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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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