Re: [patch 00/23] Slab defragmentation V6

[mel@skynet.ie (Mel Gorman)]

On (08/11/07 11:12), Christoph Lameter didst pronounce:
> 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.
> 

Ok, fair enough logic and it's a bit clearer in my head how to separate them
out. Thanks

> > > 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 ....
> 

Yeah, they are great for all occasions.

In fairness when the time comes, I can do a few tests using the hugepage
allocation tests with ZONE_MOVABLE and Badari might do a few tests with
memory hot-remove. Currently, the success rates for these tests are 100%
within ZONE_MOVABLE although that is without locked pages. Hot-remove
should be able to deal with locked pages but hugepage allocation wouldn't
as lumpy-reclaim would fail. If we allow slab pages to use the zone and the
success rates drop, it'll be obvious which is a plus at least.

> ZONE_MOVABLE also contains mlocked pages that are also not reclaimable. 

True, but they are movable so for example memory hot-remove is able to
deal with them and the memory compaction patches should have been able
to deal with it too.

> 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.
> 

It certainly can be tried out. However, this is a future problem and
independent of the current patchset. I don't want to drag us down a blind
alley about a problem that isn't even at hand.

Right now, I think the set looks in good shape for wider testing and appears
to solve a major part of the slab fragmentation problem. Assuming I don't
fall down a hole testing one-zonelist and the mm-broken-out patches, I'll
get to testing these patches as well.

> > It'd still be valid to leave them as MIGRATE_RECLAIMABLE because that is
> > what they are. Arguably, MIGRATE_RECLAIMABLE could be dropped in it's
> > entirety but I'd rather not as reclaimable blocks have significantly
> > different reclaim costs to pages that are currently marked movable.
> 
> Right. That would simplify the antifrag methods. Is there any way to 
> measure the reclaim costs?
> 

Regrettably, no.

> > > V5->V6
> > > - Rediff against 2.6.24-rc2 + mm slub patches.
> > > - Add reviewed by lines.
> > > - Take out the experimental code to make slab pages movable. That
> > >   has to wait until this has been considered by Mel.
> > > 
> > 
> > I still haven't considered them properly. I've been backlogged for I
> > don't know how long at this point and this is on the increasingly large
> > todo list :( . I don't believe it is massively urgent at the moment
> > though and reclaiming to start with is perfectly adequate just as lumpy
> > reclaim is fine at the moment.
> 
> Right. We can defer this for now.
> 

Agreed.

-- 
Mel Gorman
Part-time Phd Student                          Linux Technology Center
University of Limerick                         IBM Dublin Software Lab
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