[Bug 256217] [tcp] High system load because of interrupts with RACK

https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=256217

--- Comment #7 from Christos Chatzaras <chris@cretaforce.gr> ---
I was able to reproduce the issue with a "test" server && 10 VPS running Linux.
On each VPS I run wrk (benchmarking tool):

wrk -c 1000 -d 3600s http://url

This creates 10000 concurrent connections to "test" server.

Also in my nginx.conf I replace "keepalive_timeout 60;" with "keepalive_timeout
0;" so connections are not reused which helps to show more interrupts faster.

Then I kill all "wrk" and after 1 minute:

sockstat -sSPtcp | grep rack | wc -l
    7421

At that moment "top" shows 20% interrupts and "netstat 1" show 1-10 packets /
sec which I believe is my ssh session, so no activity. Also at that moment tcp
"rack" states were FIN_WAIT_1 and CLOSING.

After few minutes most "rack" connections close and "top" shows ~ 0.7%
interrupts. At that moment I had 4 stuck connections in LAST_ACK state which I
drop using "tcpdrop -s LAST_ACK" and finally "top" shows ~ 0.0% interrupts.

If it helps I can give root access to the "test" server.

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