Date:	Tue, 19 May 2009 08:47:19 +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 Tue, May 19, 2009 at 01:07:41AM +0800, Rafael J. Wysocki wrote:
> On Monday 18 May 2009, Wu Fengguang wrote:
> > On Mon, May 18, 2009 at 05:14:29AM +0800, Rafael J. Wysocki wrote:
> > > On Sunday 17 May 2009, Wu Fengguang wrote:
> > > > 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.
> > > 
> > > I have verified that the appended patch works reasonably well.
> > 
> > This is illogical: in previous email you complained the formula
> > 
> >         TOTAL - MAPPED - ACTIVE_ANON - SLAB/2
> > 
> > gives too high number, while 
> > 
> >         TOTAL - MAPPED
> > 
> > in this patch is OK.  (I'm not claiming the first formula to be fine.)
> 
> I wasn't precise enough. :-)
> 
> The problem with the first formula is that it's not really useful when used
> _before_ running shrink_all_memory(), becuase it may give arbitraty result
> in that case (everything depends on the preceding memory usage pattern).
> However, if it is used _after_ running shrink_all_memory(<all saveable pages>),
> the resulting minimum image size is usually (most often) below the real minimum
> number of saveable pages that can stay in memory.
> 
> The second formula, OTOH, doesn't depend so much on the preceding memory usage
> pattern and therefore it seems to be suitable for computing the estimate of the
> minimum image size _before_ running shrink_all_memory().  Still, when used
> _after_ running shrink_all_memory(<all saveable pages>), it will give a number
> below the actual minimum number of saveable pages (ie. not a really suitable
> one).
> 
> Now, since we're going to get rid of shrink_all_memory() at one point, I think
> we should be looking for a formula suitable for using before it's called.
> This, IMO, the second one is just about right. :-)

Ah OK, thanks for the explanation!

> > > The value returned as the minimum image size is usually too high, but not very
> > > much (on x86_64 usually about 20%) and there are no "magic" coefficients
> > 
> > It is _OK_ for the minimum image size to be higher, that margin serves
> > as a safety margin as well as the working set size we want to preserve.
> 
> I didn't say it wasn't OK. :-)  It's totally fine by me.

Great!

> > > involved any more and the computation of the minimum image size is carried out
> > > before calling shrink_all_memory() (so it's still going to be useful after
> > > we've dropped shrink_all_memory() at one point).
> > 
> > That's OK. Because shrink_all_memory() shrinks memory in a prioritized
> > list-after-list order.
> > 
> > > ---
> > > From: Rafael J. Wysocki <rjw@...k.pl>
> > > Subject: PM/Hibernate: Do not try to allocate too much memory too hard (rev. 2)
> > > 
> > > We want to avoid attempting to free too much memory too hard during
> > > hibernation, so estimate the minimum size of the image to use as the
> > > lower limit for preallocating memory.
> > 
> > I'd like to advocate to add "working set preservation" as another goal
> > of this function, and I can even do with the formula in this patch :-)
> >
> > That means, when one day more accurate working set estimation is
> > possible, we can extend this function to support that goal.
> 
> OK, so do you think it's fine to go with the patch below for now?

Sure, I'm fine with it.

Acked-by: Wu Fengguang <fengguang.wu@...el.com> 

Thanks,
Fengguang

> > > The approach here is based on the (experimental) observation that we
> > > can't free more page frames than the sum of:
> > > 
> > > * 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)
> > > 
> > > minus
> > > 
> > > * global_page_state(NR_FILE_MAPPED)
> > > 
> > > Namely, if this number is subtracted from the number of saveable
> > > pages in the system, we get a good estimate of the minimum reasonable
> > > size of a hibernation image.
> > > 
> > > Signed-off-by: Rafael J. Wysocki <rjw@...k.pl>
> > > ---
> > >  kernel/power/snapshot.c |   43 +++++++++++++++++++++++++++++++++++++++----
> > >  1 file changed, 39 insertions(+), 4 deletions(-)
> > > 
> > > Index: linux-2.6/kernel/power/snapshot.c
> > > ===================================================================
> > > --- linux-2.6.orig/kernel/power/snapshot.c
> > > +++ linux-2.6/kernel/power/snapshot.c
> > > @@ -1204,6 +1204,36 @@ static void free_unnecessary_pages(void)
> > >  }
> > >  
> > >  /**
> > > + * minimum_image_size - Estimate the minimum acceptable size of an image
> > > + * @saveable: Number of saveable pages in the system.
> > > + *
> > > + * We want to avoid attempting to free too much memory too hard, so estimate the
> > > + * minimum acceptable size of a hibernation image to use as the lower limit for
> > > + * preallocating memory.
> > > + *
> > > + * We assume that the minimum image size should be proportional to
> > > + *
> > > + * [number of saveable pages] - [number of pages that can be freed in theory]
> > > + *
> > > + * where the second term is the sum of (1) reclaimable slab pages, (2) active
> > > + * and (3) inactive anonymouns pages, (4) active and (5) inactive file pages,
> > > + * minus mapped file pages.
> > > + */
> > > +static unsigned long minimum_image_size(unsigned long saveable)
> > > +{
> > > +	unsigned long size;
> > > +
> > > +	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)
> > > +		- global_page_state(NR_FILE_MAPPED);
> > > +
> > > +	return saveable <= size ? 0 : saveable - size;
> > > +}
> > > +
> > > +/**
> > >   * hibernate_preallocate_memory - Preallocate memory for hibernation image
> > >   *
> > >   * To create a hibernation image it is necessary to make a copy of every page
> > > @@ -1220,8 +1250,8 @@ static void free_unnecessary_pages(void)
> > >   *
> > >   * If image_size is set below the number following from the above formula,
> > >   * the preallocation of memory is continued until the total number of saveable
> > > - * pages in the system is below the requested image size or it is impossible to
> > > - * allocate more memory, whichever happens first.
> > > + * pages in the system is below the requested image size or the minimum
> > > + * acceptable image size returned by minimum_image_size(), whichever is greater.
> > >   */
> > >  int hibernate_preallocate_memory(void)
> > >  {
> > > @@ -1282,6 +1312,11 @@ int hibernate_preallocate_memory(void)
> > >  		goto out;
> > >  	}
> > >  
> > > +	/* Estimate the minimum size of the image. */
> > > +	pages = minimum_image_size(saveable);
> > > +	if (size < pages)
> > > +		size = min_t(unsigned long, pages, max_size);
> > > +
> > >  	/*
> > >  	 * Let the memory management subsystem know that we're going to need a
> > >  	 * large number of page frames to allocate and make it free some memory.
> > > @@ -1294,8 +1329,8 @@ int hibernate_preallocate_memory(void)
> > >  	 * The number of saveable pages in memory was too high, so apply some
> > >  	 * pressure to decrease it.  First, make room for the largest possible
> > >  	 * image and fail if that doesn't work.  Next, try to decrease the size
> > > -	 * of the image as much as indicated by image_size using allocations
> > > -	 * from highmem and non-highmem zones separately.
> > > +	 * of the image as much as indicated by 'size' using allocations from
> > > +	 * highmem and non-highmem zones separately.
> > >  	 */
> > >  	pages_highmem = preallocate_image_highmem(highmem / 2);
> > >  	max_size += pages_highmem;
--
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