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

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Message-ID: <20100519155505.GD2516@laptop>
Date:	Thu, 20 May 2010 01:55:05 +1000
From:	Nick Piggin <npiggin@...e.de>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	Miklos Szeredi <miklos@...redi.hu>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	mathieu.desnoyers@...icios.com, peterz@...radead.org,
	fweisbec@...il.com, tardyp@...il.com, mingo@...e.hu,
	acme@...hat.com, tzanussi@...il.com, paulus@...ba.org,
	linux-kernel@...r.kernel.org, arjan@...radead.org,
	ziga.mahkovec@...il.com, davem@...emloft.net, linux-mm@...ck.org,
	akpm@...ux-foundation.org, kosaki.motohiro@...fujitsu.com,
	cl@...ux-foundation.org, tj@...nel.org, jens.axboe@...cle.com
Subject: Re: Unexpected splice "always copy" behavior observed

On Wed, May 19, 2010 at 11:45:42AM -0400, Steven Rostedt wrote:
> On Wed, 2010-05-19 at 17:33 +0200, Miklos Szeredi wrote:
> > On Wed, 19 May 2010, Linus Torvalds wrote:
> > > 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.
> > 
> > Virtual machines might reasonably need this for splicing to a disk
> > image.
> 
> This comes down to balancing speed and complexity. Perhaps a copy is
> fine in this case.
> 
> I'm concerned about high speed tracing, where we are always just taking
> pages from the trace ring buffer and appending them to a file or sending
> them off to the network. The slower this is, the more likely you will
> lose events.
> 
> If the "move only on append to file" is easy to implement, I would
> really like to see that happen. The speed of splicing a disk image for a
> virtual machine only impacts the patience of the user. The speed of
> splicing tracing output, impacts how much you can trace without losing
> events.

It's not "easy" to implement :) What's your ring buffer look like?
Is it a normal user address which the kernel does copy_to_user()ish
things into? Or a mmapped special driver?

If the latter, it get's even harder again. But either way if the
source pages just have to be regenerated anyway (eg. via page fault
on next access), then it might not even be worthwhile to do the
splice move.

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