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

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Message-ID: <20100519155732.GB2039@Krystal>
Date:	Wed, 19 May 2010 11:57:32 -0400
From:	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	Miklos Szeredi <miklos@...redi.hu>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	npiggin@...e.de, 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

* Steven Rostedt (rostedt@...dmis.org) 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.

I'm with Steven here. I only care about appending full pages at the end of a
file. If possible, I'd also like to steal back the pages after waiting for the
writeback I/O to complete so we can put them back in the ring buffer without
stressing the page cache and the page allocator needlessly.

Thanks,

Mathieu


-- 
Mathieu Desnoyers
Operating System Efficiency R&D Consultant
EfficiOS Inc.
http://www.efficios.com
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