I210/igb is limited by flow control and reset fc setting on MTU change (11-STABLE)

Mon Sep 24 12:00:12 UTC 2018

 My 11-STABLE system with igb (I210) can not send more than 710Mbit/s
with single TCP stream until I disable flow control (sysctl
dev.igb.0.fc=0). After that it shows full-speed and there is no collisions.

 Other system with I210 doesn't show this behavior.

 What is worse, after

 ifconfig igb0 mtu 1500 down up
 ifconfig igb0 mtu 9000 down up

 it is limited again, looks like this action sequence reset flow control
setting (but not sysctl, it is still 0).

 sysctl dev.igb.0.fc=3 dev.igb.0.fc=0

 helps till next MTU switch.

-- 
// Lev Serebryakov