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

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Date:	Mon, 17 Mar 2008 10:32:15 -0700 (PDT)
From:	Christoph Lameter <clameter@....com>
To:	"Zhang, Yanmin" <yanmin_zhang@...ux.intel.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 Mon, 17 Mar 2008, Zhang, Yanmin wrote:

> 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.

Interesting. What is the optimal configuration for your 8p? Could you 
figure out the optimal configuration for an 4p and a 2p configuration?

> 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.

Well for a 4k cpu configu this would set min_objects to 8192. So I think 
we could implement a form of logarithmic scaling based on cpu 
counts comparable to what is done for the statistics update in vmstat.c

fls(num_online_cpus()) = 4

So maybe

slub_min_objects= 8 + (2 + fls(num_online_cpus())) * 4
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