netdev - Re: [RFC] skb align patch

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Date:	Mon, 21 Sep 2009 21:30:11 -0700
From:	Stephen Hemminger <shemminger@...tta.com>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	Jesse Brandeburg <jesse.brandeburg@...il.com>,
	Jesper Dangaard Brouer <hawk@...u.dk>, netdev@...r.kernel.org
Subject: Re: [RFC] skb align patch

On Mon, 21 Sep 2009 08:13:20 +0200
Eric Dumazet <eric.dumazet@...il.com> wrote:

> Stephen Hemminger a écrit :
> > Based on the Intel suggestion that PCI-express overhead is
> > a significant cost.
> > 
> > Would people doing performance please measure the impact of
> > changing SKB alignment (64 bit only).
> 
> I had this idea some time ago when I hit a limit on bnx2 adapter
> (Giga bit link, BCM5708S), with small packets. pktgen was able
> to send ~500 Mbps 'only', or 700kps if I remember well.
> So I tried to align the pktgen build packet to a cache line,
> it gave no difference at all, but it was on a 32 bit kernel.
> (Thus my patch was for pktgen only, not a generic one as yours)
> 
> Could you elaborate why this change could be useful on 64bit ?
> 

It is useful on all architecture where unaligned CPU access is
relatively cheap.

The issue is that a unaligned DMA requires a read/modify/write
cache line access versus just a write access. I am not a bus
expert, but writes are probably more pipelined as well.

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