linux-kernel - Re: [patch 10/21] buffer heads: Support slab defrag

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Message-ID: <20080520231942.GX103491721@sgi.com>
Date:	Wed, 21 May 2008 09:19:42 +1000
From:	David Chinner <dgc@....com>
To:	Evgeniy Polyakov <johnpol@....mipt.ru>
Cc:	David Chinner <dgc@....com>, Christoph Lameter <clameter@....com>,
	akpm@...ux-foundation.org, linux-kernel@...r.kernel.org,
	linux-fsdevel@...r.kernel.org, Mel Gorman <mel@...net.ie>,
	andi@...stfloor.org, Rik van Riel <riel@...hat.com>,
	Pekka Enberg <penberg@...helsinki.fi>, mpm@...enic.com
Subject: Re: [patch 10/21] buffer heads: Support slab defrag

On Wed, May 21, 2008 at 02:25:05AM +0400, Evgeniy Polyakov wrote:
> On Wed, May 21, 2008 at 07:46:17AM +1000, David Chinner (dgc@....com) wrote:
> > Oh, god no. Let's not put the inode_lock right at the top of the VM page
> > cleaning path. We don't need to modify inode state, the superblock dirty
> > lists, etc - all we need to do is write dirty pages on a given mapping in
> > a more efficient manner.
> 
> I'm not advocating that, but having swap on reclaim does not hurt anyone,
> this is essentially the same, but with different underlying storage.

Sure. But my point is simply that sync_inode() is far too
heavy-weight to be used in a reclaim context. The fact that it holds
the inode_lock will interfere with normal writeback via pdflush and
that could potentially slow down writeback even more.

e.g. think of kswapd threads running on 20 nodes of a NUMA machine
all at once writing back dirty memory (yes, it happens). If we use
sync_inode() to write back dirty mappings we would then have at
least 20 CPUs serialising on the inode_lock trying to write back
pages. If we instead use a thin wrapper around ->writepages() then
they can all run in parallel through the filesystem(s), block
devices, etc rather than being serialised at the highest possible
layer....

> System
> will do that anyway sooner or later during usual writeback, which in turn
> can be a result of the same reclaim...

Cheers,

Dave.
-- 
Dave Chinner
Principal Engineer
SGI Australian Software Group
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