linux-kernel - Re: [RFC PATCH] fuse: support splice() reading from fuse device

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Message-Id: <E1OFAsd-0000Ra-1V@pomaz-ex.szeredi.hu>
Date:	Thu, 20 May 2010 20:54:03 +0200
From:	Miklos Szeredi <miklos@...redi.hu>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
CC:	miklos@...redi.hu, linux-fsdevel@...r.kernel.org,
	linux-mm@...ck.org, linux-kernel@...r.kernel.org,
	jens.axboe@...cle.com, akpm@...ux-foundation.org
Subject: Re: [RFC PATCH] fuse: support splice() reading from fuse device

On Thu, 20 May 2010, Linus Torvalds wrote:
> Are there actual real loads that get improved? I don't care if it means 
> that the improvement goes from three orders of magnitude to just a couple 
> of percent. The "couple of percent on actual loads" is a lot more 
> important than "many orders of magnitude on a made-up benchmark".

The reason I've been looking at zero copy for fuse is that embedded
people have been complaining about fuse's large CPU overhead for I/O.
So large in fact that it was having a performance impact even for
relatively slow devices.  And most of that overhead comes from copying
data around.

So it's not the 20GB/s throughput that's interesting but the reduced
CPU overhead, especially on slower processors.  Apart from cache
effects 20GB/s throughput with a null filesystem means 1% CPU at
200MB/s transfer speed with _any_ filesystem.  Without bigger requests
that translates to 4% overhead and without zero copy about 15%.
That's on a core2/1.8GHz, with an embedded CPU the overhead reduction
would be even more significant.

Miklos
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