Re: [PATCH 3/3] mm,vmscan: Reclaim order-0 and compact instead oflumpy reclaim when under light pressure

[Mel Gorman]

On Sun, Nov 14, 2010 at 02:59:31PM +0900, KOSAKI Motohiro wrote:
> > Lumpy reclaim is disruptive. It reclaims both a large number of pages
> > and ignores the age of the majority of pages it reclaims. This can incur
> > significant stalls and potentially increase the number of major faults.
> > 
> > Compaction has reached the point where it is considered reasonably stable
> > (meaning it has passed a lot of testing) and is a potential candidate for
> > displacing lumpy reclaim. This patch reduces the use of lumpy reclaim when
> > the priority is high enough to indicate low pressure. The basic operation
> > is very simple. Instead of selecting a contiguous range of pages to reclaim,
> > lumpy compaction reclaims a number of order-0 pages and then calls compaction
> > for the zone. If the watermarks are not met, another reclaim+compaction
> > cycle occurs.
> > 
> > Signed-off-by: Mel Gorman <mel@xxxxxxxxx>
> > ---
> >  include/linux/compaction.h |    9 ++++++++-
> >  mm/compaction.c            |    2 +-
> >  mm/vmscan.c                |   38 ++++++++++++++++++++++++++------------
> >  3 files changed, 35 insertions(+), 14 deletions(-)
> > 
> > diff --git a/include/linux/compaction.h b/include/linux/compaction.h
> > index 5ac5155..2ae6613 100644
> > --- a/include/linux/compaction.h
> > +++ b/include/linux/compaction.h
> > @@ -22,7 +22,8 @@ extern int sysctl_extfrag_handler(struct ctl_table *table, int write,
> >  extern int fragmentation_index(struct zone *zone, unsigned int order);
> >  extern unsigned long try_to_compact_pages(struct zonelist *zonelist,
> >  			int order, gfp_t gfp_mask, nodemask_t *mask);
> > -
> > +extern unsigned long compact_zone_order(struct zone *zone,
> > +			int order, gfp_t gfp_mask);
> >  /* Do not skip compaction more than 64 times */
> >  #define COMPACT_MAX_DEFER_SHIFT 6
> >  
> > @@ -59,6 +60,12 @@ static inline unsigned long try_to_compact_pages(struct zonelist *zonelist,
> >  	return COMPACT_CONTINUE;
> >  }
> >  
> > +static inline unsigned long compact_zone_order(struct zone *zone,
> > +			int order, gfp_t gfp_mask)
> > +{
> > +	return 0;
> > +}
> > +
> >  static inline void defer_compaction(struct zone *zone)
> >  {
> >  }
> > diff --git a/mm/compaction.c b/mm/compaction.c
> > index 4d709ee..f987f47 100644
> > --- a/mm/compaction.c
> > +++ b/mm/compaction.c
> > @@ -418,7 +418,7 @@ static int compact_zone(struct zone *zone, struct compact_control *cc)
> >  	return ret;
> >  }
> >  
> > -static unsigned long compact_zone_order(struct zone *zone,
> > +unsigned long compact_zone_order(struct zone *zone,
> >  						int order, gfp_t gfp_mask)
> >  {
> >  	struct compact_control cc = {
> > diff --git a/mm/vmscan.c b/mm/vmscan.c
> > index ffa438e..da35cdb 100644
> > --- a/mm/vmscan.c
> > +++ b/mm/vmscan.c
> > @@ -32,6 +32,7 @@
> >  #include <linux/topology.h>
> >  #include <linux/cpu.h>
> >  #include <linux/cpuset.h>
> > +#include <linux/compaction.h>
> >  #include <linux/notifier.h>
> >  #include <linux/rwsem.h>
> >  #include <linux/delay.h>
> > @@ -56,6 +57,7 @@ typedef unsigned __bitwise__ lumpy_mode;
> >  #define LUMPY_MODE_ASYNC		((__force lumpy_mode)0x02u)
> >  #define LUMPY_MODE_SYNC			((__force lumpy_mode)0x04u)
> >  #define LUMPY_MODE_CONTIGRECLAIM	((__force lumpy_mode)0x08u)
> > +#define LUMPY_MODE_COMPACTION		((__force lumpy_mode)0x10u)
> >  
> >  struct scan_control {
> >  	/* Incremented by the number of inactive pages that were scanned */
> > @@ -274,25 +276,27 @@ unsigned long shrink_slab(unsigned long scanned, gfp_t gfp_mask,
> >  static void set_lumpy_reclaim_mode(int priority, struct scan_control *sc,
> >  				   bool sync)
> >  {
> > -	lumpy_mode mode = sync ? LUMPY_MODE_SYNC : LUMPY_MODE_ASYNC;
> > +	lumpy_mode syncmode = sync ? LUMPY_MODE_SYNC : LUMPY_MODE_ASYNC;
> >  
> >  	/*
> > -	 * Some reclaim have alredy been failed. No worth to try synchronous
> > -	 * lumpy reclaim.
> > +	 * Initially assume we are entering either lumpy reclaim or lumpy
> > +	 * compaction. Depending on the order, we will either set the sync
> > +	 * mode or just reclaim order-0 pages later.
> >  	 */
> > -	if (sync && sc->lumpy_reclaim_mode & LUMPY_MODE_SINGLE)
> > -		return;
> > +	if (COMPACTION_BUILD)
> > +		sc->lumpy_reclaim_mode = LUMPY_MODE_COMPACTION;
> > +	else
> > +		sc->lumpy_reclaim_mode = LUMPY_MODE_CONTIGRECLAIM;
> >  
> >  	/*
> >  	 * If we need a large contiguous chunk of memory, or have
> >  	 * trouble getting a small set of contiguous pages, we
> >  	 * will reclaim both active and inactive pages.
> >  	 */
> > -	sc->lumpy_reclaim_mode = LUMPY_MODE_CONTIGRECLAIM;
> >  	if (sc->order > PAGE_ALLOC_COSTLY_ORDER)
> > -		sc->lumpy_reclaim_mode |= mode;
> > +		sc->lumpy_reclaim_mode |= syncmode;
> >  	else if (sc->order && priority < DEF_PRIORITY - 2)
> > -		sc->lumpy_reclaim_mode |= mode;
> > +		sc->lumpy_reclaim_mode |= syncmode;
> 
> Does "LUMPY_MODE_COMPACTION | LUMPY_MODE_SYNC" have any benefit?
> I haven't understand this semantics. please elaborate?
> 

