Re: [chrony-users] xleave and HW timestamping

[Miroslav Lichvar <mlichvar@xxxxxxxxxx>]

On Fri, Apr 09, 2021 at 07:31:21AM -0700, Joe Williams wrote:
> It sounds like having both RX and TX HW timestamps reduces the chance of
> asymmetry since both are done in HW, rather than RX done in the kernel and
> TX done in HW which ensures some amount of difference. In that case does it
> make sense to disable HW timestamping completely if both RX and TX cannot
> be done in HW or is there still an advantage of only having TX in HW? Seems
> like the same asymmetry would happen in interleave mode when only one side
> has full HW timestamping, is that correct?

It depends on what is more important for you, accuracy or stability?
With partial HW timestamping you will likely have a better stability,
but accuracy may be worse. It doesn't have to be, it depends on the
server and client HW and configuration.

Usually, there always is some asymmetry. Even when both RX and TX are
HW-timestamped, there will likely be an error of a few hundred
nanoseconds. An exception is the I210, which has the timestamping
errors compensated in the driver.

What matters for accuracy is how those errors cancel out between the
server and client. The best case is that they both have the same
hardware and configuration, so any asymmetries cancel out.

Maybe you have a number of identical clients and you need their clocks
to be close to each other, but don't care much about accuracy wrt the
server? In such case you would prefer partial HW timestamping
over no HW timestamping.

-- 
Miroslav Lichvar


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