linux-kernel - Re: [patch] vmscan: initialize sc.order in indirect shrink

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Message-ID: <20090216220302.GA3415@cmpxchg.org>
Date:	Mon, 16 Feb 2009 23:03:02 +0100
From:	Johannes Weiner <hannes@...xchg.org>
To:	Mel Gorman <mel@....ul.ie>
Cc:	Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org
Subject: Re: [patch] vmscan: initialize sc.order in indirect shrink_list() users

On Mon, Feb 16, 2009 at 02:53:49PM +0000, Mel Gorman wrote:
> On Wed, Feb 11, 2009 at 02:52:27AM +0100, Johannes Weiner wrote:
> > [added Mel to CC]
> > 
> > On Tue, Feb 10, 2009 at 04:29:48PM -0800, Andrew Morton wrote:
> > > On Tue, 10 Feb 2009 17:51:35 +0100
> > > Johannes Weiner <hannes@...xchg.org> wrote:
> > > 
> > > > shrink_all_memory() and __zone_reclaim() currently don't initialize
> > > > the .order field of their scan control.
> > > > 
> > > > Both of them call into functions which use that field and make certain
> > > > decisions based on a random value.
> > > > 
> > > > The functions depending on the .order field are marked with a star,
> > > > the faulty entry points are marked with a percentage sign:
> > > > 
> > > > * shrink_page_list()
> > > >   * shrink_inactive_list()
> > > >   * shrink_active_list()
> > > >     shrink_list()
> > > >       shrink_all_zones()
> > > >         % shrink_all_memory()
> > > >       shrink_zone()
> > > >         % __zone_reclaim()
> > > > 
> > > > Initialize .order to zero in shrink_all_memory().  Initialize .order
> > > > to the order parameter in __zone_reclaim().
> > > > 
> > > > Signed-off-by: Johannes Weiner <hannes@...xchg.org>
> > > > ---
> > > >  mm/vmscan.c |    2 ++
> > > >  1 files changed, 2 insertions(+), 0 deletions(-)
> > > > 
> > > > diff --git a/mm/vmscan.c b/mm/vmscan.c
> > > > index 4422301..9ce85ea 100644
> > > > --- a/mm/vmscan.c
> > > > +++ b/mm/vmscan.c
> > > > @@ -2112,6 +2112,7 @@ unsigned long shrink_all_memory(unsigned long nr_pages)
> > > >  		.may_unmap = 0,
> > > >  		.swap_cluster_max = nr_pages,
> > > >  		.may_writepage = 1,
> > > > +		.order = 0,
> > > >  		.isolate_pages = isolate_pages_global,
> > > >  	};
> > > >  
> > > > @@ -2294,6 +2295,7 @@ static int __zone_reclaim(struct zone *zone, gfp_t gfp_mask, unsigned int order)
> > > >  					SWAP_CLUSTER_MAX),
> > > >  		.gfp_mask = gfp_mask,
> > > >  		.swappiness = vm_swappiness,
> > > > +		.order = order,
> > > >  		.isolate_pages = isolate_pages_global,
> > > >  	};
> > > >  	unsigned long slab_reclaimable;
> > > 
> > > The second hunk might fix something, but it would need a correcter
> > > changelog, and some thought about what its runtimes effects are likely
> > > to be, please.
> > 
> > zone_reclaim() is used by the watermark rebalancing of the buddy
> > allocator right before trying to do an allocation.  Even though it
> > tries to reclaim at least 1 << order pages, it doesn't raise sc.order
> > to increase clustering efforts.
> > 
> 
> This affects lumpy reclaim. Direct reclaim via try_to_free_pages() and
> kswapd() is still working but the earlier reclaim attempt via zone_reclaim()
> on unmapped file and slab pages is ignoring teh order. While it'd be tricky
> to measure any difference, it does make sense that __zone_reclaim() initialse
> the order with what the caller requested.
> 
> > I think this happens with the assumption that the upcoming allocation
> > can still succeed and in that case we don't want to lump too
> > aggressively to refill the zone. 
> 
> I don't get what you mean here. The caller requested the higher order so
> the work has been requested.

I meant the buffered_rmqueue() might still succeed even without lumpy
reclaim in the case of low watermarks reached.  And if it does, we
reclaimed 'in aggressive mode without reason'.  If it does NOT, we
still drop into direct reclaim with lumpy reclaim.  Well, this is at
least what I had in mind when writing the above.

> > The allocation might succeed on
> > another zone and now we have evicted precious pages due to clustering
> > while we are still not sure it's even needed.
> > 
> 
> Also not sure what you are getting at here. zone_reclaim() is called for the
> preferred zones in order. Attempts are made to free within the preferred zone
> and then allocate from it. Granted, it might evict pages and the clustering
> was ineffective, but this is the cost of high-order reclaim.

Sure, agreed.  I was just wondering whether higher-order reclaim was
needed up-front when the low watermarks are reached or if it was
enough when direct reclaim is lumpy in case the allocation fails.

	Hannes
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