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

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Message-Id: <1205810884.3215.543.camel@ymzhang>
Date:	Tue, 18 Mar 2008 11:28:04 +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 Mon, 2008-03-17 at 10:32 -0700, Christoph Lameter wrote:
> 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?
I used 8-core stoakley to do testing, and tried boot kernel with maxcpus=4 and 2.

Just ran ./hackbench 100 process 2000.

processor number\slub_min_objects        |       slub_min_objects=8  |   16  |   32   | 64
--------------------------------------------------------------------------------------------
                  8p                     |               60second    |   30  |   28.5 | 26.5
--------------------------------------------------------------------------------------------
                  4p                     |               50second    |   43  |   42   | 
--------------------------------------------------------------------------------------------
                  2p                     |               92second    |   79  |        |


As stoakley is just multi-core machine and hasn't hyper-threading, I also tested it on an old
harwich machine which has 4 physical processors and 8 logical processors with hyperthreading.
processor number\slub_min_objects        |       slub_min_objects=8  |   16  |   32   | 64
--------------------------------------------------------------------------------------------
                  8p                     |               78.7second  |   77.5|        |


> 
> > 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
num_online_cpus as the input parameter is ok. A potential issue is how to consider cpu hot-plug.

When num_online_cpus()=16, fls(num_online_cpus())=5.

> 
> So maybe
> 
> slub_min_objects= 8 + (2 + fls(num_online_cpus())) * 4
So slub_min_objects= 8 + (1 + fls(num_online_cpus())) * 4.


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