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Date: Fri, 22 Feb 2002 13:44:08 -0800
From: Mike Fedyk <mfedyk@matchmail.com>
To: Alan Cox <alan@lxorguk.ukuu.org.uk>
Cc: Jamie Lokier <lk@tantalophile.demon.co.uk>, Dan Kegel <dank@kegel.com>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        Zach Brown <zab@zabbo.net>
Subject: Re: is CONFIG_PACKET_MMAP always a win?
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In-Reply-To: <20020222190431.A16926@kushida.apsleyroad.org> <E16eLoz-0002vD-00@the-village.bc.nu>
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On Fri, Feb 22, 2002 at 07:57:33PM +0000, Alan Cox wrote:
> > > You can process them in the ring buffer. If you can't keep up then you
> > > are screwed any way you look at it 8)
> > 
> > That still doesn't avoid copying: af_packet copies the whole packet (if
> > you want the whole packet) from the original skbuff to the ring buffer.
> 
> I'd make a handwaved claim that the first copy of the packet from a DMA
> receiving source is free. Its certainly pretty close to free because the
> overhead of sucking it into L1 cache will dominate and you need to do that
> anyway.
> 

Doesn't DMA access system memory directly and leave processor caches alone?
If so, then the fewer copies that have to pollute the L1/2 caches the better.

Even if it does for UP, I'd immagine that it doesn't for SMP...

Mike
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