linux-kernel - Re: hackbench regression due to commit 9dfc6e68bfe6e

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Date:	Fri, 02 Apr 2010 16:06:29 +0800
From:	"Zhang, Yanmin" <yanmin_zhang@...ux.intel.com>
To:	Christoph Lameter <cl@...ux-foundation.org>
Cc:	alex.shi@...el.com,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"Ma, Ling" <ling.ma@...el.com>,
	"Chen, Tim C" <tim.c.chen@...el.com>, Tim C <tim.c.chen@...el.com>,
	Pekka Enberg <penberg@...helsinki.fi>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: hackbench regression due to commit 9dfc6e68bfe6e

On Thu, 2010-04-01 at 10:53 -0500, Christoph Lameter wrote:
> On Thu, 1 Apr 2010, Zhang, Yanmin wrote:
> 
> > I suspect the moving of place of cpu_slab in kmem_cache causes the new cache
> > miss. But when I move it to the tail of the structure, kernel always panic when
> > booting. Perhaps there is another potential bug?
> 
> Why would that cause an additional cache miss?
> 
> 
> The node array is following at the end of the structure. If you want to
> move it down then it needs to be placed before the node field

Thanks. The moving cpu_slab to tail doesn't improve it. 

I used perf to collect statistics. Only data cache miss has a little difference.
My testing command on my 2 socket machine:
#hackbench 100 process 20000

With 2.6.33, it takes for about 96 seconds while 2.6.34-rc2 (or the latest tip tree)
takes for about 101 seconds.

perf shows some functions around SLUB have more cpu utilization, while some other
SLUB functions have less cpu utilization.


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