linux-kernel - Re: dentries: dentry defragmentation

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Date:	Mon, 1 Feb 2010 21:16:45 +1100
From:	Nick Piggin <npiggin@...e.de>
To:	Andi Kleen <andi@...stfloor.org>
Cc:	Al Viro <viro@...IV.linux.org.uk>,
	Christoph Lameter <cl@...ux-foundation.org>,
	Dave Chinner <david@...morbit.com>,
	Alexander Viro <viro@....linux.org.uk>,
	Christoph Hellwig <hch@...radead.org>,
	Christoph Lameter <clameter@....com>,
	Rik van Riel <riel@...hat.com>,
	Pekka Enberg <penberg@...helsinki.fi>,
	akpm@...ux-foundation.org, Miklos Szeredi <miklos@...redi.hu>,
	Nick Piggin <nickpiggin@...oo.com.au>,
	Hugh Dickins <hugh@...itas.com>, linux-kernel@...r.kernel.org
Subject: Re: dentries: dentry defragmentation

On Mon, Feb 01, 2010 at 11:10:13AM +0100, Andi Kleen wrote:
> On Mon, Feb 01, 2010 at 06:08:35PM +1100, Nick Piggin wrote:
> > I always preferred to do defrag in the opposite way. Ie. query the
> > slab allocator from existing shrinkers rather than opposite way
> > around. This lets you reuse more of the locking and refcounting etc.
> 
> I looked at this for hwpoison soft offline.
> 
> But it works really badly because the LRU list ordering 
> has nothing to do with the actual ordering inside the slab pages.

No, you don't *have* to follow LRU order. The most important thing
is if you followed what I wrote is to get a pin on the objects and
the slabs via the regular shrinker path first, then querying slab
rather than calling into all these subsystems from an atomic, and
non-slab-reentrant path.

Following LRU order would just be the first and simplest cut at
this.

 
> Christoph's basic approach is more efficient.

I want to see numbers because it is also the far more complex
approach.

 
> > So you have a pin on the object somehow via the normal shrinker path,
> > and therefore you get a pin on the underlying slab. I would just like
> > to see even performance of a real simple approach that just asks
> > whether we are in this slab defrag mode, and if so, whether the slab
> > is very sparse. If yes, then reclaim aggressively.
> 
> The typical result is that you need to get through most of the LRU
> list (and prune them all) just to free the page.

Really? If you have a large proportion of slabs which are quite
internally fragmented, then I would have thought it would give a
significant improvement (aggressive reclaim, that is).


> > If that doesn't perform well enough and you have to go further and
> 
> It doesn't.

Can we see your numbers? And the patches you tried?

Thanks,
Nick
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