linux-kernel - Re: [PATCH 2/7] Export unusable free space index via /proc/pagetypeinfo

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Message-ID: <20100208121048.GB23680@csn.ul.ie>
Date:	Mon, 8 Feb 2010 12:10:48 +0000
From:	Mel Gorman <mel@....ul.ie>
To:	David Rientjes <rientjes@...gle.com>
Cc:	Andrea Arcangeli <aarcange@...hat.com>,
	Christoph Lameter <cl@...ux-foundation.org>,
	Adam Litke <agl@...ibm.com>, Avi Kivity <avi@...hat.com>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH 2/7] Export unusable free space index via
	/proc/pagetypeinfo

On Fri, Feb 05, 2010 at 01:40:21PM -0800, David Rientjes wrote:
> On Fri, 5 Feb 2010, Mel Gorman wrote:
> 
> > > > +	/*
> > > > +	 * Index should be a value between 0 and 1. Return a value to 3
> > > > +	 * decimal places.
> > > > +	 *
> > > > +	 * 0 => no fragmentation
> > > > +	 * 1 => high fragmentation
> > > > +	 */
> > > > +	return ((info->free_pages - (info->free_blocks_suitable << order)) * 1000) / info->free_pages;
> > > > +
> > > 
> > > This value is only for userspace consumption via /proc/pagetypeinfo, so 
> > > I'm wondering why it needs to be exported as an index.  Other than a loss 
> > > of precision, wouldn't this be easier to understand (especially when 
> > > coupled with the free page counts already exported) if it were multipled 
> > > by 100 rather than 1000 and shown as a percent of _usable_ free memory at 
> > > each order? 
> > 
> > I find it easier to understand either way, but that's hardly a surprise.
> > The 1000 is because of the loss of precision. I can make it a 100 but I
> > don't think it makes much of a difference.
> > 
> 
> This suggestion was coupled with the subsequent note that there is no 
> documentation of what "unusuable free space index" is, except by the 
> implementation itself.  Since the value isn't used by the kernel,  I think 
> exporting the value as a percent would be easier understood by the user 
> without looking up the semantics.  I don't have strong feelings either 
> way, however.
> 

I'm writing documentation. I'm keeping with the 1000 value because a) I
like the precision and b) the fragmentation index is not related to
percentages and I think having one as a percentage and the other as an
index would cause confusion. Thanks

-- 
Mel Gorman
Part-time Phd Student                          Linux Technology Center
University of Limerick                         IBM Dublin Software Lab
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