linux-kernel - Re: [PATCH 1/3] clean up setup_per_zone_pages

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Date:	Wed, 20 May 2009 11:47:39 +0100
From:	Mel Gorman <mel@....ul.ie>
To:	Minchan Kim <minchan.kim@...il.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	LKML <linux-kernel@...r.kernel.org>,
	linux-mm <linux-mm@...ck.org>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	Rik van Riel <riel@...hat.com>,
	Johannes Weiner <hannes@...xchg.org>
Subject: Re: [PATCH 1/3] clean up setup_per_zone_pages_min

On Wed, May 20, 2009 at 07:30:45PM +0900, Minchan Kim wrote:
> On Wed, 20 May 2009 11:21:29 +0100
> Mel Gorman <mel@....ul.ie> wrote:
> 
> > On Wed, May 20, 2009 at 06:58:03PM +0900, Minchan Kim wrote:
> > > Hi, Mel. 
> > > 
> > > On Wed, 20 May 2009 09:54:16 +0100
> > > Mel Gorman <mel@....ul.ie> wrote:
> > > 
> > > > On Wed, May 20, 2009 at 04:18:53PM +0900, Minchan Kim wrote:
> > > > > 
> > > > > Mel changed zone->pages_[high/low/min] with zone->watermark array.
> > > > > So, setup_per_zone_pages_min also have to be changed.
> > > > > 
> > > > 
> > > > Just to be clear, this is a function renaming to match the new zone
> > > > field name, not something I missed. As the function changes min, low and
> > > > max, a better name might have been setup_per_zone_watermarks but whether
> > > 
> > > At first, I thouht, too. But It's handle of min_free_kbytes.
> > > Documentation said, it's to compute a watermark[WMARK_MIN]. 
> > > I think many people already used that knob to contorl pages_min to keep the 
> > > low pages. 
> > 
> > Which documentation?
> 
> Documentation/sysctl/vm.txt - min_free_kbytes.
> 

That documentation states

==== 
This is used to force the Linux VM to keep a minimum number
of kilobytes free.  The VM uses this number to compute a pages_min
value for each lowmem zone in the system.  Each lowmem zone gets
a number of reserved free pages based proportionally on its size.
====

This is true. It just happens in the implementation that sets pages_min
(or it's renamed value) also sets the low and high watermarks are also set
based on the value of the minimum value. It doesn't need to be updated.

> > I'm looking at the function comment and see
> > 
> >  * setup_per_zone_pages_min - called when min_free_kbytes changes.
> >  *
> >  * Ensures that the pages_{min,low,high} values for each zone are set
> >  * correctly with respect to min_free_kbytes.
> > 
> > So, the values of all the watermarks are updated by that function depending
> > on what the new value of min_free_kbytes is. It is a bit wrong I suppose as
> > it missed memory hot-add
> > 
> > setup_per_zone_pages_min - called when min_free_kbytes changes or when memory is hot-added
> 
> God!. I changed this function's comments with memory hot plug. 
> With my mistake, My patch 3/3 lost it. 
> I will add comment about memory hotplug.
> 
> Thanks for pointing me out. :)
> 
> 
> > > 
> > > So, I determined function name is proper now. 
> > > If setup_per_zone_watermark is better than it, we also have to change with 
> > > documentation. 
> > > 
> > > > you go with that name or not, this is better than what is there so;
> > > > 
> > > > Acked-by: Mel Gorman <mel@....ul.ie>
> > > 
> > > 
> > > -- 
> > > Kinds Regards
> > > Minchan Kim
> > > 
> > 
> > -- 
> > Mel Gorman
> > Part-time Phd Student                          Linux Technology Center
> > University of Limerick                         IBM Dublin Software Lab
> 
> 
> -- 
> Kinds Regards
> Minchan Kim
> 

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