linux-kernel - Re: [RFC][PATCH 6/6] PM/Hibernate: Do not try to allocate too much memory too hard

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Date:	Sun, 17 May 2009 22:07:12 +0800
From:	Wu Fengguang <fengguang.wu@...el.com>
To:	"Rafael J. Wysocki" <rjw@...k.pl>
Cc:	pm list <linux-pm@...ts.linux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	LKML <linux-kernel@...r.kernel.org>, Pavel Machek <pavel@....cz>,
	Nigel Cunningham <nigel@...onice.net>,
	David Rientjes <rientjes@...gle.com>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>
Subject: Re: [RFC][PATCH 6/6] PM/Hibernate: Do not try to allocate too much
	memory too hard

On Sun, May 17, 2009 at 08:55:05PM +0800, Rafael J. Wysocki wrote:
> On Sunday 17 May 2009, Wu Fengguang wrote:

> > > +static unsigned long minimum_image_size(unsigned long saveable)
> > > +{
> > > +	unsigned long size;
> > > +
> > > +	/* Compute the number of saveable pages we can free. */
> > > +	size = global_page_state(NR_SLAB_RECLAIMABLE)
> > > +		+ global_page_state(NR_ACTIVE_ANON)
> > > +		+ global_page_state(NR_INACTIVE_ANON)
> > > +		+ global_page_state(NR_ACTIVE_FILE)
> > > +		+ global_page_state(NR_INACTIVE_FILE);
> > 
> > For example, we could drop the 1.25 ratio and calculate the above
> > reclaimable size with more meaningful constraints:
> > 
> >         /* slabs are not easy to reclaim */
> > 	size = global_page_state(NR_SLAB_RECLAIMABLE) / 2;
> 
> Why 1/2?

Also a very coarse value:
- we don't want to stress icache/dcache too much
  (unless they grow too large)
- my experience was that the icache/dcache are scanned in a slower
  pace than lru pages.
- most importantly, inside the NR_SLAB_RECLAIMABLE pages, maybe half
  of the pages are actually *in use* and cannot be freed:
        % cat /proc/sys/fs/inode-nr     
        30450   16605
        % cat /proc/sys/fs/dentry-state 
        41598   35731   45      0       0       0
  See? More than half entries are in-use. Sure many of them will actually
  become unused when dentries are freed, but in the mean time the internal
  fragmentations in the slabs can go up.

> >         /* keep NR_ACTIVE_ANON */
> > 	size += global_page_state(NR_INACTIVE_ANON);
> 
> Why exactly did you omit ACTIVE_ANON?

To keep the "core working set" :)
  	
> >         /* keep mapped files */
> > 	size += global_page_state(NR_ACTIVE_FILE);
> > 	size += global_page_state(NR_INACTIVE_FILE);
> >         size -= global_page_state(NR_FILE_MAPPED);
> > 
> > That restores the hard core working set logic in the reverse way ;)
> 
> I think the 1/2 factor for NR_SLAB_RECLAIMABLE may be too high in some cases,
> but I'm going to check that.

Yes, after updatedb. In that case simple magics numbers may not help.
In that case we should really first call shrink_slab() in a loop to
cut down the slab pages to a sane number.

Thanks,
Fengguang
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