linux-kernel - Re: [00/41] Large Blocksize Support V7 (adds memmap support)

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Message-ID: <20070916174657.GA2393@lazybastard.org>
Date:	Sun, 16 Sep 2007 19:46:58 +0200
From:	Jörn Engel <joern@...fs.org>
To:	Andrea Arcangeli <andrea@...e.de>
Cc:	Goswin von Brederlow <brederlo@...ormatik.uni-tuebingen.de>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Joern Engel <joern@...fs.org>,
	Nick Piggin <nickpiggin@...oo.com.au>,
	Christoph Lameter <clameter@....com>,
	torvalds@...ux-foundation.org, linux-fsdevel@...r.kernel.org,
	linux-kernel@...r.kernel.org, Christoph Hellwig <hch@....de>,
	Mel Gorman <mel@...net.ie>,
	William Lee Irwin III <wli@...omorphy.com>,
	David Chinner <dgc@....com>,
	Jens Axboe <jens.axboe@...cle.com>,
	Badari Pulavarty <pbadari@...il.com>,
	Maxim Levitsky <maximlevitsky@...il.com>,
	Fengguang Wu <fengguang.wu@...il.com>,
	swin wang <wangswin@...il.com>, totty.lu@...il.com,
	hugh@...itas.com
Subject: Re: [00/41] Large Blocksize Support V7 (adds memmap support)

On Sun, 16 September 2007 00:30:32 +0200, Andrea Arcangeli wrote:
> 
> Movable? I rather assume all slab allocations aren't movable. Then
> slab defrag can try to tackle on users like dcache and inodes. Keep in
> mind that with the exception of updatedb, those inodes/dentries will
> be pinned and you won't move them, which is why I prefer to consider
> them not movable too... since there's no guarantee they are.

I have been toying with the idea of having seperate caches for pinned
and movable dentries.  Downside of such a patch would be the number of
memcpy() operations when moving dentries from one cache to the other.
Upside is that a fair amount of slab cache can be made movable.
memcpy() is still faster than reading an object from disk.

Most likely the current reaction to such a patch would be to shoot it
down due to overhead, so I didn't pursue it.  All I have is an old patch
to seperate never-cached from possibly-cached dentries.  It will
increase the odds of freeing a slab, but provide no guarantee.

But the point here is: dentries/inodes can be made movable if there are
clear advantages to it.  Maybe they should?

Jörn

-- 
Joern's library part 2:
http://www.art.net/~hopkins/Don/unix-haters/tirix/embarrassing-memo.html
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