linux-kernel - Re: [RFC 0/3] Slab Defrag / Slab Targeted Reclaim and general Slab API changes

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Date:	Fri, 4 May 2007 22:14:46 -0700 (PDT)
From:	Christoph Lameter <clameter@....com>
To:	Eric Dumazet <dada1@...mosbay.com>
cc:	linux-kernel@...r.kernel.org, linux-mm@...ck.org, dgc@....com,
	Mel Gorman <mel@....ul.ie>
Subject: Re: [RFC 0/3] Slab Defrag / Slab Targeted Reclaim and general Slab
 API changes

On Sat, 5 May 2007, Eric Dumazet wrote:

> > C. Introduces a slab_ops structure that allows a slab user to provide
> >    operations on slabs.
> 
> Could you please make it const ?

Sure. Done.

> > All of this is really not necessary since the compiler knows how to align
> > structures and we should use this information instead of having the user
> > specify an alignment. I would like to get rid of SLAB_HWCACHE_ALIGN
> > and kmem_cache_create. Instead one would use the following macros (that
> > then result in a call to __kmem_cache_create).
> 
> Hum, the problem is the compiler sometimes doesnt know the target processor
> alignment.
> 
> Adding ____cacheline_aligned to 'struct ...' definitions might be overkill if
> you compile a generic kernel and happens to boot a Pentium III with it.

Then add ___cacheline_aligned_in_smp or specify the alignment in the 
various other ways that exist. Practice is that most slabs specify 
SLAB_HWCACHE_ALIGN. So most slabs are cache aligned today.

> G. Being able to track the number of pages in a kmem_cache
> 
> 
> If you look at fs/buffer.c, you'll notice the bh_accounting, recalc_bh_state()
> that might be overkill for large SMP configurations, when the real concern is
> to be able to limit the bh's not to exceed 10% of LOWMEM.
> 
> Adding a callback in slab_ops to track total number of pages in use by a given
> kmem_cache would be good.

Such functionality exists internal to SLUB and in the reporting tool. 
I can export that function if you need it.

> Same thing for fs/file_table.c : nr_file logic
> (percpu_counter_dec()/percpu_counter_inc() for each file open/close) could be
> simplified if we could just count the pages in use by filp_cachep kmem_cache.
> The get_nr_files() thing is not worth the pain.

Sure. What exactly do you want? The absolute number of pages of memory 
that the slab is using?

	kmem_cache_pages_in_use(struct kmem_cache *) ?

The call will not be too lightweight since we will have to loop over all 
nodes and add the counters in each per node struct for allocates slabs.

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