Re: [PATCH 04/10] mm: vmscan: Decide whether to compact the pgdat based on reclaim progress

[Kamezawa Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>]

(2013/04/09 20:06), Mel Gorman wrote:
> In the past, kswapd makes a decision on whether to compact memory after the
> pgdat was considered balanced. This more or less worked but it is late to
> make such a decision and does not fit well now that kswapd makes a decision
> whether to exit the zone scanning loop depending on reclaim progress.
> 
> This patch will compact a pgdat if at least the requested number of pages
> were reclaimed from unbalanced zones for a given priority. If any zone is
> currently balanced, kswapd will not call compaction as it is expected the
> necessary pages are already available.
> 
> Signed-off-by: Mel Gorman <mgorman@suse.de>

I like this way.

> ---
>   mm/vmscan.c | 60 ++++++++++++++++++++++++++++++------------------------------
>   1 file changed, 30 insertions(+), 30 deletions(-)
> 
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index 78268ca..a9e68b4 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -2640,7 +2640,8 @@ static bool prepare_kswapd_sleep(pg_data_t *pgdat, int order, long remaining,
>    */
>   static bool kswapd_shrink_zone(struct zone *zone,
>   			       struct scan_control *sc,
> -			       unsigned long lru_pages)
> +			       unsigned long lru_pages,
> +			       unsigned long *nr_attempted)
>   {
>   	unsigned long nr_slab;
>   	struct reclaim_state *reclaim_state = current->reclaim_state;
> @@ -2656,6 +2657,9 @@ static bool kswapd_shrink_zone(struct zone *zone,
>   	nr_slab = shrink_slab(&shrink, sc->nr_scanned, lru_pages);
>   	sc->nr_reclaimed += reclaim_state->reclaimed_slab;
>   
> +	/* Account for the number of pages attempted to reclaim */
> +	*nr_attempted += sc->nr_to_reclaim;
> +
>   	if (nr_slab == 0 && !zone_reclaimable(zone))
>   		zone->all_unreclaimable = 1;
>   
> @@ -2703,8 +2707,11 @@ static unsigned long balance_pgdat(pg_data_t *pgdat, int order,
>   
>   	do {
>   		unsigned long lru_pages = 0;
> +		unsigned long nr_attempted = 0;
>   		unsigned long nr_reclaimed = sc.nr_reclaimed = 0;
> +		unsigned long this_reclaimed;
>   		bool raise_priority = true;
> +		bool pgdat_needs_compaction = (order > 0);
>   
>   		/*
>   		 * Scan in the highmem->dma direction for the highest
> @@ -2752,7 +2759,21 @@ static unsigned long balance_pgdat(pg_data_t *pgdat, int order,
>   		for (i = 0; i <= end_zone; i++) {
>   			struct zone *zone = pgdat->node_zones + i;
>   
> +			if (!populated_zone(zone))
> +				continue;
> +
>   			lru_pages += zone_reclaimable_pages(zone);
> +
> +			/*
> +			 * If any zone is currently balanced then kswapd will
> +			 * not call compaction as it is expected that the
> +			 * necessary pages are already available.
> +			 */
> +			if (pgdat_needs_compaction &&
> +					zone_watermark_ok(zone, order,
> +						low_wmark_pages(zone),
> +						*classzone_idx, 0))
> +				pgdat_needs_compaction = false;
>   		}
>   
>   		/*
> @@ -2821,7 +2842,8 @@ static unsigned long balance_pgdat(pg_data_t *pgdat, int order,
>   				 * already being scanned that high
>   				 * watermark would be met at 100% efficiency.
>   				 */
> -				if (kswapd_shrink_zone(zone, &sc, lru_pages))
> +				if (kswapd_shrink_zone(zone, &sc, lru_pages,
> +						       &nr_attempted))
>   					raise_priority = false;
>   			}
>   
> @@ -2873,42 +2895,20 @@ static unsigned long balance_pgdat(pg_data_t *pgdat, int order,
>   		if (try_to_freeze() || kthread_should_stop())
>   			break;
>   
> +		/* Compact if necessary and kswapd is reclaiming efficiently */
> +		this_reclaimed = sc.nr_reclaimed - nr_reclaimed;
> +		if (pgdat_needs_compaction && this_reclaimed > nr_attempted)
> +			compact_pgdat(pgdat, order);
> +

What does "this_reclaimed" mean ?   
"the total amount of reclaimed memory - reclaimed memory at this iteration" ?

And this_reclaimed > nr_attempted means kswapd is efficient ?
What "efficient" means here ?

Thanks,
-Kame

>   		/*
>   		 * Raise priority if scanning rate is too low or there was no
>   		 * progress in reclaiming pages
>   		 */
> -		if (raise_priority || sc.nr_reclaimed - nr_reclaimed == 0)
> +		if (raise_priority || !this_reclaimed)
>   			sc.priority--;
>   	} while (sc.priority >= 0 &&
>   		 !pgdat_balanced(pgdat, order, *classzone_idx));
>   
> -	/*
> -	 * If kswapd was reclaiming at a higher order, it has the option of
> -	 * sleeping without all zones being balanced. Before it does, it must
> -	 * ensure that the watermarks for order-0 on *all* zones are met and
> -	 * that the congestion flags are cleared. The congestion flag must
> -	 * be cleared as kswapd is the only mechanism that clears the flag
> -	 * and it is potentially going to sleep here.
> -	 */
> -	if (order) {
> -		int zones_need_compaction = 1;
> -
> -		for (i = 0; i <= end_zone; i++) {
> -			struct zone *zone = pgdat->node_zones + i;
> -
> -			if (!populated_zone(zone))
> -				continue;
> -
> -			/* Check if the memory needs to be defragmented. */
> -			if (zone_watermark_ok(zone, order,
> -				    low_wmark_pages(zone), *classzone_idx, 0))
> -				zones_need_compaction = 0;
> -		}
> -
> -		if (zones_need_compaction)
> -			compact_pgdat(pgdat, order);
> -	}
> -
>   out:
>   	/*
>   	 * Return the order we were reclaiming at so prepare_kswapd_sleep()
> 


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