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- To: chrony-users@xxxxxxxxxxxxxxxxxxxx
- Subject: Re: [chrony-users] Shared PTP clock
- From: Miroslav Lichvar <mlichvar@xxxxxxxxxx>
- Date: Tue, 29 Nov 2022 11:17:30 +0100
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On Mon, Nov 28, 2022 at 03:39:32PM -0500, Aaron Ball wrote:
> On some new time servers, we're using an Intel E810 NIC with two ports that
> share a single clock, with PTP coming from a GPS appliance on one interface
> and hardware-timestamped NTP being served on the other. Are there any risks
> we should be aware of around the shared clock in this application? (All our
> existing time servers have a unique clock per interface.)
Stability of the PHC synchronized by PTP will impact stability of
chrony's tracking of the PHC. If this is the main time source that
chrony will use/serve, it shouldn't matter much.
Ideally, you would use a virtual clock for PTP in order to let the
physical clock run free. This is supported since Linux 5.14 and in the
latest code of linuxptp (with a pending bug fix posted on -devel).
You can create a vclock with
echo 1 > /sys/class/ptp/ptp0/n_vclocks
and then start:
ptp4l -i eth0 --phc_index $VCLOCK_PHC
to use the vclock instead of the physical clock, which can be used by
chronyd.
However, the ice driver (used for E810) currently doesn't work well
with vclocks due to a locking issue. There are frequent "BUG:
scheduling while atomic" errors reported. It was reported on the
wired-lan list and it might be fixed with some planned changes soon.
There is also an issue with large delays in getting of the TX
timestamp on the E810. It sometimes takes tens of milliseconds for the
timestamp to be available, which causes frequent fallbacks to software
timestamping. Not an ideal HW for NTP, at least with the current
driver/firmware.
--
Miroslav Lichvar
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