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

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Message-ID: <alpine.DEB.2.00.0903311014140.10407@chino.kir.corp.google.com>
Date:	Tue, 31 Mar 2009 10:21:42 -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 Tue, 31 Mar 2009, Christoph Lameter wrote:

> > @@ -3833,7 +3830,9 @@ static ssize_t order_store(struct kmem_cache *s,
> >  	if (order > slub_max_order || order < slub_min_order)
> >  		return -EINVAL;
> >
> > +	thrash_ratio = s->min_free_watermark * 100 / oo_objects(s->oo);
> >  	calculate_sizes(s, order);
> > +	s->min_free_watermark = oo_objects(s->oo) * thrash_ratio / 100;
> >  	return length;
> >  }
> >
> 
> Hmmm.. Still calculating the trash ratio based on existing objects per
> slab and then resetting the objects per slab to a different number.
> Shouldnt the trash_ratio simply be zapped to an initial value if the
> number of objects per slab changes?
> 

Each cache with >= 20 objects per slab will get a default 
slab_thrash_ratio of 20 in v2 of the series.  If the order of a cache is 
subsequently tuned, the default slab_thrash_ratio would be cleared without 
knowledge to the user.

I'd agree that it should be cleared if the tunable had object units 
instead of a ratio, but the ratio simply applies to any given order.
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