linux-kernel - Re: [PATCH] mm: limit direct reclaim for higher order allocations

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Date:	Wed, 24 Feb 2016 17:17:56 -0500
From:	Rik van Riel <riel@...hat.com>
To:	David Rientjes <rientjes@...gle.com>
Cc:	linux-kernel@...r.kernel.org, hannes@...xchg.org,
	akpm@...ux-foundation.org, vbabka@...e.cz, mgorman@...e.de
Subject: Re: [PATCH] mm: limit direct reclaim for higher order allocations

On Wed, 2016-02-24 at 14:15 -0800, David Rientjes wrote:
> On Wed, 24 Feb 2016, Rik van Riel wrote:
> 
> > For multi page allocations smaller than PAGE_ALLOC_COSTLY_ORDER,
> > the kernel will do direct reclaim if compaction failed for any
> > reason. This worked fine when Linux systems had 128MB RAM, but
> > on my 24GB system I frequently see higher order allocations
> > free up over 3GB of memory, pushing all kinds of things into
> > swap, and slowing down applications.
> > 
> 
> Just curious, are these higher order allocations typically done by
> the 
> slub allocator or where are they coming from?

These are slab allocator ones, indeed.

The allocations seem to be order 2 and 3, mostly
on behalf of the inode cache and alloc_skb.

> > It would be much better to limit the amount of reclaim done,
> > rather than cause excessive pageout activity.
> > 
> > When enough memory is free to do compaction for the highest order
> > allocation possible, bail out of the direct page reclaim code.
> > 
> > On smaller systems, this may be enough to obtain contiguous
> > free memory areas to satisfy small allocations, continuing our
> > strategy of relying on luck occasionally. On larger systems,
> > relying on luck like that has not been working for years.
> > 
> > Signed-off-by: Rik van Riel <riel@...hat.com>
> > ---
> >  mm/vmscan.c | 19 ++++++++-----------
> >  1 file changed, 8 insertions(+), 11 deletions(-)
> > 
> > diff --git a/mm/vmscan.c b/mm/vmscan.c
> > index fc62546096f9..8dd15d514761 100644
> > --- a/mm/vmscan.c
> > +++ b/mm/vmscan.c
> > @@ -2584,20 +2584,17 @@ static bool shrink_zones(struct zonelist
> *zonelist, struct scan_control *sc)
> >                               continue;       /* Let kswapd poll it
> */
> >  
> >                       /*
> > -                      * If we already have plenty of memory free
> for
> > -                      * compaction in this zone, don't free any
> more.
> > -                      * Even though compaction is invoked for any
> > -                      * non-zero order, only frequent costly order
> > -                      * reclamation is disruptive enough to become
> a
> > -                      * noticeable problem, like transparent huge
> > -                      * page allocations.
> > +                      * For higher order allocations, free enough
> memory
> > +                      * to be able to do compaction for the
> largest possible
> > +                      * allocation. On smaller systems, this may
> be enough
> > +                      * that smaller allocations can skip
> compaction, if
> > +                      * enough adjacent pages get freed.
> >                        */
> > -                     if (IS_ENABLED(CONFIG_COMPACTION) &&
> > -                         sc->order > PAGE_ALLOC_COSTLY_ORDER &&
> > +                     if (IS_ENABLED(CONFIG_COMPACTION) && sc-
> >order &&
> >                           zonelist_zone_idx(z) <= requested_highidx
> &&
> > -                         compaction_ready(zone, sc->order)) {
> > +                         compaction_ready(zone, MAX_ORDER)) {
> >                               sc->compaction_ready = true;
> > -                             continue;
> > +                             return true;
> >                       }
> >  
> >                       /*
> > 
-- 
All Rights Reversed.


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