11-STABLE still livelocks under combined network & disk activity — patch from D7538 helps

[Lev Serebryakov]

 My home server (and torrent box) is rather beefy (E3-1220v3, 16GiB RAM,
radiz5, SSD L2ARC), NIC is igb (I210).

 This server expieince live-lock when there is fast and large download
with multiple streams (some very popular large torrent). In this state
it is not accessible by SSH, USB is failed (UPS, mouse and keyboard are
flaky), existing ssh connection is not simply "slow" but almost
unusable: 1-2 keypresses PER MINUTE, etc.

 top(1) shows (with 1 "frame" per minute update) that 100% of CPU is
consumed by "intr{irqXXX: igb0:que X}" in this state, memory is really
low (less than 100MiB) and pmc-based stats shows, that it is very hard
lock congestion somewhere in memory subsystem.

 As soon as download is finished, everything become normal again.

 Please note, that it is 100Mb/s download at max, not 1Gb/s!

  I've tried to tune different sysctls and even tuned UDP buckets, but
it doesn't help.

 This problem could be solved by https://reviews.freebsd.org/D7538.
Maybe, it could be integrated to system?

 Or there are other solutions?

-- 
// Lev Serebryakov

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