linux-kernel - Re: [PATCH 2.6.19-rc3] VFS: per-sb dentry lru list

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Date:	Mon, 30 Oct 2006 16:08:25 +0100
From:	Eric Dumazet <dada1@...mosbay.com>
To:	Vasily Averin <vvs@...ru>
Cc:	Andrew Morton <akpm@...l.org>, David Howells <dhowells@...hat.com>,
	Neil Brown <neilb@...e.de>, Jan Blunck <jblunck@...e.de>,
	Olaf Hering <olh@...e.de>, Balbir Singh <balbir@...ibm.com>,
	Kirill Korotaev <dev@...nvz.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	devel@...nvz.org
Subject: Re: [PATCH 2.6.19-rc3] VFS: per-sb dentry lru list

On Monday 30 October 2006 15:24, Vasily Averin wrote:
> Andrew Morton wrote:
> > On Fri, 27 Oct 2006 18:05:50 +0400
> >
> > Vasily Averin <vvs@...ru> wrote:
> >> Virtuozzo/OpenVZ linux kernel team has discovered that umount/remount
> >> can last for hours looping in shrink_dcache_sb() without much successes.
> >> Since during shrinking s_umount semaphore is taken lots of other
> >> unrelated operations like sync can stop working until shrink finished.
> >
> > Did you consider altering shrink_dcache_sb() so that it holds onto
> > dcache_lock and moves all the to-be-pruned dentries onto a private list
> > in a single pass, then prunes them all outside the lock?
>
> At the first glance it is wrong because of 2 reasons:
> 1) it continues to check the whole global LRU list (we propose to use
> per-sb LRU, it will provide very quick search)

Quick search maybe, but your patch adds 2 pointers to each dentry in the 
system... That's pretty expensive, as dentries are already using a *lot* of 
ram.

Maybe an alternative would be to not have anymore a global dentry_unused, but 
only per-sb unused dentries lists ?


> 2) we have not any guarantee that someone will add new unused dentries to
> the list when we prune it outside the lock. And to the contrary, some of
> unused dentries can be used again. As far as I understand we should hold
> dcache_lock beginning at the removing dentry from unused_list until
> dentry_iput() call.
>
> David did it inside shrink_dcache_for_umount() just because it have
> guarantee that all the filesystem operations are finished and new ones
> cannot be started.

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