unable to set mtu 9000 on Intel x520-t2 adapter

[Jack Vogel]

Its a system resource thing, when you increase to a 9K mtu the driver will
use 9K
mbuf clusters, and with the queues and rings you have its unable to get
enough.

Very important to look at kern.ipc.nbm*

In your case nmbjumbo9 is the pool, if you had 4K it would those clusters.
Change the defaults in /etc/sysctl.conf.

Cheers,

Jack


On Fri, Jan 13, 2012 at 9:22 AM, Iordan Iordanov <iordan at cdf.toronto.edu>wrote:

> Hello,
>
> We are testing FreeBSD 9.0-RELEASE on a pair of machines with 10GBASE-T
> Intel x520-t2 adapters interconnected with a straight-through cat6 rj45
> cable. With mtu 1500, we are achieving aproximately 4 gigabits/s both
> directions as demonstrated with iperf. We wanted to test with mtu 9000 to
> see whether there would be any difference, but we encountered a problem
> setting it. We ran the following commands to alter the MTU to 9000 on the
> two machines (fw3 and fw4):
>
> fw3# ifconfig ix1 192.168.1.100 mtu 9000
> fw4# ifconfig ix1 192.168.1.200 mtu 9000
>
> However, instead of getting mtu set to 9000, what we see is the following
> message in the kernel output of both machines:
>
> ix1: Could not setup receive structures
> ix1: Could not setup receive structures
>
> and no traffic getting through. The moment we run:
>
> fw3# ifconfig ix1 192.168.1.100 mtu 1500
> fw4# ifconfig ix1 192.168.1.200 mtu 1500
>
> we see:
>
> ix1: link state changed to DOWN
> ix1: link state changed to UP
>
> and traffic starts getting through as before. More information about our
> hardware is:
>
> Motherboard: Supermicro H8DGU-F
> CPU:         AMD 6128 8-core
> Memory:      16GB DDR3
> NICs:        Intel x520-t2 10GbaseT adapters
>
> An ifconfig ix1 output for one of the machines:
>
> fw4# ifconfig ix1
> ix1: flags=8843<UP,BROADCAST,**RUNNING,SIMPLEX,MULTICAST> metric 0 mtu
> 1500
>
> options=401bb<RXCSUM,TXCSUM,**VLAN_MTU,VLAN_HWTAGGING,JUMBO_**
> MTU,VLAN_HWCSUM,TSO4,VLAN_**HWTSO>
>        ether 00:1b:21:d6:19:51
>        inet 192.168.1.200 netmask 0xffffff00 broadcast 192.168.1.255
>        nd6 options=29<PERFORMNUD,**IFDISABLED,AUTO_LINKLOCAL>
>        media: Ethernet autoselect (autoselect <full-duplex>)
>        status: active
>
>
> Many thanks for any help/action on this!
>
> Sincerely,
>
> Iordan Iordanov
> CDF System Administrator
> University of Toronto
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