
round-trip latency across a modern 10G+ can be brought down to 30-40usec on
modern hardware with relatively low effort or specialized equipment
(e.g. https://blog.cloudflare.com/how-to-achieve-low-latency/), and can be
driven as low as 3-5 usec with specialized hardware and software stacks
(kernel bypass, etc) (e.g.
http://www.mellanox.com/related-docs/whitepapers/HP_Mellanox_FSI%20Benchmarking%20Report%20for%2010%20%26%2040GbE.pdf).
A trivial round trip ("what time do you have? [my time is X]" to "My clock
shows Y for your request sent at X" [recieved at Z]". would allow you to
measure the delta between the perceived wall clock difference between two
machines to within the round trip latency. e.g. The difference between the
clocks (at the time measured) in the above sequence is known to be (Z-Y)
+/- (Z-X). You can use various statistical techniques to more closely
estimate the bound when repeating the round trip queries many times and
across periods of time. E.g. the amazingly effective techniques used
(decades ago) by NTP to synchronize clocks to within milliseconds across
wide geographical distances and slow/jittery networks still apply even at
low latency scales (e.g. start with something
like http://www.ntp.org/ntpfaq/NTP-s-algo.htm or
https://www.cisco.com/c/en/us/about/press/internet-protocol-journal/back-issues/table-contents-58/154-ntp.html
and dig into references if interested).
Keep in mind that at the levels you are looking at clock skew and drift are
very real things. And then there is jitter...
On Tuesday, October 23, 2018 at 5:05:22 AM UTC-7, Himanshu Sharma wrote:
>
> As the title suggests, consider 2 servers connected via an L3 switch. How
> can we find the absolute time difference between the clocks running on the
> servers. I want to go as close as possible.
>
> Actually syncing the clocks is not possible due to some constraints so I
> want to know the time difference. Is there any opensource tool I can use
> readily.
>
>
> Many thanks in advance
>
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