if_em, legacy nic and GbE saturation

[Harald Schmalzbauer]

 Hello,

I recycled an older box and put an i350-2 together with a second 82541GI
(PCI-slot, one already on-board) into it.
The two i350-ports are used with VMDq for ESXi5.1.
The two 82541GI are used as lagg-nics by a 9.2-RC (amd64) guest as
passthrou PCI device.
Always had good results with such setups, but found out, that nics which
use the legacy driver part of if_em max out at ~0.6Gbits/s (1500 MTU).

There's another NIC on board of this recycle-box, a 82566-PHY (ICH9
integrated MAC).
This one uses also if_em, but not legacy code, it reports version 7.3.8
(compared to 1.0.6).
And it has no problem fully saturating GbE (~925Mbits/s, no jumbo Frames
support anyways).

I'm using iperf, with and without lagg (doesn't change anyhing, like it
doesn't influence tests on some other boxes with 82576 and i350 (igb))
I see enough idle cycles so CPU shouldn't limit the legacy if_em nics.
Also, I see the 82541 consuming arround 8k irqs. Same does the
82566-PHY, but with much higher throughput...

I'd like to know if I can't generally expect to saturate older (PCI) GbE
nics the line for any reason... I can remember tigeon cards from more
than a decade ago, which indeed seemd to lack the performance to gain
GbE, but I thought that was no issue shortly later and no "modern"
Intel-GbE card had such constraints!?
Is there any special tuning for legacy if_em (no need for any TCP
tuning, 82566 doesn't have any issue)?

Thanks,

-Harry

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