Re: [PATCH 8/8] writeback: Do not sleep on the congestion queue if there are no congested BDIs or if significant congestion is not being encountered in the current zone

[Andrew Morton <akpm@linux-foundation.org>]

On Wed, 15 Sep 2010 13:27:51 +0100
Mel Gorman <mel@csn.ul.ie> wrote:

> If wait_iff_congested() is called with no BDI congested, the function simply
> calls cond_resched(). In the event there is significant writeback happening
> in the zone that is being reclaimed, this can be a poor decision as reclaim
> would succeed once writeback was completed. Without any backoff logic,
> younger clean pages can be reclaimed resulting in more reclaim overall and
> poor performance.

This is because cond_resched() is a no-op, and we skip around the
under-writeback pages and go off and look further along the LRU for
younger clean pages, yes?

> This patch tracks how many pages backed by a congested BDI were found during
> scanning. If all the dirty pages encountered on a list isolated from the
> LRU belong to a congested BDI, the zone is marked congested until the zone
> reaches the high watermark.

High watermark, or low watermark?

The terms are rather ambiguous so let's avoid them.  Maybe "full"
watermark and "empty"?

>
> ...
>
> @@ -706,6 +726,7 @@ static unsigned long shrink_page_list(struct list_head *page_list,
>  			goto keep;
>  
>  		VM_BUG_ON(PageActive(page));
> +		VM_BUG_ON(page_zone(page) != zone);

?

>  		sc->nr_scanned++;
>  
>
> ...
>
> @@ -903,6 +928,15 @@ keep_lumpy:
>  		VM_BUG_ON(PageLRU(page) || PageUnevictable(page));
>  	}
>  
> +	/*
> +	 * Tag a zone as congested if all the dirty pages encountered were
> +	 * backed by a congested BDI. In this case, reclaimers should just
> +	 * back off and wait for congestion to clear because further reclaim
> +	 * will encounter the same problem
> +	 */
> +	if (nr_dirty == nr_congested)
> +		zone_set_flag(zone, ZONE_CONGESTED);

The implicit "100%" there is a magic number.  hrm.