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

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

--- Comment #5 from Christos Chatzaras <chris@cretaforce.gr> ---
To avoid reboot I did this which I believe is the same:

sysctl net.inet.tcp.functions_default
net.inet.tcp.functions_default: freebsd

kldload tcp_rack

sysctl net.inet.tcp.functions_default=rack

I restart all services so new connections (mostly nginx) use "rack".

I wait for few minutes and interrupts increase to ~5%

sysctl net.inet.tcp.functions_default=freebsd

tcpdrop -S rack

Interrupts decrease to ~ 0.3% (looks like less connections = less interrupts)

New connections still use "rack", so I restart again all services.

Now all connections use "freebsd" and interrupts decrease to ~ 0.0%

----

Also something I notice is that during the issue if I run "sysctl
kern.eventtimer.timer=HPET" then interrupts immediately increase. If I run
"sysctl kern.eventtimer.timer=LAPIC" the interrupts immediately decrease.

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So it looks like that active rack connections cause the issue.

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