linux-kernel - Re: [PATCHv11 2/4] zbud: add to mm/

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Date:	Thu, 23 May 2013 10:00:07 +0800
From:	Bob Liu <bob.liu@...cle.com>
To:	Mel Gorman <mgorman@...e.de>
CC:	Seth Jennings <sjenning@...ux.vnet.ibm.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Nitin Gupta <ngupta@...are.org>,
	Minchan Kim <minchan@...nel.org>,
	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
	Dan Magenheimer <dan.magenheimer@...cle.com>,
	Robert Jennings <rcj@...ux.vnet.ibm.com>,
	Jenifer Hopper <jhopper@...ibm.com>,
	Johannes Weiner <jweiner@...hat.com>,
	Rik van Riel <riel@...hat.com>,
	Larry Woodman <lwoodman@...hat.com>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	Dave Hansen <dave@...1.net>, Joe Perches <joe@...ches.com>,
	Joonsoo Kim <iamjoonsoo.kim@....com>,
	Cody P Schafer <cody@...ux.vnet.ibm.com>,
	Hugh Dickens <hughd@...gle.com>,
	Paul Mackerras <paulus@...ba.org>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org, devel@...verdev.osuosl.org
Subject: Re: [PATCHv11 2/4] zbud: add to mm/

Hi Mel & Seth,

On 05/21/2013 04:10 PM, Mel Gorman wrote:
> On Mon, May 20, 2013 at 10:42:25AM -0500, Seth Jennings wrote:
>> On Mon, May 20, 2013 at 02:54:39PM +0100, Mel Gorman wrote:
>>> On Sun, May 19, 2013 at 03:52:19PM -0500, Seth Jennings wrote:
>>>> My first guess is that the external fragmentation situation you are referring to
>>>> is a workload in which all pages compress to greater than half a page.  If so,
>>>> then it doesn't matter what NCHUCNKS_ORDER is, there won't be any pages the
>>>> compress enough to fit in the < PAGE_SIZE/2 free space that remains in the
>>>> unbuddied zbud pages.
>>>>
>>>
>>> There are numerous aspects to this, too many to write them all down.
>>> Modelling the external fragmentation one and how it affects swap IO
>>> would be a complete pain in the ass so lets consider the following
>>> example instead as it's a bit clearer.
>>>
>>> Three processes. Process A compresses by 75%, Process B compresses to 15%,
>>> Process C pages compress to 15%. They are all adding to zswap in lockstep.
>>> Lets say that zswap can hold 100 physical pages.
>>>
>>> NCHUNKS == 2
>>> 	All Process A pages get rejected.
>>
>> Ah, I think this is our disconnect.  Process A pages will not be rejected.
>> They will be stored in a zbud page, and that zbud page will be added
>> to the 0th unbuddied list.  This list maintains a list of zbud pages
>> that will never be buddied because there are no free chunks.
>>
> 
> D'oh, good point. Unfortunately, the problem then still exists at the
> writeback end which I didn't bring up in the previous mail. 

What's your opinion if we write back the whole compressed page to swap disk?

-- 
Regards,
-Bob
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