netdev - Re: [RFC 2/2] shrink size of scatterlist on common i386/x86-64

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Message-Id: <20070705.171535.105173068.davem@davemloft.net>
Date:	Thu, 05 Jul 2007 17:15:35 -0700 (PDT)
From:	David Miller <davem@...emloft.net>
To:	shemminger@...ux-foundation.org
Cc:	netdev@...r.kernel.org
Subject: Re: [RFC 2/2] shrink size of scatterlist on common i386/x86-64

From: Stephen Hemminger <shemminger@...ux-foundation.org>
Date: Thu, 5 Jul 2007 17:00:51 -0700

> On Thu, 05 Jul 2007 16:43:08 -0700 (PDT)
> David Miller <davem@...emloft.net> wrote:
> 
> > From: Stephen Hemminger <shemminger@...ux-foundation.org>
> > Date: Thu, 05 Jul 2007 16:14:14 -0700
> > 
> > > The scatterlist only needs 16 bits for length/offset because
> > > PAGE_SIZE is 4K
> > > 
> > > Signed-off-by: Stephen Hemminger <shemminger@...ux-foundation.org>
> > 
> > Unfortunately I don't think this can be done, even on i386.
> > 
> > It is legal to use order!=0 pages and multi-page areas in a single
> > scatterlist entry.
> > 
> 
> Okay, but then using SG lists makes skbuff's much bigger.
>     
> 	fraglist	scatterlist		   per skbuff
> 32 bit	8		20		+12 * 18 = +216!
> 64 bit	16		32		+16 * 18 = +288
> 
> So never mind...

I know, this is why nobody ever really tries to tackle this.

> I'll do a fraglist to scatter list set of routines, but not sure
> if it's worth it.

It's better to add dma_map_skb() et al. interfaces to be honest.

Also even with the scatterlist idea, we'd still need to do two
map calls, one for skb->data and one for the page vector.

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