Recommended settings for chrony on KVM

1

Hello,

I operate chrony 4.2 on Debian 11 in a KVM virtualized system with 4 dedicated CPU cores (KVM host is a AMD EPYC 7452).

Are there any recommended settings for the operating system and/or chrony in such an environment?
To name a few chrony settings, what about filter / minsamples / clockprecision?

The reason for my question is: I’ve noticed that the values in the columns NP and NR of chronys’ sourcestats command conspicuously often fall back to the minimum values (6|3), and they also rarely get high.

2

You can not control the local-system-clock from a virtual system.

Just run it with server or pool params and it will work fine.

3

You’re right. I use a dedicated server now, and maybe you have a hint for me, I really don’t get it…

The server has a Intel i210 NIC, ethtool -T shows

Capabilities:
        hardware-transmit
        software-transmit
        hardware-receive
        software-receive
        software-system-clock
        hardware-raw-clock
PTP Hardware Clock: 0
Hardware Transmit Timestamp Modes:
        off
        on
Hardware Receive Filter Modes:
        none
        all

After starting the system there’s a /dev/pps0 and /dev/ptp0.
Running chrony with hwtimestamp eth0 reports Enabled HW timestamping on eth0.

What I don’t know and really don’t understand now, should or do I have to install the package linuxptp to use

  • ptp4l ?
  • phc2sys ?

And even without linuxptp installed, is it helpful/recommended to configure something like this in chrony:

refclock PHC /dev/ptp0 pps

4

There is no special configuration.
Sync your hypervisor with a Reflcock or (Pool)server and on the client do the same. That’s it.
Or you can sync the hypervisor and use a client clock which sync with the hypervisor. IIRC Amazon EC2 is doing likely the same.

Maybe Red Hat have some good suggestions:

5

I don’t have one anymore, see previous post :slight_smile:

6

It’s my understanding that your NIC doesn’t have a clock, it just stamps network packages in order to minimise the offset.
Also, for it to work, all NIC’s need hardware timestamping else it’s ignored as far as I know.

In order to get accurate time from a refclock, you still need a GPS with PPS.

As for onboard clocks, forget about those, none of them is accurate and they wander all over the place.

7

Only if there is PTP provided in the local network and you would want to use it as a time source.

No, the clock would need to be synchronized (e.g. by ptp4l) to be useful as a reference clock.