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

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Date:	Wed, 19 May 2010 12:31:25 -0700 (PDT)
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
cc:	Nick Piggin <npiggin@...e.de>,
	Steven Rostedt <rostedt@...dmis.org>,
	Miklos Szeredi <miklos@...redi.hu>, 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, 19 May 2010, Mathieu Desnoyers wrote:
> 
> Good point. This discard flag might do the trick and let us keep things simple.
> The major concern here is to keep the page cache disturbance relatively low.
> Which of new page allocation or stealing back the page has the lowest overhead
> would have to be determined with benchmarks.

We could probably make it easier somehow to do the writeback and discard 
thing, but I have had _very_ good experiences with even a rather trivial 
file writer that basically used (iirc) 8MB windows, and the logic was very 
trivial:

 - before writing a new 8M window, do "start writeback" 
   (SYNC_FILE_RANGE_WRITE) on the previous window, and do 
   a wait (SYNC_FILE_RANGE_WAIT_AFTER) on the window before that.

in fact, in its simplest form, you can do it like this (this is from my 
"overwrite disk images" program that I use on old disks):

	for (index = 0; index < max_index ;index++) {
		if (write(fd, buffer, BUFSIZE) != BUFSIZE)
			break;
		/* This won't block, but will start writeout asynchronously */
		sync_file_range(fd, index*BUFSIZE, BUFSIZE, SYNC_FILE_RANGE_WRITE);
		/* This does a blocking write-and-wait on any old ranges */
		if (index)
			sync_file_range(fd, (index-1)*BUFSIZE, BUFSIZE, SYNC_FILE_RANGE_WAIT_BEFORE|SYNC_FILE_RANGE_WRITE|SYNC_FILE_RANGE_WAIT_AFTER);
	}

and even if you don't actually do a discard (maybe we should add a 
SYNC_FILE_RANGE_DISCARD bit, right now you'd need to do a separate 
fadvise(FADV_DONTNEED) to throw it out) the system behavior is pretty 
nice, because the heavy writer gets good IO performance _and_ leaves only 
easy-to-free pages around after itself.

		Linus
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