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

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Date:	Wed, 14 Jan 2009 11:03:30 +0100
From:	Volker Lendecke <Volker.Lendecke@...Net.DE>
To:	Eric Dumazet <dada1@...mosbay.com>
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?

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

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