linux-kernel - Re: [PATCH 2/3] Properly account for the number of page cache pages zone

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Date:	Tue, 9 Jun 2009 09:47:46 +0100
From:	Mel Gorman <mel@....ul.ie>
To:	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>
Cc:	Rik van Riel <riel@...hat.com>,
	Christoph Lameter <cl@...ux-foundation.org>,
	yanmin.zhang@...el.com, Wu Fengguang <fengguang.wu@...el.com>,
	linuxram@...ibm.com, linux-mm <linux-mm@...ck.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 2/3] Properly account for the number of page cache
	pages zone_reclaim() can reclaim

On Tue, Jun 09, 2009 at 05:19:41PM +0900, KOSAKI Motohiro wrote:
> Hi
> 
> > On NUMA machines, the administrator can configure zone_relcaim_mode that
> > is a more targetted form of direct reclaim. On machines with large NUMA
> > distances for example, a zone_reclaim_mode defaults to 1 meaning that clean
> > unmapped pages will be reclaimed if the zone watermarks are not being met.
> > 
> > There is a heuristic that determines if the scan is worthwhile but the
> > problem is that the heuristic is not being properly applied and is basically
> > assuming zone_reclaim_mode is 1 if it is enabled.
> > 
> > This patch makes zone_reclaim() makes a better attempt at working out how
> > many pages it might be able to reclaim given the current reclaim_mode. If it
> > cannot clean pages, then NR_FILE_DIRTY number of pages are not candidates. If
> > it cannot swap, then NR_FILE_MAPPED are not. This indirectly addresses tmpfs
> > as those pages tend to be dirty as they are not cleaned by pdflush or sync.
> > 
> > The ideal would be that the number of tmpfs pages would also be known
> > and account for like NR_FILE_MAPPED as swap is required to discard them.
> > A means of working this out quickly was not obvious but a comment is added
> > noting the problem.
> > 
> > Signed-off-by: Mel Gorman <mel@....ul.ie>
> > ---
> >  mm/vmscan.c |   18 ++++++++++++++++--
> >  1 files changed, 16 insertions(+), 2 deletions(-)
> > 
> > diff --git a/mm/vmscan.c b/mm/vmscan.c
> > index ba211c1..ffe2f32 100644
> > --- a/mm/vmscan.c
> > +++ b/mm/vmscan.c
> > @@ -2380,6 +2380,21 @@ int zone_reclaim(struct zone *zone, gfp_t gfp_mask, unsigned int order)
> >  {
> >  	int node_id;
> >  	int ret;
> > +	int pagecache_reclaimable;
> > +
> > +	/*
> > +	 * Work out how many page cache pages we can reclaim in this mode.
> > +	 *
> > +	 * NOTE: Ideally, tmpfs pages would be accounted as if they were
> > +	 *       NR_FILE_MAPPED as swap is required to discard those
> > +	 *       pages even when they are clean. However, there is no
> > +	 *       way of quickly identifying the number of tmpfs pages
> > +	 */
> 
> I think I and you tackle the same issue.
> Please see vmscan-change-the-number-of-the-unmapped-files-in-zone-reclaim.patch in -mm.
> 

Awesome. This is why I posted the patches a bit earlier than I would
normally. Stuff like this is found :D

> My intension mean, tmpfs page and swapcache increased NR_FILE_PAGES.
> but they can't be reclaimed by zone_reclaim_mode==1.
> 

Sounds familiar!

> Then, I decide to use following calculation.
> 
> +	nr_unmapped_file_pages = zone_page_state(zone, NR_INACTIVE_FILE) +
> +				 zone_page_state(zone, NR_ACTIVE_FILE) -
> +				 zone_page_state(zone, NR_FILE_MAPPED);
> 

That should now be in a helper. If I use that calculation, it'll appear
in three different places. I'll do the shuffling.

> 
> > +	pagecache_reclaimable = zone_page_state(zone, NR_FILE_PAGES);
> > +	if (!(zone_reclaim_mode & RECLAIM_WRITE))
> > +		pagecache_reclaimable -= zone_page_state(zone, NR_FILE_DIRTY);
> > +	if (!(zone_reclaim_mode & RECLAIM_SWAP))
> > +		pagecache_reclaimable -= zone_page_state(zone, NR_FILE_MAPPED);
> 
> if you hope to solve tmpfs issue, RECLAIM_WRITE/RECLAIM_SWAP are unrelated, I think.

For reclaim_zone() to do anything useful, the pages have to be cleaned
and swapped and that needs RECLAIM_WRITE and RECLAIM_SWAP. So, how are
they unrelated?

> Plus, Could you please see vmscan-zone_reclaim-use-may_swap.patch in -mm?
> it improve RECLAIM_SWAP by another way.
> 

Looking now, I'm going to rebase this patchset on top of -mm where I can
take advantage of that patch.

Thanks a lot

> 
> 
> >  
> >  	/*
> >  	 * Zone reclaim reclaims unmapped file backed pages and
> > @@ -2391,8 +2406,7 @@ int zone_reclaim(struct zone *zone, gfp_t gfp_mask, unsigned int order)
> >  	 * if less than a specified percentage of the zone is used by
> >  	 * unmapped file backed pages.
> >  	 */
> > -	if (zone_page_state(zone, NR_FILE_PAGES) -
> > -	    zone_page_state(zone, NR_FILE_MAPPED) <= zone->min_unmapped_pages
> > +	if (pagecache_reclaimable <= zone->min_unmapped_pages
> >  	    && zone_page_state(zone, NR_SLAB_RECLAIMABLE)
> >  			<= zone->min_slab_pages)
> >  		return 0;

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