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SubjectRe: [PATCH 2/6] mm: kswapd: Keep kswapd awake for high-order allocations until a percentage of the node is balanced
On Fri, Dec 10, 2010 at 12:42:58AM +0900, Minchan Kim wrote:
> On Thu, Dec 09, 2010 at 11:18:16AM +0000, Mel Gorman wrote:
> > When reclaiming for high-orders, kswapd is responsible for balancing a
> > node but it should not reclaim excessively. It avoids excessive reclaim by
> > considering if any zone in a node is balanced then the node is balanced. In
> > the cases where there are imbalanced zone sizes (e.g. ZONE_DMA with both
> > ZONE_DMA32 and ZONE_NORMAL), kswapd can go to sleep prematurely as just
> > one small zone was balanced.
> >
> > This alters the sleep logic of kswapd slightly. It counts the number of pages
> > that make up the balanced zones. If the total number of balanced pages is
> > more than a quarter of the zone, kswapd will go back to sleep. This should
> > keep a node balanced without reclaiming an excessive number of pages.
> >
> > Signed-off-by: Mel Gorman <mel@csn.ul.ie>
> Reviewed-by: Minchan Kim <minchan.kim@gmail.com>
>
> Just below trivials.
>
> > ---
> > mm/vmscan.c | 43 ++++++++++++++++++++++++++++++++++---------
> > 1 files changed, 34 insertions(+), 9 deletions(-)
> >
> > diff --git a/mm/vmscan.c b/mm/vmscan.c
> > index 25cb373..b4472a1 100644
> > --- a/mm/vmscan.c
> > +++ b/mm/vmscan.c
> > @@ -2117,10 +2117,26 @@ unsigned long try_to_free_mem_cgroup_pages(struct mem_cgroup *mem_cont,
> > }
> > #endif
> >
> > +/*
> > + * pgdat_balanced is used when checking if a node is balanced for high-order
> > + * allocations. Only zones that meet watermarks make up "balanced".
> > + * The total of balanced pages must be at least 25% of the node for the
>
> Could you write down why you select 25% in description?
> It could help someone who try to change that watermark in future.
>

Sure, how about;

+/*
+ * pgdat_balanced is used when checking if a node is balanced for high-order
+ * allocations. Only zones that meet watermarks make up "balanced".
+ * The total of balanced pages must be at least 25% of the node for the
+ * node to be considered balanced. Forcing all zones to be balanced for high
+ * orders can cause excessive reclaim when there are imbalanced zones.
+ * The choice of 25% is due to
+ * o a 16M DMA zone that is balanced will not balance a zone on any
+ * reasonable sized machine
+ * o On all other machines, the top zone must be at least a reasonable
+ * precentage of the middle zones. For example, on 32-bit x86, highmem
+ * would need to be at least 256M for it to be balance a whole node.
+ * Similarly, on x86-64 the Normal zone would need to be at least 1G
+ * to balance a node on its own. These seemed like reasonable ratios.
+ */
+static bool pgdat_balanced(pg_data_t *pgdat, unsigned long balanced)
+{
+ return balanced > pgdat->node_present_pages / 4;
+}
?

> > + * node to be considered balanced. Forcing all zones to be balanced for high
> > + * orders can cause excessive reclaim when there are imbalanced zones.
> > + * Similarly, we do not want kswapd to go to sleep because ZONE_DMA happens
> > + * to be balanced when ZONE_DMA32 is huge in comparison and unbalanced
> > + */
> > +static bool pgdat_balanced(pg_data_t *pgdat, unsigned long balanced)
>
> I think balanced_pages is more clear.
> But it's a preference.
>

Updated.

> > +{
> > + return balanced > pgdat->node_present_pages / 4;
> > +}
> > +
> > /* is kswapd sleeping prematurely? */
> > static int sleeping_prematurely(pg_data_t *pgdat, int order, long remaining)
> > {
> > int i;
> > + unsigned long balanced = 0;
> > + bool all_zones_ok = true;
> >
> > /* If a direct reclaimer woke kswapd within HZ/10, it's premature */
> > if (remaining)
> > @@ -2138,10 +2154,19 @@ static int sleeping_prematurely(pg_data_t *pgdat, int order, long remaining)
> >
> > if (!zone_watermark_ok(zone, order, high_wmark_pages(zone),
> > 0, 0))
> > - return 1;
> > + all_zones_ok = false;
> > + else
> > + balanced += zone->present_pages;
> > }
> >
> > - return 0;
> > + /*
> > + * For high-order requests, any zone meeting the watermark allows
> > + * kswapd to sleep. For order-0, all zones must be balanced
>
> The comment isn't exact.
> could allow?
>

Correct;

+ /*
+ * For high-order requests, the balanced zones must contain at least
+ * 25% of the nodes pages for kswapd to sleep. For order-0, all zones
+ * must be balanced
+ */
+ if (order)
+ return pgdat_balanced(pgdat, balanced);
+ else
+ return !all_zones_ok;
Thanks

> > + */
> > + if (order)
> > + return pgdat_balanced(pgdat, balanced);
> > + else
> > + return !all_zones_ok;
> > }
> >
> > /*
> > @@ -2169,7 +2194,7 @@ static unsigned long balance_pgdat(pg_data_t *pgdat, int order,
> > int classzone_idx)
> > {
> > int all_zones_ok;
> > - int any_zone_ok;
> > + unsigned long balanced;
> > int priority;
> > int i;
> > int end_zone = 0; /* Inclusive. 0 = ZONE_DMA */
> > @@ -2203,7 +2228,7 @@ loop_again:
> > disable_swap_token();
> >
> > all_zones_ok = 1;
> > - any_zone_ok = 0;
> > + balanced = 0;
> >
> > /*
> > * Scan in the highmem->dma direction for the highest
> > @@ -2314,11 +2339,11 @@ loop_again:
> > */
> > zone_clear_flag(zone, ZONE_CONGESTED);
> > if (i <= classzone_idx)
> > - any_zone_ok = 1;
> > + balanced += zone->present_pages;
> > }
> >
> > }
> > - if (all_zones_ok || (order && any_zone_ok))
> > + if (all_zones_ok || (order && pgdat_balanced(pgdat, balanced)))
> > break; /* kswapd: all done */
> > /*
> > * OK, kswapd is getting into trouble. Take a nap, then take
> > @@ -2344,10 +2369,10 @@ out:
> >
> > /*
> > * order-0: All zones must meet high watermark for a balanced node
> > - * high-order: Any zone below pgdats classzone_idx must meet the high
> > - * watermark for a balanced node
> > + * high-order: Balanced zones must make up at least 25% of the node
> > + * for the node to be balanced
> > */
> > - if (!(all_zones_ok || (order && any_zone_ok))) {
> > + if (!(all_zones_ok || (order && pgdat_balanced(pgdat, balanced)))) {
> > cond_resched();
> >
> > try_to_freeze();

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


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