ixg(4) performances

[Emmanuel Dreyfus <manu%netbsd.org@localhost>]

Hi

ixgb(4) has poor performances, even on latest -current. Here is the
dmesg output:
ixg1 at pci5 dev 0 function 1: Intel(R) PRO/10GbE PCI-Express Network Driver, 
Version - 2.3.10
ixg1: clearing prefetchable bit
ixg1: interrupting at ioapic0 pin 9
ixg1: PCI Express Bus: Speed 2.5Gb/s Width x8

The interface is configued with:
ifconfig ixg1 mtu 9000 tso4 ip4csum tcp4csum-tx udp4csum-tx

And sysctl:
kern.sbmax = 67108864
kern.somaxkva = 67108864
net.inet.udp.sendspace = 2097152
net.inet.udp.recvspace = 2097152
net.inet.tcp.sendspace = 2097152
net.inet.tcp.recvspace = 2097152
net.inet.tcp.recvbuf_auto = 0
net.inet.tcp.sendbuf_auto = 0

netperfs shows a maximum performance of 2.3 Gb/s. That let me with
the feeling that only a PCI lane is used. Is it possible?

I also found this page that tackles the same problem on Linux:
http://dak1n1.com/blog/7-performance-tuning-intel-10gbe

They tweak the PCI MMRBC. Anyone has an idea of how it could be 
done on NetBSD? I thought about borrowing code from src/sys/dec/pci/if_dge.c
but I am not sure what pci_conf_read/pci_conf_write commands should be used.

Any other idea on how to improve performance?

-- 
Emmanuel Dreyfus
manu%netbsd.org@localhost