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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 Thu,  9 Dec 2010 11:18:16 +0000
Mel Gorman <mel@csn.ul.ie> 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>

Hmm, does this work well in

for example, x86-32,
DMA: 16MB
NORMAL: 700MB
HIGHMEM: 11G
?

At 1st look, it's balanced when HIGHMEM has enough free pages...
This is not good for NICs which requests high-order allocations.

Can't we take claszone_idx into account at checking rather than
node->present_pages ?

as
balanced > present_pages_below_classzone_idx(node, classzone_idx)/4
?
Thanks,
-Kame

> ---
> 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
> + * 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)
> +{
> + 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
> + */
> + 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();
> --
> 1.7.1
>
> --
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