Low default setting of UDBHASHSIZE leads to unresponsive system

[Lev Serebryakov]

 Now 11-STABLE (and 12-CURRENT too) have this:

sys/netinet/udp_usrreq.c:#define   UDBHASHSIZE     128

 Looks like such low value could lead to 100% consumption of CPU by
interrupt threads (igb queues in my case) on heavy incoming UDP traffic
(torrents with uTP in my case).

 My system (E3-1220v3 with I210 NICs) becomes completely unresponsive
(nut complains about lost connection to UPS, ssh to system times out,
etc) when system downloads torrent with many uTP (UDP) peers. Four igb0
queues consume 100% CPU each in this scenario.

 Total traffic could be very low like 500KiB/s (yes, 500KiB/s, not
MiB/s!), I don't speak about 1Gbit/s or even 100Mbit/s here!

  Rebuilding kernel with UDBHASHSIZE=16384 seems to help.

  Why is this value so low and why I need to patch sources to change it?
Many such settings are changeable via sysctl and/or tunables, but this
one looks hardcoded.

-- 
// Lev Serebryakov

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