linux-kernel - Re: [this_cpu_xx V6 7/7] this_cpu: slub aggressive use of this

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Date:	Tue, 13 Oct 2009 13:15:27 -0700 (PDT)
From:	David Rientjes <rientjes@...gle.com>
To:	Christoph Lameter <cl@...ux-foundation.org>
cc:	Pekka Enberg <penberg@...helsinki.fi>, Tejun Heo <tj@...nel.org>,
	linux-kernel@...r.kernel.org,
	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>,
	Mel Gorman <mel@....ul.ie>,
	Zhang Yanmin <yanmin_zhang@...ux.intel.com>
Subject: Re: [this_cpu_xx V6 7/7] this_cpu: slub aggressive use of this_cpu
 operations in the hotpaths

On Tue, 13 Oct 2009, Christoph Lameter wrote:

> > I wonder how reliable these numbers are. We did similar testing a while back
> > because we thought kmalloc-96 caches had weird cache behavior but finally
> > figured out the anomaly was explained by the order of the tests run, not cache
> > size.
> 
> Well you need to look behind these numbers to see when the allocator uses
> the fastpath or slow path. Only the fast path is optimized here.
> 

With the netperf -t TCP_RR -l 60 benchmark I ran, CONFIG_SLUB_STATS shows 
the allocation fastpath is utilized quite a bit for a couple of key 
caches:

	cache		ALLOC_FASTPATH	ALLOC_SLOWPATH
	kmalloc-256	98125871	31585955
	kmalloc-2048	77243698	52347453

For an optimized fastpath, I'd expect such a workload would result in at 
least a slightly higher transfer rate.

I'll try the irqless patch, but this particular benchmark may not 
appropriately demonstrate any performance gain because of the added code 
in the also significantly-used slowpath.
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