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Date:      Wed, 19 Aug 2015 17:13:08 +0900
From:      Yonghyeon PYUN <pyunyh@gmail.com>
To:        Hans Petter Selasky <hps@selasky.org>
Cc:        Rick Macklem <rmacklem@uoguelph.ca>, FreeBSD stable <freebsd-stable@freebsd.org>, FreeBSD Net <freebsd-net@freebsd.org>, Slawa Olhovchenkov <slw@zxy.spb.ru>, Christopher Forgeron <csforgeron@gmail.com>, Daniel Braniss <danny@cs.huji.ac.il>
Subject:   Re: ix(intel) vs mlxen(mellanox) 10Gb performance
Message-ID:  <20150819081308.GC964@michelle.fasterthan.com>
In-Reply-To: <55D43590.8050508@selasky.org>
References:  <473274181.23263108.1439814072514.JavaMail.zimbra@uoguelph.ca> <7F892C70-9C04-4468-9514-EDBFE75CF2C6@cs.huji.ac.il> <805850043.24018217.1439848150695.JavaMail.zimbra@uoguelph.ca> <9D8B0503-E8FA-43CA-88F0-01F184F84D9B@cs.huji.ac.il> <1721122651.24481798.1439902381663.JavaMail.zimbra@uoguelph.ca> <55D333D6.5040102@selasky.org> <1325951625.25292515.1439934848268.JavaMail.zimbra@uoguelph.ca> <55D429A4.3010407@selasky.org> <20150819074212.GB964@michelle.fasterthan.com> <55D43590.8050508@selasky.org>

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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.

> >
> >>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.
> 
> You're right it's complicated. Not sure if bus_dma can provide an API 
> for this though.
> 
> --HPS



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