Subject: Re: [patch 00/23] Slab defragmentation V6
From: Lee Schermerhorn <Lee.Schermerhorn@hp.com>
To: Christoph Lameter <clameter@sgi.com>
Cc: Mel Gorman <mel@csn.ul.ie>, akpm@linux-foundatin.org,
       linux-kernel@vger.kernel.org, linux-mm@kvack.org
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Organization: HP/OSLO
Date: Thu, 08 Nov 2007 15:58:15 -0500
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On Thu, 2007-11-08 at 11:12 -0800, Christoph Lameter wrote:
> On Thu, 8 Nov 2007, Mel Gorman wrote:
> 
> > On Tue, 2007-11-06 at 17:11 -0800, Christoph Lameter wrote:
> > > Slab defragmentation is mainly an issue if Linux is used as a fileserver
> > 
> > Was hoping this would get renamed to SLUB Targetted Reclaim from
> > discussions at VM Summit. As no copying is taking place, it's confusing
> > to call it defragmentation to me anyway. Not a major deal but it made
> > reading the patches a little confusing.
> 
> The problem is that people are focusing on one feature here and forget 
> about the rest. Targetted reclaim is one feature that was added later when 
> lumpy reclaim was added to the kernel. The primary intend of this patchset 
> was always to reduce the fragmentation. The name is appropriate and the 
> patchset will support copying of objects as soon as support for that is 
> added to the kick(). In that case the copying you are looking for will be 
> there. The simple implementation for the kick() methods is to simply copy
> pieces of the reclaim code. That is what is included here.
> 
> > > With lumpy reclaim slab defragmentation can be used to enhance the
> > > ability to recover larger contiguous areas of memory. Lumpy reclaim currently
> > > cannot do anything if a slab page is encountered. With slab defragmentation
> > > that slab page can be removed and a large contiguous page freed. It may
> > > be possible to have slab pages also part of ZONE_MOVABLE (Mel's defrag
> > > scheme in 2.6.23)
> > 
> > More terminology nit-pick - ZONE_MOVABLE is not defragmenting anything.
> > It's just partitioning memory. The slab pages need to be 100%
> > reclaimable or movable for that to happen but even with targetted
> > reclaim, some dentries such as the root directory one cannot be
> > reclaimed, right?
> 
> 100%? I am so fond of these categorical statements ....
> 
> ZONE_MOVABLE also contains mlocked pages that are also not reclaimable. 
> The question is at what level would it be possible to make them MOVABLE? 
> It may take some improvements to the kick() methods to make eviction more 
> reliable. Allowing the moving of objects in the kick() methods will 
> likely get usthere.

Christoph:  Although mlocked pages are not reclaimable, they ARE
migratable.  You fixed that a long time ago.  [And I just verified with
memtoy.]  Doesn't this make them "movable"?

Lee

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