UDP buffer drops / statistics collector

Hi all,

I'm seeing sporadic (but frequent) UDP buffer drops on a host that so far
I've not been able to resolve.

The drops are originating from postgres processes, and from what I know -
the only UDP traffic generated by postgres should be consumed by the
statistics collector - but for whatever reason, it's failing to read the
packets quickly enough.

Interestingly, I'm seeing these drops occur even when the system is idle..
but every 15 minutes or so (not consistently enough to isolate any
particular activity) we'll see in the order of ~90 packets dropped at a
time.

I'm running 9.6.2, but the issue was previously occurring on 9.2.4 (on the
same hardware)

If it's relevant.. there are two instances of postgres running (and
consequently, 2 instances of the stats collector process) though 1 of those
instances is most definitely idle for most of the day.

In an effort to try to resolve the problem, I've increased (x2) the UDP
recv buffer sizes on the host - but it seems to have had no effect.

cat /proc/sys/net/core/rmem_max
1677216

The following parameters are configured

track_activities on
track_counts on
track_functions none
track_io_timing off

There are approximately 80-100 connections at any given time.

It seems that the issue started a few weeks ago, around the time of a
reboot on the given host... but it's difficult to know what (if anything)
has changed, or why :-/

Incidentally... the documentation doesn't seem to have any mention of UDP
whatsoever. I'm going to use this as an opportunity to dive into the
source - but perhaps it's worth improving the documentation around this?

My next step is to try disabling track_activities and track_counts to see
if they improve matters any, but I wouldn't expect these to generate enough
data to flood the UDP buffers :-/

Any ideas?