netdev - Re: zero copy for relay server

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Date:	Mon, 28 Mar 2011 18:52:18 +0200
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Viral Mehta <Viral.Mehta@...infotech.com>
Cc:	"netdev@...r.kernel.org" <netdev@...r.kernel.org>
Subject: Re: zero copy for relay server

Le lundi 28 mars 2011 à 21:57 +0530, Viral Mehta a écrit :
> Hi,
> I am implementing a particular application where
> my application acts nothing but like Relay Server.
> 
> Relay server accepts connection from machine A.
> It also accepts connection from Machine B.
> 
> Machine A and B are on different LAN/subnnets.
> Now, there are two connections.
> What server is supposed to do is RECV packets from machine A and SEND same
> to machine B.
> 
> Pseudo Code is something like,
> while(1)
> {
> recvagain:
>    n =3D recv(incoming_fd, &buf, 8192, ...)
>    if(n < 0)
>         goto recvagain;
>    send(outgoing_fd, &buf, n, ...);
> }
> 
> Now the question is,
> I want to avoid kernel-user copy for such application.
> I found that a syscall like "sendfile"; I wanted to know if there is any
> similar thing exists in-kernel which can take 2 socket descriptors....
> 
> If not, is it possible ? I would like to implement the same if someone
> can suggest some pointers.

linux way (if you want to avoid netfilter stuff and use userland code)
is to use splice() system call, and a pipe between two sockets.


/* skeleton : must add error checking to exit the loop properly */
int fds[2];
pipe(fds);

while (1) {
	splice(incoming_fd, NULL, fds[1], NULL, 65536, 0);
	splice(fds[0], NULL, outgoing_fd, NULL, 65536, 0);
}

This way, messages dont cross kernel<>user boundary.

The pipe is acting as a buffer between the two sockets.



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