linux-kernel - Re: Free memory never fully used, swapping

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Date:	Fri, 26 Nov 2010 11:31:42 +0900 (JST)
From:	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>
To:	Shaohua Li <shaohua.li@...el.com>
Cc:	kosaki.motohiro@...fujitsu.com, Mel Gorman <mel@....ul.ie>,
	Simon Kirby <sim@...tway.ca>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	Dave Hansen <dave@...ux.vnet.ibm.com>
Subject: Re: Free memory never fully used, swapping

> record the order seems not sufficient. in balance_pgdat(), the for look
> exit only when:
> priority <0 or sc.nr_reclaimed >= SWAP_CLUSTER_MAX.
> but we do if (sc.nr_reclaimed < SWAP_CLUSTER_MAX)
>                         order = sc.order = 0;
> this means before we set order to 0, we already reclaimed a lot of
> pages, so I thought we need set order to 0 earlier before there are
> enough free pages. below is a debug patch.
> 
> 
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index d31d7ce..ee5d2ed 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -2117,6 +2117,26 @@ unsigned long try_to_free_mem_cgroup_pages(struct mem_cgroup *mem_cont,
>  }
>  #endif
>  
> +static int all_zone_enough_free_pages(pg_data_t *pgdat)
> +{
> +	int i;
> +
> +	for (i = 0; i < pgdat->nr_zones; i++) {
> +		struct zone *zone = pgdat->node_zones + i;
> +
> +		if (!populated_zone(zone))
> +			continue;
> +
> +		if (zone->all_unreclaimable)
> +			continue;
> +
> +		if (!zone_watermark_ok(zone, 0, high_wmark_pages(zone) * 8,
> +								0, 0))
> +			return 0;
> +	}
> +	return 1;
> +}
> +
>  /* is kswapd sleeping prematurely? */
>  static int sleeping_prematurely(pg_data_t *pgdat, int order, long remaining)
>  {
> @@ -2355,7 +2375,8 @@ out:
>  		 * back to sleep. High-order users can still perform direct
>  		 * reclaim if they wish.
>  		 */
> -		if (sc.nr_reclaimed < SWAP_CLUSTER_MAX)
> +		if (sc.nr_reclaimed < SWAP_CLUSTER_MAX ||
> +		    (order > 0 && all_zone_enough_free_pages(pgdat)))
>  			order = sc.order = 0;

Ummm. this doesn't work. this place is processed every 32 pages reclaimed.
(see below code and comment). Theresore your patch break high order reclaim
logic.


                /*
                 * We do this so kswapd doesn't build up large priorities for
                 * example when it is freeing in parallel with allocators. It
                 * matches the direct reclaim path behaviour in terms of impact
                 * on zone->*_priority.
                 */
                if (sc.nr_reclaimed >= SWAP_CLUSTER_MAX)
                        break;


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