linux-kernel - Re: Dropping NETIF_F_SG since no checksum feature.

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Message-ID: <20061011142957.5bd42784@freekitty>
Date:	Wed, 11 Oct 2006 14:29:57 -0700
From:	Stephen Hemminger <shemminger@...l.org>
To:	"Michael S. Tsirkin" <mst@...lanox.co.il>
Cc:	Steven Whitehouse <steve@...gwyn.com>,
	David Miller <davem@...emloft.net>,
	linux-kernel@...r.kernel.org, netdev@...r.kernel.org,
	openib-general@...nib.org, rolandd@...co.com
Subject: Re: Dropping NETIF_F_SG since no checksum feature.

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@...l.org>
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