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

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Date:	Mon, 30 Mar 2009 17:19:45 -0400 (EDT)
From:	Christoph Lameter <cl@...ux.com>
To:	David Rientjes <rientjes@...gle.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, David Rientjes 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.
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