linux-kernel - Re: [PATCH 06/11] Export fragmentation index via /proc/extfrag

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Message-Id: <20100325200919.6C8F.A69D9226@jp.fujitsu.com>
Date:	Thu, 25 Mar 2010 20:20:04 +0900 (JST)
From:	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>
To:	Mel Gorman <mel@....ul.ie>
Cc:	kosaki.motohiro@...fujitsu.com,
	Andrew Morton <akpm@...ux-foundation.org>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Christoph Lameter <cl@...ux-foundation.org>,
	Adam Litke <agl@...ibm.com>, Avi Kivity <avi@...hat.com>,
	David Rientjes <rientjes@...gle.com>,
	Rik van Riel <riel@...hat.com>, linux-kernel@...r.kernel.org,
	linux-mm@...ck.org
Subject: Re: [PATCH 06/11] Export fragmentation index via /proc/extfrag_index

> On Thu, Mar 25, 2010 at 11:47:17AM +0900, KOSAKI Motohiro wrote:
> > > On Tue, Mar 23, 2010 at 09:22:04AM +0900, KOSAKI Motohiro wrote:
> > > > > > > +	/*
> > > > > > > +	 * Index is between 0 and 1 so return within 3 decimal places
> > > > > > > +	 *
> > > > > > > +	 * 0 => allocation would fail due to lack of memory
> > > > > > > +	 * 1 => allocation would fail due to fragmentation
> > > > > > > +	 */
> > > > > > > +	return 1000 - ( (1000+(info->free_pages * 1000 / requested)) / info->free_blocks_total);
> > > > > > > +}
> > > > > > 
> > > > > > Dumb question.
> > > > > > your paper (http://portal.acm.org/citation.cfm?id=1375634.1375641) says
> > > > > > fragmentation_index = 1 - (TotalFree/SizeRequested)/BlocksFree
> > > > > > but your code have extra '1000+'. Why?
> > > > > 
> > > > > To get an approximation to three decimal places.
> > > > 
> > > > Do you mean this is poor man's round up logic?
> > > 
> > > Not exactly.
> > > 
> > > The intention is to have a value of 968 instead of 0.968231. i.e.
> > > instead of a value between 0 and 1, it'll be a value between 0 and 1000
> > > that matches the first three digits after the decimal place.
> > 
> > Let's consider extream case.
> > 
> > free_pages: 1
> > requested: 1
> > free_blocks_total: 1
> > 
> > frag_index = 1000  - ((1000 + 1*1000/1))/1 = -1000
> > 
> > This is not your intension, I guess. 
> 
> Why not?
> 
> See this comment
> 
> /* Fragmentation index only makes sense when a request would fail */
> 
> In your example, there is a free page of the requested size so the allocation
> would succeed. In this case, fragmentation index does indeed go negative
> but the value is not useful.
>
> > Probably we don't need any round_up/round_down logic. because fragmentation_index
> > is only used "if (fragindex >= 0 && fragindex <= 500)" check in try_to_compact_pages().
> > +1 or -1 inaccurate can be ignored. iow, I think we can remove '1000+' expression.
> > 
> 
> This isn't about rounding, it's about having a value that normally is
> between 0 and 1 expressed as a number between 0 and 1000 because we
> can't use double in the kernel.

Sorry, My example was wrong. new example is here.

free_pages: 4
requested: 2
free_blocks_total: 4

theory: 1 - (TotalFree/SizeRequested)/BlocksFree
            = 1 - (4/2)/4 = 0.5

code : 1000 - ((1000 + 4*1000/2))/4 = 1000 - (1000 + 2000)/4 = 1000/4 = 250


I don't think this is three decimal picking up code. This seems might makes
lots compaction invocation rather than theory.


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