linux-kernel - Re: [PATCH 0/8] Reduce latencies and improve overall reclaim efficiency v2

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Date:	Wed, 03 Nov 2010 11:50:35 +0100
From:	Christian Ehrhardt <ehrhardt@...ux.vnet.ibm.com>
To:	Mel Gorman <mel@....ul.ie>
CC:	Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
	linux-fsdevel@...r.kernel.org,
	Linux Kernel List <linux-kernel@...r.kernel.org>,
	Johannes Weiner <hannes@...xchg.org>,
	Minchan Kim <minchan.kim@...il.com>,
	Wu Fengguang <fengguang.wu@...el.com>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>
Subject: Re: [PATCH 0/8] Reduce latencies and improve overall reclaim efficiency
 v2



On 10/18/2010 03:55 PM, Mel Gorman wrote:
> On Thu, Oct 14, 2010 at 05:28:33PM +0200, Christian Ehrhardt wrote:
> 
>> Seing the patches Mel sent a few weeks ago I realized that this series
>> might be at least partially related to my reports in 1Q 2010 - so I ran my
>> testcase on a few kernels to provide you with some more backing data.
> 
> Thanks very much for revisiting this.
> 
>> Results are always the average of three iozone runs as it is known to be somewhat noisy - especially when affected by the issue I try to show here.
>> As discussed in detail in older threads the setup uses 16 disks and scales the number of concurrent iozone processes.
>> Processes are evenly distributed so that it always is one process per disk.
>> In the past we reported 40% to 80% degradation for the sequential read case based on 2.6.32 which can still be seen.
>> What we found was that the allocations for page cache with GFP_COLD flag loop a long time between try_to_free, get_page, reclaim as free makes some progress and due to that GFP_COLD allocations can loop and retry.
>> In addition my case had no writes at all, which forced congestion_wait to wait the full timeout all the time.
>>
>> Kernel (git)                   4          8         16   deviation #16 case                           comment
>> linux-2.6.30              902694    1396073    1892624                 base                              base
>> linux-2.6.32              752008     990425     932938               -50.7%     impact as reported in 1Q 2010
>> linux-2.6.35               63532      71573      64083               -96.6%                    got even worse
>> linux-2.6.35.6            176485     174442     212102               -88.8%  fixes useful, but still far away
>> linux-2.6.36-rc4-trace    119683     188997     187012               -90.1%                         still bad
>> linux-2.6.36-rc4-fix      884431    1114073    1470659               -22.3%            Mels fixes help a lot!
>>
[...]
> If all goes according to plan,
> kernel 2.6.37-rc1 will be of interest. Thanks again.

Here a measurement with 2.6.37-rc1 as confirmation of progress:
   linux-2.6.37-rc1          876588    1161876    1643430               -13.1%       even better than 2.6.36-fix

That means 2.6.37-rc1 really shows what we hoped for.
And it eventually even turned out a little bit better than 2.6.36 + your fixes.

 

-- 

Grüsse / regards, Christian Ehrhardt
IBM Linux Technology Center, System z Linux Performance 
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