Functional differences when using ntpd as NTP client on NTP-NetBSD 9.3<-->10 ?

[Matthias Petermann <mp%petermann-it.de@localhost>]

Hello all,

I use ntpd in my Qemu/nvmm VMs as a client to synchronise the (otherwise 
lagging) clocks. For this purpose, ntpd runs on the host and 
synchronises on the internet. The ntpd in the VM only knows the host as 
the only time source and is configured in such a way that it does not 
give up even in the case of major deviations. This means, I use mostly 
the defaults with the following adjustments:

```
$ doas vi /etc/ntp.conf

    tinker panic 0 stepout 30

    #tos minsane 2

    server 192.168.2.10 burst minpoll 4 maxpoll 6 true
```

This works reliably in the VMs with NetBSD 9.3 as guest. Deviations are 
regularly compensated for by stepping.

In the VMs with NetBSD 9.99.99 as guest, for some reason no stepping 
takes place. As a consequence, in a short time deviations of several 
seconds up to minutes occur.

NetBSD 9.3 contains ntpd 4.2.8p11. In my build of NetBSD 9.99.99, ntpd 
4.2.8p14 is included, but I have also seen that p15 is now in the tree. 
Does anyone know if there have been any functional changes between these 
three versions that could account for this behaviour?

(I am aware that such questions are tricky in the virtualisation 
context. For what it's worth, the host is running on NetBSD 9.3 with 
HZ=1000, all VMs are running with HZ=100, and the Qemu processes are set 
to "near real-time" priority in the host via Schedctl (SCHED_FIFO). 
Since I have been using this setup with NetBSD 9.3 VMs, I have had no 
more problems with clock synchronisation. With said NetBSD 9.99.99 VM, 
all these host-side optimisations don't seem to help. Since the external 
configuration is the same (same virtual CPU count), I suspect the cause 
is more on the side of ntpd (or other internal changes that I'm not 
thinking about at all right now).

As always, I'm grateful for any helpful hints - chrony is compiling in 
the background and I'll test it tonight as a workaround.

Kind regards
Matthias