netdev - Re: Fwd: Packet mmap: TX RING and zero copy

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Date:	Fri, 5 Sep 2008 19:38:11 +0400
From:	Evgeniy Polyakov <johnpol@....mipt.ru>
To:	Johann Baudy <johaahn@...il.com>
Cc:	netdev@...r.kernel.org
Subject: Re: Fwd: Packet mmap: TX RING and zero copy

Hi.

On Fri, Sep 05, 2008 at 05:30:39PM +0200, Johann Baudy (johaahn@...il.com) wrote:
> > Well, udp_sendpage() needs to be extended to only append page when there
> > is anough free space there, otherwise push given frame and create next
> > packet.
> >
> Ok, I'll do a patch and let you know result.

Great, thank you. But it should take into account UDP nature: data is
not allowed to be split between ip packets like with TCP.

> > I meant you get a pointer by mapping some file in tmpfs (for example)
> > and then use some offset variable to store where you put your last data
> > (either packet header, or data itself), so that any subsequent write to
> > that area (either new packet header or dma data placement) would put
> > data just after the previous chunk. Thus after you have put number of
> > headers and appropriate data chunks, you could call sendfile() and reset
> > offset to the beginning of the mapped area.
> 
> If I understand well, there is no link between start of ethernet frame
> and packet header ?

Ethernet header is appended by the network core itself, likely core will
just allocate skb with small data area, put there an ethernet and udp/ip
headers and attach pages from the file. If hardware does not support
checksumming and scatter/gather, things will be different.

> App protocol must support packet loss ^^

I think matter of packet loss relevance here is just the same like with
any other sending method.

-- 
	Evgeniy Polyakov
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