At the moment, it doesn't have any benefit. In the future, we might pass
the flags down to migration which currently always behaves in a sync fashion.
For now, I think it's better to flag what we expect the behaviour to be
even if it's not responded to appropriately.

> 
> >  	else
> >  		sc->lumpy_reclaim_mode = LUMPY_MODE_SINGLE | LUMPY_MODE_ASYNC;
> >  }
> > @@ -1366,11 +1370,18 @@ shrink_inactive_list(unsigned long nr_to_scan, struct zone *zone,
> >  	lru_add_drain();
> >  	spin_lock_irq(&zone->lru_lock);
> >  
> > +	/*
> > +	 * If we are lumpy compacting, we bump nr_to_scan to at least
> > +	 * the size of the page we are trying to allocate
> > +	 */
> > +	if (sc->lumpy_reclaim_mode & LUMPY_MODE_COMPACTION)
> > +		nr_to_scan = max(nr_to_scan, (1UL << sc->order));
> > +
> >  	if (scanning_global_lru(sc)) {
> >  		nr_taken = isolate_pages_global(nr_to_scan,
> >  			&page_list, &nr_scanned, sc->order,
> > -			sc->lumpy_reclaim_mode & LUMPY_MODE_SINGLE ?
> > -					ISOLATE_INACTIVE : ISOLATE_BOTH,
> > +			sc->lumpy_reclaim_mode & LUMPY_MODE_CONTIGRECLAIM ?
> > +					ISOLATE_BOTH : ISOLATE_INACTIVE,
> >  			zone, 0, file);
> >  		zone->pages_scanned += nr_scanned;
> >  		if (current_is_kswapd())
> > @@ -1382,8 +1393,8 @@ shrink_inactive_list(unsigned long nr_to_scan, struct zone *zone,
> >  	} else {
> >  		nr_taken = mem_cgroup_isolate_pages(nr_to_scan,
> >  			&page_list, &nr_scanned, sc->order,
> > -			sc->lumpy_reclaim_mode & LUMPY_MODE_SINGLE ?
> > -					ISOLATE_INACTIVE : ISOLATE_BOTH,
> > +			sc->lumpy_reclaim_mode & LUMPY_MODE_CONTIGRECLAIM ?
> > +					ISOLATE_BOTH : ISOLATE_INACTIVE,
> >  			zone, sc->mem_cgroup,
> >  			0, file);
> >  		/*
> > @@ -1416,6 +1427,9 @@ shrink_inactive_list(unsigned long nr_to_scan, struct zone *zone,
> >  
> >  	putback_lru_pages(zone, sc, nr_anon, nr_file, &page_list);
> >  
> > +	if (sc->lumpy_reclaim_mode & LUMPY_MODE_COMPACTION)
> > +		compact_zone_order(zone, sc->order, sc->gfp_mask);
> > +
> 
> If free pages are very little, compaction may not work. don't we need to
> check NR_FREE_PAGES?
> 

Yes, it's on my TODO list to split out the logic used in
try_to_compact_pages to decide if compact_zone_order() should be called
or not.

Well spotted!

> 
> >  	trace_mm_vmscan_lru_shrink_inactive(zone->zone_pgdat->node_id,
> >  		zone_idx(zone),
> >  		nr_scanned, nr_reclaimed,
> > -- 
> > 1.7.1
> > 
> 
> 
> 

-- 
Mel Gorman
Part-time Phd Student                          Linux Technology Center
University of Limerick                         IBM Dublin Software Lab
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