Re: Unexpected splice "always copy" behavior observed

From: Rick Sherm
Date: Wed May 19 2010 - 16:59:23 EST

Sorry for deleting CC'd addresses. yahoo was whining...

--- On Wed, 5/19/10, Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx> wrote:

> From: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
> Subject: Re: Unexpected splice "always copy" behavior observed
> To: "Steven Rostedt" <rostedt@xxxxxxxxxxx>
> Cc: "Nick Piggin" <npiggin@xxxxxxx>, "Mathieu Desnoyers" <mathieu.desnoyers@xxxxxxxxxxxx>, "Peter Zijlstra" <peterz@xxxxxxxxxxxxx>, "Frederic Weisbecker" <fweisbec@xxxxxxxxx>, "Pierre Tardy" <tardyp@xxxxxxxxx>, "Ingo Molnar" <mingo@xxxxxxx>, "Arnaldo Carvalho de Melo" <acme@xxxxxxxxxx>, "Tom Zanussi" <tzanussi@xxxxxxxxx>, "Paul Mackerras" <paulus@xxxxxxxxx>, linux-kernel@xxxxxxxxxxxxxxx, arjan@xxxxxxxxxxxxx, ziga.mahkovec@xxxxxxxxx, "davem" <davem@xxxxxxxxxxxxx>, linux-mm@xxxxxxxxx, "Andrew Morton" <akpm@xxxxxxxxxxxxxxxxxxxx>, "KOSAKI Motohiro" <kosaki.motohiro@xxxxxxxxxxxxxx>, "Christoph Lameter" <cl@xxxxxxxxxxxxxxxxxxxx>, "Tejun Heo" <tj@xxxxxxxxxx>, "Jens Axboe" <jens.axboe@xxxxxxxxxx>
> Date: Wednesday, May 19, 2010, 2:59 PM
> 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.

I've a similar 'append' use case:

My mmapped buffers are pinned down.

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