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Subject: Re: ix(intel) vs mlxen(mellanox) 10Gb performance
To: Rick Macklem <rmacklem@uoguelph.ca>, Daniel Braniss <danny@cs.huji.ac.il>
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Cc: FreeBSD Net <freebsd-net@freebsd.org>, Slawa Olhovchenkov
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From: Hans Petter Selasky <hps@selasky.org>
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Date: Tue, 18 Aug 2015 15:32:06 +0200
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On 08/18/15 14:53, Rick Macklem wrote:
> 2572 	  	         ifp->if_hw_tsomax = 65518;
>> >2573 	  	         ifp->if_hw_tsomaxsegcount = IXGBE_82599_SCATTER;
>> >2574 	  	         ifp->if_hw_tsomaxsegsize = 2048;

Hi,

If IXGBE_82599_SCATTER is the maximum scatter/gather entries the 
hardware can do, remember to subtract one fragment for the TCP/IP-header 
mbuf!

I think there is an off-by-one here:

ifp->if_hw_tsomax = 65518;
ifp->if_hw_tsomaxsegcount = IXGBE_82599_SCATTER - 1;
ifp->if_hw_tsomaxsegsize = 2048;

Refer to:

>          *
>          * NOTE: The TSO limits only apply to the data payload part of
>          * a TCP/IP packet. That means there is no need to subtract
>          * space for ethernet-, vlan-, IP- or TCP- headers from the
>          * TSO limits unless the hardware driver in question requires
>          * so.

In sys/net/if_var.h

Thank you!

--HPS