netdev - Re: [RFC] [TCP 0/3] Receive from socket into bio without copying

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Date:	Mon, 2 Jul 2012 15:41:19 -0400
From:	chetan loke <loke.chetan@...il.com>
To:	Andreas Gruenbacher <agruen@...bit.com>
Cc:	Eric Dumazet <eric.dumazet@...il.com>, netdev@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	Herbert Xu <herbert@...dor.apana.org.au>,
	"David S. Miller" <davem@...emloft.net>
Subject: Re: [RFC] [TCP 0/3] Receive from socket into bio without copying

On Mon, Jul 2, 2012 at 12:06 PM, Andreas Gruenbacher <agruen@...bit.com> wrote:
> On Mon, 2012-07-02 at 15:54 +0200, Eric Dumazet wrote:
>> So I will just say no to your patches, unless you demonstrate the
>> splice() problems, and how you can fix the alignment problem in a new
>> layer instead of in the existing zero copy standard one.
>
> Again, splice or not is not the issue here. It does not, by itself, allow zero
> copy from the network directly to disk but it could likely be made to support
> that if we can get the alignment right first.  The proposed MSG_NEW_PACKET flag
> helps with that, but maybe someone has a better idea.
>

Eric - by using splice do you mean something like:

int filedes[2];
PIPE_SIZE (64*1024)
pipe(filedes);
ret = splice (sock_fd_from, &from_offset, filedes [1], NULL, PIPE_SIZE,
                     SPLICE_F_MORE | SPLICE_F_MOVE);


ret = splice (filedes [0], NULL, file_fd_to,
                         &to_offset, ret,
                         SPLICE_F_MORE | SPLICE_F_MOVE);


i.e. splice-in from socket to pipe, and splice-out from pipe to destination?

Andreas - if the above assumption is true then can you apply the
'MSG_NEW_PACKET' on the sender and see if the above pseudo-splice code
achieves something similar to what you expect on the receive side(you
can also play w/ F_SETPIPE_SZ -  although I found very little
reduction in CPU usage)? Note: My personal experience - using splice
from an input-file-A to output-file-B bought very minimal cpu
reduction(yes, both the files used O_DIRECT). Instead, a simple
read/write w/ O_DIRECT from file-A to file-B was much much faster.


Chetan
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