linux-kernel - Re: Quadratic behavior of shrink_dcache

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Message-Id: <200611210003.13417.rjw@sisk.pl>
Date:	Tue, 21 Nov 2006 00:03:12 +0100
From:	"Rafael J. Wysocki" <rjw@...k.pl>
To:	Miklos Szeredi <miklos@...redi.hu>
Cc:	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
	Andrew Morton <akpm@...l.org>,
	Nigel Cunningham <ncunningham@...uxmail.org>,
	Pavel Machek <pavel@....cz>
Subject: Re: Quadratic behavior of shrink_dcache_parent()

On Monday, 20 November 2006 12:10, Miklos Szeredi wrote:
> The shrink_dcache_parent() can take a very long time for deep
> directory trees: minutes for depth of 100,000, probably hours for
> depth of 1,000,000.
> 
> The reason is that after dropping a leaf, it starts again from the
> root.

Oh, well.  So that's the reason why the shrinking of memory in swsusp can
take so much time.

> Filesystems affected include FUSE, NFS, CIFS.  Others I haven't
> checked.  NFS and to a lesser extent CIFS don't seem to efficiently
> handle lookups within such a deep hierarchy, so they're sort of
> immune.
> 
> But with FUSE it's pretty easy to DoS the system.
> 
> Limiting the depth to some sane value could work around this problem,
> but that would mean having to traverse subtrees in rename().
> 
> Any better ideas?

None, for now.  It looks like I need to learn that code ...

Greetings,
Rafael


-- 
You never change things by fighting the existing reality.
		R. Buckminster Fuller
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