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Date:      Wed, 19 Aug 2015 16:42:12 +0900
From:      Yonghyeon PYUN <pyunyh@gmail.com>
To:        Hans Petter Selasky <hps@selasky.org>
Cc:        Rick Macklem <rmacklem@uoguelph.ca>, Daniel Braniss <danny@cs.huji.ac.il>, FreeBSD Net <freebsd-net@freebsd.org>, Slawa Olhovchenkov <slw@zxy.spb.ru>, FreeBSD stable <freebsd-stable@freebsd.org>, Christopher Forgeron <csforgeron@gmail.com>
Subject:   Re: ix(intel) vs mlxen(mellanox) 10Gb performance
Message-ID:  <20150819074212.GB964@michelle.fasterthan.com>
In-Reply-To: <55D429A4.3010407@selasky.org>
References:  <1D52028A-B39F-4F9B-BD38-CB1D73BF5D56@cs.huji.ac.il> <17871443-E105-4434-80B1-6939306A865F@cs.huji.ac.il> <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>

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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.



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