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

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Date:	Mon, 20 May 2013 10:42:25 -0500
From:	Seth Jennings <sjenning@...ux.vnet.ibm.com>
To:	Mel Gorman <mgorman@...e.de>
Cc:	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 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.

In other words, changing NCHUNKS has no effect on the acceptable size
of allocations.

Seth

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