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

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Message-ID: <20090330155419.GC27169@csn.ul.ie>
Date:	Mon, 30 Mar 2009 16:54:19 +0100
From:	Mel Gorman <mel@....ul.ie>
To:	Pekka Enberg <penberg@...helsinki.fi>
Cc:	David Rientjes <rientjes@...gle.com>,
	Christoph Lameter <cl@...ux-foundation.org>,
	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, Mar 30, 2009 at 10:11:31AM +0300, Pekka Enberg wrote:
> On Sun, 2009-03-29 at 22:43 -0700, David Rientjes wrote:
> > The slab_thrash_ratio for each cache do not have non-zero defaults
> > (yet?).
> 
> If we're going to merge this code, I think it would be better to put a
> non-zero default there; otherwise we won't be able to hit potential
> performance regressions or bugs. Furthermore, the optimization is not
> very useful on large scale if people need to enable it themselves.
> 
> Maybe stick 20 there and run tbench, sysbench, et al to see if it makes
> a difference? I'm cc'ing Mel in case he has some suggestions how to test
> it.
> 

netperf and tbench will both pound the sl*b allocator far more than sysbench
will in my opinion although I don't have figures on-hand to back that up. In
the case of netperf, it might be particular obvious if the client is on one
CPU and the server on another because I believe that means all allocs happen
on one CPU and all frees on another.

I have a vague concern that such a tunable needs to exist at all though
and wonder what workloads it can hurt when set to 20 for example versus any
other value.

-- 
Mel Gorman
Part-time Phd Student                          Linux Technology Center
University of Limerick                         IBM Dublin Software Lab
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