linux-kernel - Re: [patch 1/3] slub: add per-cache slab thrash ratio

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Message-ID: <alpine.DEB.2.00.0903301545200.30928@chino.kir.corp.google.com>
Date:	Mon, 30 Mar 2009 15:48:44 -0700 (PDT)
From:	David Rientjes <rientjes@...gle.com>
To:	Christoph Lameter <cl@...ux.com>
cc:	Pekka Enberg <penberg@...helsinki.fi>,
	Nick Piggin <nickpiggin@...oo.com.au>,
	Martin Bligh <mbligh@...gle.com>, linux-kernel@...r.kernel.org
Subject: Re: [patch 1/3] slub: add per-cache slab thrash ratio

On Mon, 30 Mar 2009, Christoph Lameter wrote:

> > > > +	if (oo_objects(s->oo))
> > > > +		thrash_ratio = s->min_free_watermark * 100 / oo_objects(s->oo);
> > > >  	s->oo = oo_make(order, size);
> > >
> > > s->oo is set *after* you check it. Check oo_objects after the value has
> > > been set please.
> > >
> >
> > It's actually right the way I implemented it, oo_objects(s->oo) will be 0
> > when this is called for kmem_cache_open() meaning there is no preexisting
> > slab_thrash_ratio.  But this check is required when calculate_sizes() is
> > called from order_store() to adjust the slab_thrash_ratio for the new
> > objects per slab.  The above check is saving the old thrash ratio so
> > the new s->min_free_watermark value can be set following the
> > oo_make().  This was mentioned in the changelog for this patch:
> >
> > 	The value is stored in terms of the number of objects that the
> > 	ratio represents, not the ratio itself.  This avoids costly
> > 	arithmetic in the slowpath for a calculation that could otherwise
> > 	be done only when `slab_thrash_ratio' or `order' is changed.
> 
> Then its the wrong place to set it. Initializations are done in
> kmem_cache_open() after calculate_sizes are called.
> 

The way the code is currently written, this acts as an initialization when 
there was no previous object count (i.e. its coming from 
kmem_cache_open()) and acts as an adjustment when there was a previous 
count (i.e. /sys/kernel/slab/cache/order was changed).  The only way to 
avoid adding this to calculate_sizes() would be to add logic to 
order_store() to adjust the watermark when the order changes, but that 
duplicates the same calculation that is required for initialization if 
s->min_free_watermark does get a default value in kmem_cache_open() as 
Pekka suggested.
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