Re: [patch 00/23] Slab defragmentation V6

[Mel Gorman <mel@csn.ul.ie>]

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.

> and large amounts of dentries, inodes and buffer heads accumulate. In some
> load situations the slabs become very sparsely populated so that a lot of
> memory is wasted by slabs that only contain one or a few objects. In
> extreme cases the performance of a machine will become sluggish since
> we are continually running reclaim. Slab defragmentation adds the
> capability to recover wasted memory.
> 

When reading this first, I expected to find how slab objects get copied
around and packed which is my problem with the defragmentation name.
Again, not really that relevant to the code.

> 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?

>
>  or the MOVABLE areas (antifrag patches in mm).
> 

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.

> The patchset is also available via git
> 
> git pull git://git.kernel.org/pub/scm/linux/kernel/git/christoph/slab.git defrag
> 
> 
> Currently memory reclaim from the following slab caches is possible:
> 
> 1. dentry cache
> 2. inode cache (with a generic interface to allow easy setup of more
>    filesystems than the currently supported ext2/3/4 reiserfs, XFS
>    and proc)
> 3. buffer_heads
> 
> One typical mechanism that triggers slab defragmentation on my systems
> is the daily run of
> 
> 	updatedb
> 
> Updatedb scans all files on the system which causes a high inode and dentry
> use. After updatedb is complete we need to go back to the regular use
> patterns (typical on my machine: kernel compiles). Those need the memory now
> for different purposes. The inodes and dentries used for updatedb will
> gradually be aged by the dentry/inode reclaim algorithm which will free
> up the dentries and inode entries randomly through the slabs that were
> allocated. As a result the slabs will become sparsely populated. If they
> become empty then they can be freed but a lot of them will remain sparsely
> populated. That is where slab defrag comes in: It removes the slabs with
> just a few entries reclaiming more memory for other uses.
> 
> 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.

> V4->V5:
> - Support lumpy reclaim for slabs
> - Support reclaim via slab_shrink()
> - Add constructors to insure a consistent object state at all times.
> 
> V3->V4:
> - Optimize scan for slabs that need defragmentation
> - Add /sys/slab/*/defrag_ratio to allow setting defrag limits
>   per slab.
> - Add support for buffer heads.
> - Describe how the cleanup after the daily updatedb can be
>   improved by slab defragmentation.
> 
> V2->V3
> - Support directory reclaim
> - Add infrastructure to trigger defragmentation after slab shrinking if we
>   have slabs with a high degree of fragmentation.
> 
> V1->V2
> - Clean up control flow using a state variable. Simplify API. Back to 2
>   functions that now take arrays of objects.
> - Inode defrag support for a set of filesystems
> - Fix up dentry defrag support to work on negative dentries by adding
>   a new dentry flag that indicates that a dentry is not in the process
>   of being freed or allocated.
> 

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