linux-kernel - Re: [PATCH 1/3] slub: set a criteria for slub node partial adding

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Date:	Tue, 6 Dec 2011 13:06:07 -0800 (PST)
From:	David Rientjes <rientjes@...gle.com>
To:	"Alex,Shi" <alex.shi@...el.com>
cc:	Christoph Lameter <cl@...ux.com>,
	"penberg@...nel.org" <penberg@...nel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	Andi Kleen <ak@...ux.intel.com>
Subject: Re: [PATCH 1/3] slub: set a criteria for slub node partial adding

On Mon, 5 Dec 2011, Alex,Shi wrote:

> Previous testing depends on 3.2-rc1, that show hackbench performance has
> no clear change, and netperf get some benefit. But seems after
> irqsafe_cpu_cmpxchg patch, the result has some change. I am collecting
> these results. 
> 

netperf will also degrade with this change on some machines, there's no 
clear heuristic that can be used to benefit all workloads when deciding 
where to add a partial slab into the list.  Cache hotness is great but 
your patch doesn't address situations where frees happen to a partial slab 
such that they may be entirely free (or at least below your 1:4 inuse to 
nr_objs threshold) at the time you want to deactivate the cpu slab.

I had a patchset that iterated the partial list and found the "most free" 
partial slab (and terminated prematurely if a threshold had been reached, 
much like yours) and selected that one, and it helped netperf 2-3% in my 
testing.  So I disagree with determining where to add a partial slab to 
the list at the time of free because it doesn't infer its state at the 
time of cpu slab deactivation.
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