linux-kernel - Re: Unexpected splice "always copy" behavior observed

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Message-ID: <alpine.LFD.2.00.1005190758070.23538@i5.linux-foundation.org>
Date:	Wed, 19 May 2010 07:59:48 -0700 (PDT)
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Steven Rostedt <rostedt@...dmis.org>
cc:	Nick Piggin <npiggin@...e.de>,
	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Pierre Tardy <tardyp@...il.com>, Ingo Molnar <mingo@...e.hu>,
	Arnaldo Carvalho de Melo <acme@...hat.com>,
	Tom Zanussi <tzanussi@...il.com>,
	Paul Mackerras <paulus@...ba.org>,
	linux-kernel@...r.kernel.org, arjan@...radead.org,
	ziga.mahkovec@...il.com, davem <davem@...emloft.net>,
	linux-mm@...ck.org, Andrew Morton <akpm@...ux-foundation.org>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	Christoph Lameter <cl@...ux-foundation.org>,
	Tejun Heo <tj@...nel.org>, Jens Axboe <jens.axboe@...cle.com>
Subject: Re: Unexpected splice "always copy" behavior observed



On Wed, 19 May 2010, Steven Rostedt wrote:

> On Wed, 2010-05-19 at 07:39 -0700, Linus Torvalds wrote:
> 
> > The real limitation is likely always going to be the fact that it has to 
> > be page-aligned and a full page. For a lot of splice inputs, that simply 
> > won't be the case, and you'll end up copying for alignment reasons anyway.
> 
> That's understandable. For the use cases of splice I use, I work to make
> it page aligned and full pages. Anyone else using splice for
> optimizations, should do the same. It only makes sense.
> 
> The end of buffer may not be a full page, but then it's the end anyway,
> and I'm not as interested in the speed.

Btw, since you apparently have a real case - is the "splice to file" 
always just an append? IOW, if I'm not right in assuming that the only 
sane thing people would reasonable care about is "append to a file", then 
holler now.

		Linus
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