Re: [chrony-users] xleave and HW timestamping

Thanks for the response!

> As compared to a fully HW timestamping?

Yes, that's right. Having both RX and TX HW timestamping vs only having TX HW timestamping.

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?

-Joe

On Tue, Apr 6, 2021 at 1:07 AM Miroslav Lichvar <mlichvar@xxxxxxxxxx> wrote:

On Thu, Apr 01, 2021 at 06:13:08PM -0700, Joe Williams wrote:
> Hello list,
>
> I have chrony working great using interleaved mode between two machines
> that have both RX and TX hardware timestamping. It's far more common for
> NICs to support TX timestamps in HW rather than both TX and RX. It seems
> like in that case the RX timestamps would fall back to happening in the
> kernel, while TX would stay in hardware. The xleave docs only mention TX
> timestamps, while the RFC draft mentions both RX and TX. So it's not
> totally clear to me how much HW RX timestamps matter to interleaved mode.

Interleaved mode matters only for TX timestamps. RX timestamps are
the same in basic and interleaved mode.

> For clients or servers that don't support HW RX timestamping (only TX) how
> much precision would I be giving up?

As compared to a fully HW timestamping? The main issue would be the
asymmetry. RX timestamps will likely have a much larger error, e.g.
few microseconds vs sub-microsecond in the TX timestamp. The loss of
stability will depend on which direction has more network jitter. If
it's mostly on the RX path, it might not be so bad as chrony would
stick to the more stable TX direction.

--
Miroslav Lichvar

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