lagg(8) causes ghost queue with igb(4)

[Harald Schmalzbauer]

 Hi all,

while investigating a watchdog timeout problem (on FreeBSD-10.1-stable
r276295) I noticed "ghost" queues consuming interrupts with
igb(4)[82576] if igb(4) is member of lagg(4) (laggproto loadbalance
lagghash l2). Mysteriously only sometimes (1 of 2 runs).

Sending machine has only one ssh session open, where I do the following:
'dd if=/dev/zero | nc vegashare 3333' (of course on vegashare is a
listener [nc -l vegashare 3333 > /dev/null])

I'm transfering 123.5*10^6 Bytes/sec and on the sender, systat reports:
2682 igb0:que 0 (1 of 4 queues causes moderate irq load hw.igb.aim=1,
otherhise it were 16k irqs/s, nothing on the other 3 queues)

But roughly every second run, I see a another queue consuming much more
irqs/s while transferring exactly the same at exactly the same speed:
7740 igb0:que 0
2640 igb0:que 3

Here's again one queue with 2.6k irqs/s, but another one with 7.7k
irqs/s which I can't understand what this is doing. It's useless for
sure, because with only one queue I get the same payload transported on
the same hardware with the same speed and 3 queues idle…

I removed lagg(4) from the party and the 2nd queue hasn't showed up for
at least 20 test runs. So I guess it's caused by lagg(4).

Does anybody have an explanation for this?

Thanks,

-Harry





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