Re: [PATCH 2/3] Properly account for the number of page cachepages zone_reclaim() can reclaim

[Wu Fengguang]

On Mon, Jun 08, 2009 at 09:01:29PM +0800, Mel Gorman wrote:
> On NUMA machines, the administrator can configure zone_relcaim_mode that
> is a more targetted form of direct reclaim. On machines with large NUMA
> distances for example, a zone_reclaim_mode defaults to 1 meaning that clean
> unmapped pages will be reclaimed if the zone watermarks are not being met.
> 
> There is a heuristic that determines if the scan is worthwhile but the
> problem is that the heuristic is not being properly applied and is basically
> assuming zone_reclaim_mode is 1 if it is enabled.
> 
> This patch makes zone_reclaim() makes a better attempt at working out how
> many pages it might be able to reclaim given the current reclaim_mode. If it
> cannot clean pages, then NR_FILE_DIRTY number of pages are not candidates. If
> it cannot swap, then NR_FILE_MAPPED are not. This indirectly addresses tmpfs
> as those pages tend to be dirty as they are not cleaned by pdflush or sync.

No, tmpfs pages are not accounted in NR_FILE_DIRTY because of the
BDI_CAP_NO_ACCT_AND_WRITEBACK bits.

> The ideal would be that the number of tmpfs pages would also be known
> and account for like NR_FILE_MAPPED as swap is required to discard them.
> A means of working this out quickly was not obvious but a comment is added
> noting the problem.

I'd rather prefer it be accounted separately than to muck up NR_FILE_MAPPED :)

> +	int pagecache_reclaimable;
> +
> +	/*
> +	 * Work out how many page cache pages we can reclaim in this mode.
> +	 *
> +	 * NOTE: Ideally, tmpfs pages would be accounted as if they were
> +	 *       NR_FILE_MAPPED as swap is required to discard those
> +	 *       pages even when they are clean. However, there is no
> +	 *       way of quickly identifying the number of tmpfs pages
> +	 */

So can you remove the note on NR_FILE_MAPPED?

> +	pagecache_reclaimable = zone_page_state(zone, NR_FILE_PAGES);
> +	if (!(zone_reclaim_mode & RECLAIM_WRITE))
> +		pagecache_reclaimable -= zone_page_state(zone, NR_FILE_DIRTY);

> +	if (!(zone_reclaim_mode & RECLAIM_SWAP))
> +		pagecache_reclaimable -= zone_page_state(zone, NR_FILE_MAPPED);

So the "if" can be removed because NR_FILE_MAPPED is not related to swapping?

Thanks,
Fengguang

>  	/*
>  	 * Zone reclaim reclaims unmapped file backed pages and
> @@ -2391,8 +2406,7 @@ int zone_reclaim(struct zone *zone, gfp_t gfp_mask, unsigned int order)
>  	 * if less than a specified percentage of the zone is used by
>  	 * unmapped file backed pages.
>  	 */
> -	if (zone_page_state(zone, NR_FILE_PAGES) -
> -	    zone_page_state(zone, NR_FILE_MAPPED) <= zone->min_unmapped_pages
> +	if (pagecache_reclaimable <= zone->min_unmapped_pages
>  	    && zone_page_state(zone, NR_SLAB_RECLAIMABLE)
>  			<= zone->min_slab_pages)
>  		return 0;
> -- 
> 1.5.6.5
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