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

[Rick Macklem]

Hans Petter Selasky wrote:
> On 08/19/15 09:42, Yonghyeon PYUN wrote:
> > 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.
> 
> Don't forget that not all drivers in the tree set the TSO limits before
> if_attach(), so possibly the subtraction of one TSO fragment needs to go
> into ip_output() ....
> 
I think setting them before a call to ether_ifattach() should be required and
any driver that doesn't do that needs to be fixed.

Also, I notice that "32 * MCLBYTES - (ETHER_HDR_LEN + ETHER_VLAN_ENCAP_LEN)"
is getting written as "65536 - (ETHER_HDR_LEN + ETHER_VLAN_ENCAP_LEN)" which
obscures the reason it is the default. It probably isn't the correct default
for any driver that sets if_hw_tsomaxsegcount, but is close to IP_MAXPACKET,
so the breakage is mostly theoretical.

rick

> --HPS
> 
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