netdev - Re: maximum buffer size for splice(2) tcp->pipe?

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Date:	Wed, 14 Jan 2009 11:17:57 +0100
From:	Eric Dumazet <dada1@...mosbay.com>
To:	Volker.Lendecke@...Net.DE
CC:	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org, Steven French <sfrench@...ibm.com>,
	Jens Axboe <jens.axboe@...cle.com>, netdev@...r.kernel.org
Subject: Re: maximum buffer size for splice(2) tcp->pipe?

Volker Lendecke a écrit :
> On Wed, Jan 14, 2009 at 10:13:34AM +0100, Eric Dumazet wrote:
>> for (;;) {
>> 	struct pollfd pfd;
>> 	pfd.fd = socket;
>> 	pfd.events = POLLIN;
>> 	if (poll(&pfd, 1, -1) != 1)
>> 		continue;
>> 	res = splice(socket, NULL, pipefds[1], NULL, 65536, SPLICE_F_MOVE|SPLICE_F_NONBLOCK);
>> 	if (res > 0)
>> 		nwritten = splice(pipefds[0], NULL, file_fd, NULL, res, SPLICE_F_MOVE|SPLICE_F_MORE);
>> }
> 
> Doesn't this reduce performance again? I thought the whole
> point of splice() was to increase performance by avoiding
> memory copies. If I have to do a poll syscall for each call
> to splice, doesn't the context switch eat that performance
> advantage again?
> 
> Or was splice designed only for multi-threaded applications
> (which at least Samba is not)?
> 
> Volker

splice() avoids memory copies yes, but on typical 1460 bytes
frames its a small gain.

But if no data is available on socket,
you still have to wait (and have a context switch later).

Waiting in poll() or splice() has same context switch cost.

Only cost is the extra syscall of course, but it is mandatory
if you want to avoid a possible deadlock in current splice()
implementation.

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