ix(intel) vs mlxen(mellanox) 10Gb performance

[Yonghyeon PYUN]

On Wed, Aug 19, 2015 at 09:00:52AM +0200, Hans Petter Selasky wrote:
> On 08/18/15 23:54, Rick Macklem wrote:
> >Ouch! Yes, I now see that the code that counts the # of mbufs is before the
> >code that adds the tcp/ip header mbuf.
> >
> >In my opinion, this should be fixed by setting if_hw_tsomaxsegcount to 
> >whatever
> >the driver provides - 1. It is not the driver's responsibility to know if 
> >a tcp/ip
> >header mbuf will be added and is a lot less confusing that expecting the 
> >driver
> >author to know to subtract one. (I had mistakenly thought that 
> >tcp_output() had
> >added the tc/ip header mbuf before the loop that counts mbufs in the list. 
> >Btw,
> >this tcp/ip header mbuf also has leading space for the MAC layer header.)
> >
> 
> Hi Rick,
> 
> Your question is good. With the Mellanox hardware we have separate 
> so-called inline data space for the TCP/IP headers, so if the TCP stack 
> subtracts something, then we would need to add something to the limit, 
> because then the scatter gather list is only used for the data part.
> 

I think all drivers in tree don't subtract 1 for
if_hw_tsomaxsegcount.  Probably touching Mellanox driver would be
simpler than fixing all other drivers in tree.

> Maybe it can be controlled by some kind of flag, if all the three TSO 
> limits should include the TCP/IP/ethernet headers too. I'm pretty sure 
> we want both versions.
> 

Hmm, I'm afraid it's already complex.  Drivers have to tell almost
the same information to both bus_dma(9) and network stack.