Re: [patch 00/23] Slab defragmentation V6

Lee Schermerhorn
2007-11-08T21:20:40Z

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