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SubjectRe: [PATCH 3/3] mm,vmscan: Reclaim order-0 and compact instead of lumpy reclaim when under light pressure
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@csn.ul.ie>
> > ---
> > 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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