unbound request list exceeded stat ubuntu 20 vs ubuntu 22

[Shanmuga Rao]

Our ubuntu 20 vms run unbound version 1.9.4
received new ubuntu 22 vms and installed unbound version 1.13.1 via package
manager.  I have started noticing that on the ubuntu 22 machines, the
*total.requestlist.exceeded* stat started getting non-zero values. unbound
v1.9.4 in ubuntu 20, always shows 0.

The queries received by each node looks to be  at most 20k or less and are
distributed equally. All sysctl parameters are identical between ubuntu 20
and ubuntu 22 vms. Unbound config is identical on both. Most unbound.conf
parameters are left to defaults, the following are the important ones I
believe influence unbound performance.

 num-threads: 8
 outgoing-range: 60000
 num-queries-per-thread: 30000
 so-reuseport: yes
 module-config: "iterator"
 msg-cache-slabs: 2
 rrset-cache-slabs: 2
 infra-cache-slabs: 2
 key-cache-slabs: 2
 rrset-cache-size: 512m
 msg-cache-size: 256m
 so-rcvbuf: 4m
 so-sndbuf: 4m

Unbound service starts and occupies available ports and FDs. CPU and memory
utilisation on both are very minimal. less than 25%
# ulimit -n
1048576
unbound[764890:0] debug: total of 59463 outgoing ports available
*Ubuntu 20 stats_noreset: *
total.num.queries=15979

*total.requestlist.max=5**total.requestlist.exceeded=0*

*total.requestlist.current.all=9total.requestlist.current.user=7*

*Ubuntu22 **stats_noreset**: *
total.num.queries=9157

*total.requestlist.max=5*

*total.requestlist.exceeded=3total.requestlist.current.all=3total.requestlist.current.user=3*

ubuntu 22 systems constantly have non-zero values. Ubuntu 20 systems and
even Centos7 ( unbound version 1.7.1) always have zero. As per my
understanding of the config parameters, the system itself has enough
resources available to handle atleast 2x the current traffic.

As a test I have compiled version 1.13.1 on a ubuntu 20 VM and still the
request list exceeded stat is zero.  On Ubuntu 22 I have also compiled and
installed the latest unbound version 1.20.0 and it's still the same - non
zero values.

I can see that the queries dropped is less than 1 req/s but it's still
confusing since its only seen on ubuntu 22 vms with little resource
utilisation.

Has anyone run into something similar ? Do you know if any ubuntu 22
specific parameters should be modified ? Happy to provide more information
if needed.
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