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

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Date:	Thu, 23 May 2013 10:52:15 +0100
From:	Mel Gorman <mgorman@...e.de>
To:	Bob Liu <bob.liu@...cle.com>
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/

On Thu, May 23, 2013 at 10:00:07AM +0800, Bob Liu wrote:
> 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?
> 

I'm not sure how to answer that sensibly. If the compressed page is
written to swap, in my opinion then there will be IO :/ . It will be a
maximum of two pages of IO with zbud (or zpair or whatever) as currently
implemented. With zsmalloc it potentially was more.

-- 
Mel Gorman
SUSE Labs
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