Re: [PATCH] Revert "mm: vmscan: scale number of pages reclaimed byreclaim/compaction based on failures"

[Mel Gorman]

On Tue, Nov 06, 2012 at 11:15:54AM +0100, Johannes Hirte wrote:
> Am Mon, 5 Nov 2012 14:24:49 +0000
> schrieb Mel Gorman <mgorman@xxxxxxx>:
> 
> > Jiri Slaby reported the following:
> > 
> > 	(It's an effective revert of "mm: vmscan: scale number of
> > pages reclaimed by reclaim/compaction based on failures".) Given
> > kswapd had hours of runtime in ps/top output yesterday in the morning
> > 	and after the revert it's now 2 minutes in sum for the last
> > 24h, I would say, it's gone.
> > 
> > The intention of the patch in question was to compensate for the loss
> > of lumpy reclaim. Part of the reason lumpy reclaim worked is because
> > it aggressively reclaimed pages and this patch was meant to be a sane
> > compromise.
> > 
> > When compaction fails, it gets deferred and both compaction and
> > reclaim/compaction is deferred avoid excessive reclaim. However, since
> > commit c6543459 (mm: remove __GFP_NO_KSWAPD), kswapd is woken up each
> > time and continues reclaiming which was not taken into account when
> > the patch was developed.
> > 
> > Attempts to address the problem ended up just changing the shape of
> > the problem instead of fixing it. The release window gets closer and
> > while a THP allocation failing is not a major problem, kswapd chewing
> > up a lot of CPU is. This patch reverts "mm: vmscan: scale number of
> > pages reclaimed by reclaim/compaction based on failures" and will be
> > revisited in the future.
> > 
> > Signed-off-by: Mel Gorman <mgorman@xxxxxxx>
> > ---
> >  mm/vmscan.c |   25 -------------------------
> >  1 file changed, 25 deletions(-)
> > 
> > diff --git a/mm/vmscan.c b/mm/vmscan.c
> > index 2624edc..e081ee8 100644
> > --- a/mm/vmscan.c
> > +++ b/mm/vmscan.c
> > @@ -1760,28 +1760,6 @@ static bool in_reclaim_compaction(struct
> > scan_control *sc) return false;
> >  }
> >  
> > -#ifdef CONFIG_COMPACTION
> > -/*
> > - * If compaction is deferred for sc->order then scale the number of
> > pages
> > - * reclaimed based on the number of consecutive allocation failures
> > - */
> > -static unsigned long scale_for_compaction(unsigned long
> > pages_for_compaction,
> > -			struct lruvec *lruvec, struct scan_control
> > *sc) -{
> > -	struct zone *zone = lruvec_zone(lruvec);
> > -
> > -	if (zone->compact_order_failed <= sc->order)
> > -		pages_for_compaction <<= zone->compact_defer_shift;
> > -	return pages_for_compaction;
> > -}
> > -#else
> > -static unsigned long scale_for_compaction(unsigned long
> > pages_for_compaction,
> > -			struct lruvec *lruvec, struct scan_control
> > *sc) -{
> > -	return pages_for_compaction;
> > -}
> > -#endif
> > -
> >  /*
> >   * Reclaim/compaction is used for high-order allocation requests. It
> > reclaims
> >   * order-0 pages before compacting the zone.
> > should_continue_reclaim() returns @@ -1829,9 +1807,6 @@ static inline
> > bool should_continue_reclaim(struct lruvec *lruvec,
> >  	 * inactive lists are large enough, continue reclaiming
> >  	 */
> >  	pages_for_compaction = (2UL << sc->order);
> > -
> > -	pages_for_compaction =
> > scale_for_compaction(pages_for_compaction,
> > -						    lruvec, sc);
> >  	inactive_lru_pages = get_lru_size(lruvec, LRU_INACTIVE_FILE);
> >  	if (nr_swap_pages > 0)
> >  		inactive_lru_pages += get_lru_size(lruvec,
> > LRU_INACTIVE_ANON); --
> 
> Even with this patch I see kswapd0 very often on top. Much more than
> with kernel 3.6.

How severe is the CPU usage? The higher usage can be explained by "mm:
remove __GFP_NO_KSWAPD" which allows kswapd to compact memory to reduce
the amount of time processes spend in compaction but will result in the
CPU cost being incurred by kswapd.

Is it really high like the bug was reporting with high usage over long
periods of time or do you just see it using 2-6% of CPU for short
periods?

Thanks.

-- 
Mel Gorman
SUSE Labs
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