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

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Message-ID: <alpine.DEB.2.00.0903301309010.21074@chino.kir.corp.google.com>
Date:	Mon, 30 Mar 2009 13:12:55 -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:

> > @@ -2291,10 +2292,13 @@ static int calculate_sizes(struct kmem_cache *s, int forced_order)
> >  	/*
> >  	 * Determine the number of objects per slab
> >  	 */
> > +	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.
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