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

[Rick Macklem]

Yonghyeon PYUN wrote:
> On Wed, Aug 19, 2015 at 08:13:59AM -0400, Rick Macklem wrote:
> > Yonghyeon PYUN wrote:
> > > On Wed, Aug 19, 2015 at 09:51:44AM +0200, 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.
> > > > 
> > > > Hi,
> > > > 
> > > > If you change the behaviour don't forget to update and/or add comments
> > > > describing it. Maybe the amount of subtraction could be defined by some
> > > > macro? Then drivers which inline the headers can subtract it?
> > > > 
> > > 
> > > I'm also ok with your suggestion.
> > > 
> > > > Your suggestion is fine by me.
> > > > 
> > > 
> > > > The initial TSO limits were tried to be preserved, and I believe that
> > > > TSO limits never accounted for IP/TCP/ETHERNET/VLAN headers!
> > > > 
> > > 
> > > I guess FreeBSD used to follow MS LSOv1 specification with minor
> > > exception in pseudo checksum computation. If I recall correctly the
> > > specification says upper stack can generate up to IP_MAXPACKET sized
> > > packet.  Other L2 headers like ethernet/vlan header size is not
> > > included in the packet and it's drivers responsibility to allocate
> > > additional DMA buffers/segments for L2 headers.
> > > 
> > Yep. The default for if_hw_tsomax was reduced from IP_MAXPACKET to
> >   32 * MCLBYTES - max_ethernet_header_size as a workaround/hack so that
> > devices limited to 32 transmit segments would work (ie. the entire packet,
> > including MAC header would fit in 32 MCLBYTE clusters).
> > This implied that many drivers did end up using m_defrag() to copy the mbuf
> > list to one made up of 32 MCLBYTE clusters.
> > 
> > If a driver sets if_hw_tsomaxsegcount correctly, then it can set
> > if_hw_tsomax
> > to whatever it can handle as the largest TSO packet (without MAC header)
> > the
> > hardware can handle. If it can handle > IP_MAXPACKET, then it can set it to
> > that.
> > 
> 
> I thought the upper limit was still IP_MAXPACKET. If driver
> increase it (i.e. > IP_MAXPACKET,  the length field in the IP
> header would overflow which in turn may break firewalls and other
> packet handling in IPv4/IPv6 code path.
I have no idea if a bogus value in the ip_len field of the TSO segment
would break something in ip_output() or not. This would need to be checked
before anyone configures if_hw_tsomax > IP_MAXPACKET. I didn't think of
any effect this would have in ip_output(), I just knew that the hardware
would be replacing ip_len when it generated the TCP/IP segments from the TSO
segment. As you note, I vaguely recall some hardware being able to handle a
TSO segment > IP_MAXPACKET (presumably getting the TSO segment's length some
other way).

It would be nice if this was checked, but yes, the comment should specify
an upper bound on if_hw_tsomax of IP_MAXPACKET until then.

rick

> If the limit no longer apply to network stack, that's great.  Some
> controllers can handle up to 256KB TCP/UDP segmentation and
> supporting that feature wouldn't be hard.
> _______________________________________________
> freebsd-stable at freebsd.org mailing list
> https://lists.freebsd.org/mailman/listinfo/freebsd-stable
> To unsubscribe, send any mail to "freebsd-stable-unsubscribe at freebsd.org"
>