[ofa-general] How to Interpret MTU reported by "ibv_devinfo" vs "ifconfig ib0"

[Ralph Campbell]

On Mon, 2008-09-15 at 15:50 -0400, Kumar, Amit H. wrote:
> >
> > The MTU reported by "ifconfig ib0" is the MTU used by the Linux
> > TCP/IP network stack. The MTU reported by ibv_devinfo is the
> > MTU that the hardware is capable of sending. This is limited
> > to 4K by the Infiniband specification. The reason the network
> > stack can have a higher MTU is that ib_ipoib is using the RC
> > QP protocol to send IP messages larger than the hardware MTU.
> > If you use "datagram" mode for ib_ipoib, you will see that
> > the network stack MTU is limited to the hardware MTU - 4.
> 
> Thank you Ralph!!!  As far as common applications taking advantage of ib_ipoib
> Does it help using RC QP with a higher MTU than the hardware MTU?
> 
> Does an Application, which uses Sockets API, by default make use of ib_ipoib,
> if it is enabled ?
> 
> Is there any essential difference between IPoIB and ib_ipoib, or is it just a matter of usage ?
> 
> Thank you,
> Amit

IPoIB is ib_ipoib. The first is the name in the IB spec. the second is
the name of the kernel module.
ib_ipoib just looks like another sockets network device to Linux so
Sockets API calls work normally (you just need to use the IP address
of the ib0 device).
The reason a larger network MTU helps is because the Linux network
stack is more efficient when using larger MTUs.