Date: Wed, 11 Oct 2006 14:29:57 -0700
From: Stephen Hemminger <shemminger@osdl.org>
To: "Michael S. Tsirkin" <mst@mellanox.co.il>
Cc: Steven Whitehouse <steve@chygwyn.com>, David Miller <davem@davemloft.net>,
       linux-kernel@vger.kernel.org, netdev@vger.kernel.org,
       openib-general@openib.org, rolandd@cisco.com
Subject: Re: Dropping NETIF_F_SG since no checksum feature.
Message-ID: <20061011142957.5bd42784@freekitty>
In-Reply-To: <20061011212339.GH15468@mellanox.co.il>
References: <20061011135720.303f166b@freekitty>
	<20061011212339.GH15468@mellanox.co.il>
Organization: OSDL
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O
> > 
> > You might want to try ignoring the check in dev.c and testing
> > to see if there is a performance gain.  It wouldn't be hard to test
> > a modified version and validate the performance change.
> 
> Yes. With my patch, there is a huge performance gain by increasing MTU to 64K.
> And it seems the only way to do this is by S/G.
> 
> > You could even do what I suggested and use skb_checksum_help()
> > to do inplace checksumming, as a performance test.
> 
> I can. But as network algorithmics says (chapter 5)
> "Since such bus reads are expensive, the CPU might as well piggyback
> the checksum computation with the copy process".
> 
> It speaks about onboard the adapter buffers, but memory bus reads are also much slower
> than CPU nowdays.  So I think even if this works well in benchmark in real life
> single copy should better.
> 

The other alternative might be to make copy/checksum code smarter about using
fragments rather than allocating a large buffer. It should avoid second order
allocations (effective size > PAGESIZE).

-- 
Stephen Hemminger <shemminger@osdl.org>
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