linux-kernel - Re: hackbench regression since 2.6.25-rc

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Date:	Mon, 17 Mar 2008 15:50:04 +0800
From:	"Zhang, Yanmin" <yanmin_zhang@...ux.intel.com>
To:	Christoph Lameter <clameter@....com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Kay Sievers <kay.sievers@...y.org>,
	Greg Kroah-Hartman <gregkh@...e.de>,
	LKML <linux-kernel@...r.kernel.org>, Ingo Molnar <mingo@...e.hu>
Subject: Re: hackbench regression since 2.6.25-rc

On Fri, 2008-03-14 at 14:06 -0700, Christoph Lameter wrote:
> On Fri, 14 Mar 2008, Zhang, Yanmin wrote:
> 
> > On my 8-core stoakley, there is no such regression. Below data is after 
> > testing.
> 
> I was looking for the details on two slab caches. The comparison of the 
> details statistics is likely very interesting because we will be able to
> see how the doubling of processor counts affects the internal behavior of 
> slub.
I collected more data on 16-p tigerton to try to find the possible relationship
between slub_min_objects and processor number. Kernel is 2.6.25-rc5.

Command\slub_min_objects	|	slub_min_objects=8  |   16  |   32   |  64
-------------------------------------------------------------------------------------
./hackbench 100 process 2000    |  		250second   |	23  |   18.6 |  17.5
./hackbench 200 process 2000	|	      	532         |	44  |   35.6 |  33.5

The first command line will start 4000 processes and the second will start 8000 processes.

As the problemtic slab is kmalloc-512, slub_min_objects=8 is just the default configuration.

Oprofile data shows the ratio of __slab_alloc+__slab_free+add_partial has no difference
between the 2 commandline with the same kernel boot parameters.

slub_min_objects	                                   | 8      |   16   |   32   |  64
--------------------------------------------------------------------------------------------
slab(__slab_alloc+__slab_free+add_partial) cpu utilization | 88.00% | 44.00% | 13.00% | 12%

When slub_min_objects=32, we could get a reasonable value. Beyond 32, the improvement
is very small. 32 is just possible_cpu_number*2 on my tigerton.

It's hard to say hackbench simulates real applications closely. But it discloses a possible
performance bottlebeck. Last year, we once captured the kmalloc-2048 issue by tbench. So the
default slub_min_objects need to be revised. In the other hand, slab is allocated by alloc_page
when its size is equal to or more than a half page, so enlarging slub_min_objects won't create
too many slab page buffers.

As for NUMA, perhaps we could define slub_min_objects to 2*max_cpu_number_per_node.

-yanmin

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